Java OOPS Interview Questions

These are the questions I have drafted depending on my experience level. In order to be comfortable in OOPS concept, you need have a clear understanding of these concepts rather than mocking up these answers. Normally there used to be a heavy debate on these in interviews. I would appreciate to have cross questioning in these.

JAVA INTERVIEW QUESTION

                                             1. Java OOPS Interview Questions
                                             2. Core Java Interview Questions
                                             3. Java Serialization Interview Question
                                             4. Collection Framework Interview Question
                                             5. Java Multi-Threading Interview Question

What are the basic principles of OOPS?

There are three basic principles of Object Oriented Programming Structure (OOPS)

Polymorphism: Briefly described as "one interface, many implementations."

Inheritence: Process of Aquiring properties from one object to another without changes.

Encapsulation: Wrapping up of data and methods in to a single unit.

Abstraction: Abstraction is essential element of Object Oriented Programming. 

What is Encapsulation?

It is the mechanism that binds together code and data in manipulates and keeps both safe from outside interference and misuse. In short it isolates a particular code and data from all other codes and data. A well-defined interface controls the access to that particular code and data. In Java the basis of encapsulation is the class. A class defines the structure and behavior(data and code) that will be shared by a set of objects. Each object of a given class contains the structure and behavior defined by the class as if it were stamped out of a mold in the shape of a class. A class is a logical construct an object has physical reality. When you create a class you will specify the code and data that will constitute that class. Collectively these elements are called the members of the class. Specifically the data defined by the class are referred to as member variables or instance variables.The code that operates on that data is referred to as member methods or just methods which define the use of the member variables. 

Since the purpose of a class is to encapsulate complexity there are mechanisms for hiding the complexity of the implementation inside the class. Each method or variable in a class may be marked public or private. The private methods and data can only be accessed by the code that is a member of the class. The public method has all details essential for external users.

What is Inheritence?

It is the process by which one object acquires the properties of another object. This supports the hierarchical classification. Without the use of hierarchies each object would need to define all its characteristics explicitly. However by use of inheritance an object need only define those qualities that make it unique within its class. It can inherit its general attributes from its parent. A new sub-class inherits all of the attributes ofall of its ancestors.

• A class that is inherited is called a superclass.
• The class that does the inheriting is called a subclass.
• Inheritance is done by using the keyword extends.

The two most common reasons to use inheritance are: 

1. To promote code reuse 
2. To use polymorphism

What is Polymorphism?

It is a feature that allows one interface to be used for general class of actions. The specific action is determined by the exact nature of the situation.In general polymorphism means one interface multiple methods .This means that it is possible to design a generic interface to a group of related activities. This helps reduce complexity by allowing the same interface to be used to specify a general class of action. It is the compiler's job to select the specific action (that is method) as it applies to each situation.

What is Abstraction?

Abstraction is an essential element of Object Oriented Programming which manages the complexity. In a sense, when someone works on a computer, its not necessary that person should know the working of each and every part of the computer. Even without the hardware knowledge he can e-mail, type or do any other job on the computer. Thus people do not think of a computer as a unit made up of hundreds of cards and chips but as a well-defined object with its own unique behavior. This is the advantage of abstraction. 

Object-oriented programming is modeled on how in the real world objects are often made up of many kinds of smaller objects. This capability of combining objects however is only one very general aspect of object-oriented programming.

How does Java implement polymorphism ?

(Inheritance, Overloading and Overriding are used to achieve Polymorphism in java). Polymorphism manifests itself in Java in the form of multiple methods having the same name.

• In some cases, multiple methods have the same name, but different formal argument lists (overloaded methods).
• In other cases, multiple methods have the same name, same return type, and same formal argument list (overridden methods).

Explain the different forms of Polymorphism.

There are two types of polymorphism one is Compile time polymorphism and the other is run time polymorphism. Compile time polymorphism is method overloading. Runtime time polymorphism is done using inheritance. 

Note: From a practical programming viewpoint, polymorphism manifests itself in two distinct forms in Java:

• Method overloading
• Method overriding through inheritance

What is the difference between abstraction and encapsulation ?

• Abstraction focuses on the outside view of an object (i.e. the interface) Encapsulation (information hiding) prevents clients from seeing it's inside view, where the behavior of the abstraction is implemented.
• Abstraction solves the problem in the design side while Encapsulation is the Implementation.
• Encapsulation is the deliverables of Abstraction. Encapsulation barely talks about grouping up your abstraction to suit the developer needs.

How do you express an "is a" relationship and a "has a" relationship? Explain Inheritance and Composition?

The 'is a' relationship is expressed with inheritance and 'has a' relationship is expressed with composition. Both inheritance and composition allow you to place sub-objects inside your new class. Two of the main techniques for code reuse are class inheritance and object composition. 

Inheritance is uni-directional. For example House is a Building. But Building is not a House. Inheritance uses extends key word. Composition is used when House has a Bathroom. It is incorrect to say House is a Bathroom. Composition simply means using instance variables that refer to other objects. The class House will have an instance variable, which refers to a Bathroom object. 

Inheritance should be only used when subclass 'is a' superclass.

• Don't use inheritance just to get code reuse. If there is no 'is a' relationship then use composition for code reuse. Overuse of inheritance (uses the "extends" key word) can break all the subclasses, if the superclass is modified.
• Do not use inheritance just to get polymorphism. If there is no 'is a' relationship and all you want is polymorphism then use interface inheritance with composition, which gives you code reuse.

What is method Overriding ?

An method in a subclass with the same signature (name, plus the number and the type of its parameters) and return type as a method in the superclass, then the superclass's method is said to be overridden by the subclass method, or the subclass method overrides the superclass's method. 

Any time you have a class that inherits a method from a superclass, you have the opportunity to override the method, unless the method is marked final The key benefit of overriding is the ability to define behavior that's specific to a particular subclass type. 

The following example demonstrates a Horse subclass of Animal overriding the Animal version of the eat() method:

public class Animal {
 public void eat() {
 System.out.println("Generic Animal Eating Generically");
 }
}
  
class Horse extends Animal {
 public void eat() {
 System.out.println("Horse eating hay, oats, and horse treats");
 }
}

For abstract methods you inherit from a superclass, you have no choice. You must implement the method in the subclass unless the subclass is also abstract. So you could think of abstract methods as methods you're forced to override.

What are the rules for Overridding a method?

The rules for overriding a method are as follows:

• The argument list must exactly match that of the overridden method. If they don't match, you can end up with an overloaded method you didn't intend.
• The return type must be the same as, or a subtype of, the return type declared in the original overridden method in the superclass.
• The access level can't be more restrictive than the overridden method's.
• The access level CAN be less restrictive than that of the overridden method.
• Instance methods can be overridden only if they are inherited by the subclass. A subclass within the same package as the instance's superclass can override any superclass method that is not marked private or final. A subclass in a different package can override only those non-final methods marked public or protected (since protected methods are inherited by the subclass).
• The overriding method CAN throw any unchecked (runtime) exception, regardless of whether the overridden method declares the exception.
• The overriding method must NOT throw checked exceptions that are new or broader than those declared by the overridden method. For example, a method that declares a FileNotFoundException cannot be overridden by a method that declares a SQLException, Exception, or any other non-runtime exception unless it's a subclass of FileNotFoundException.
• The overriding method can throw narrower or fewer exceptions. Just because an overridden method "takes risks" doesn't mean that the overriding subclass exception takes the same risks. Bottom line: an overriding method doesn't have to declare any exceptions that it will never throw, regardless of what the overridden method declares.
• You cannot override a method marked final.
• You cannot override a method marked static.
• If a method can't be inherited, you cannot override it. Remember that overriding implies that you're reimplementing a method you inherited.

What is the difference between Overriding and Hiding Methods?

Instance Methods: 

An instance method in a subclass with the same signature (name, plus the number and the type of its parameters) and return type as an instance method in the superclass overrides the superclass's method. 

Class Methods: 

If a subclass defines a class method with the same signature as a class method in the superclass, the method in the subclass hides the one in the superclass. 

The distinction between hiding and overriding has important implications. The version of the overridden method that gets invoked is the one in the subclass. The version of the hidden method that gets invoked depends on whether it is invoked from the superclass or the subclass. Let's look at an example that contains two classes. The first is Animal, which contains one instance method and one class method:

public class Animal {
 public static void testClassMethod() {
     System.out.println("The class method in Animal.");
 }
 public void testInstanceMethod() {
     System.out.println("The instance method in Animal.");
 }
} 

The second class, a subclass of Animal, is called Cat:

public class Cat extends Animal {
 public static void testClassMethod() {
    System.out.println("The class method in Cat.");
 }
 public void testInstanceMethod() {
    System.out.println("The instance method in Cat.");
 }
 
 public static void main(String[] args) {
    Cat myCat = new Cat();
    Animal myAnimal = myCat;
    Animal.testClassMethod();
    myAnimal.testInstanceMethod();
 }
}

The Cat class overrides the instance method in Animal and hides the class method in Animal. The main method in this class creates an instance of Cat and calls testClassMethod() on the class and testInstanceMethod() on the instance. 

The output from this program is as follows: 

The class method in Animal.
The instance method in Cat.

So, the version of the hidden method that gets invoked is the one in the superclass, and the version of the overridden method that gets invoked is the one in the subclass.

Can we change the return type in the Overriding method?

As of Java 5, you're allowed to change the return type in the overriding method as long as the new return type is a subtype of the declared return type of the overridden (superclass) method.

class Alpha {
 Alpha doSomething(char c) {
     return new Alpha();
 }
}
  
class Beta extends Alpha {
 Beta doSomething(char c) { // legal override in Java 1.5
     return new Beta();
 }
}

As of Java 5, this code will compile. If you were to attempt to compile this code with a 1.4 compiler will throw error "attempting to use incompatible return type"

What are the Rules for Constructors?

• Constructors can use any access modifier, including private. (A private constructor means only code within the class itself can instantiate an object of that type, so if the private constructor class wants to allow an instance of the class to be used, the class must provide a static method or variable that allows access to an instance created from within the class.)
• The constructor name must match the name of the class.
• Constructors must not have a return type.
• It's legal (but stupid) to have a method with the same name as the class, but that doesn't make it a constructor. If you see a return type, it's a method rather than a constructor. In fact, you could have both a method and a constructor with the same name - the name of the class - in the same class, and that's not a problem for Java.
• If you don't type a constructor into your class code, a default constructor will be automatically generated by the compiler.
• The default constructor is ALWAYS a no-arg constructor.
• If you want a no-arg constructor and you've typed any other constructor(s) into your class code, the compiler won't provide the no-arg constructor (or any other constructor) for you. In other words, if you've typed in a constructor with arguments, you won't have a no-arg constructor unless you type it in yourself.
• Every constructor has, as its first statement, either a call to an overloaded constructor (this()) or a call to the superclass constructor (super()), although remember that this call can be inserted by the compiler.
• If you do type in a constructor (as opposed to relying on the compiler-generated default constructor), and you do not type in the call to super() or a call to this(), the compiler will insert a no-arg call to super() for you, as the very first statement in the constructor.
• A call to super() can be either a no-arg call or can include arguments passed to the super constructor.
• A no-arg constructor is not necessarily the default (i.e., compiler-supplied) constructor, although the default constructor is always a no-arg constructor. The default constructor is the one the compiler provides! While the default constructor is always a no-arg constructor, you're free to put in your own noarg constructor.
• You cannot make a call to an instance method, or access an instance variable, until after the super constructor runs.
• Only static variables and methods can be accessed as part of the call to super() or this(). (Example: super(Animal.NAME) is OK, because NAME is declared as a static variable.)
• Abstract classes have constructors, and those constructors are always called when a concrete subclass is instantiated.
• Interfaces do not have constructors. Interfaces are not part of an object's inheritance tree.
• The only way a constructor can be invoked is from within another constructor.

What is Coupling and Cohesion?

Coupling: Coupling is the degree to which one class knows about another class. If the only knowledge that class A has about class B, is what class B has exposed through its interface, then class A and class B are said to be loosely coupled?that's a good thing. If, on the other hand, class A relies on parts of class B that are not part of class B's interface, then the coupling between the classes is tighter?not a good thing. In other words, if A knows more than it should about the way in which B was implemented, then A and B are tightly coupled. 

Cohesion: While coupling has to do with how classes interact with each other, cohesion is all about how a single class is designed. The term cohesion is used to indicate the degree to which a class has a single, well-focused purpose. The more focused a class is, the higher its cohesiveness - a good thing. The key benefit of high cohesion is that such classes are typically much easier to maintain (and less frequently changed) than classes with low cohesion. Another benefit of high cohesion is that classes with a well-focused purpose tend to be more reusable than other classes.

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Java Multi-Threading Interview Questions

A good hands on experience is required to understand the core threads concepts. Most of the Core Java interview questions are derived from Multi-Threading & Collections framework. Here are some of the questions and their answers that I encountered doing my interviews recently. But it is very much important to understand the base concept rather than mocking the question's answer. I could not able to frame some small small questions here as those questions were asked to me as cross questions to my explanations.

JAVA INTERVIEW QUESTION

                                             1. Java OOPS Interview Questions
                                             2. Core Java Interview Questions
                                             3. Java Serialization Interview Question
                                             4. Collection Framework Interview Question
                                             5. Java Multi-Threading Interview Question

This post is a collection of some good questions on Java threads, which can be asked to a senior java developer. 

What is synchronization in respect to multi-threading in Java?

With respect to multi-threading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for one Java thread to modify a shared variable while another thread is in the process of using or updating same shared variable. This usually leads to erroneous behavior or program.

Explain different way of using thread?

A Java thread could be implemented by using Runnable interface or by extending the Thread class. The Runnable is more advantageous, when you are going for multiple inheritance. But you need to use Thread class instance in order to make a thread.

What is the difference between Thread.start() & Thread.run() method?

Thread.start() method (native method) of Thread class actually does the job of running the Thread.run() method in a thread. If we directly call Thread.run() method it will executed in same thread, so does not solve the purpose of creating a new thread.

Why do we need run() & start() method both. Can we achieve it with only run method?

We need run() & start() method both because JVM needs to create a separate thread which can not be differentiated from a normal method call. So this job is done by start method native implementation which has to be explicitly called. Another advantage of having these two methods is we can have any object run as a thread if it implements Runnable interface. This is to avoid Java’s multiple inheritance problems which will make it difficult to inherit another class with Thread. 

What is ThreadLocal class? How can it be used?

Below are some key points about ThreadLocal variables

A thread-local variable effectively provides a separate copy of its value for each thread that uses it.

ThreadLocal instances are typically private static fields in classes that wish to associate state with a thread

In case when multiple threads access a ThreadLocal instance, separate copy of Threadlocal variable is maintained for each thread.

Common use is seen in DAO pattern where the DAO class can be singleton but the Database connection can be maintained separately for each thread. (Per Thread Singleton)

When InvalidMonitorStateException is thrown? Why?

This exception is thrown when you try to call wait()/notify()/notifyAll() any of these methods for an Object from a point in your program where u are NOT having a lock on that object.(i.e. u r not executing any synchronized block/method of that object and still trying to call wait()/notify()/notifyAll()) wait(), notify() and notifyAll() all throw IllegalMonitorStateException. since This exception is a subclass of RuntimeException so we r not bound to catch it (although u may if u want to). and being a RuntimeException this exception is not mentioned in the signature of wait(), notify(), notifyAll() methods.

What is the difference between sleep(), suspend() and wait() ?

Thread.sleep() takes the current thread to a "Not Runnable" state for specified amount of time. The thread holds the monitors it has acquired. For example, if a thread is running a synchronized block or method and sleep method is called then no other thread will be able to enter this block or method. The sleeping thread can wake up when some other thread calls t.interrupt on it. Note that sleep is a static method, that means it always affects the current thread (the one executing sleep method). A common mistake is trying to call t2.sleep() where t2 is a different thread; even then, it is the current thread that will sleep, not the t2 thread. thread.suspend() is deprecated method. Its possible to send other threads into suspended state by making a suspend method call. In suspended state a thread keeps all its monitors and can not be interrupted. This may cause deadlocks therefore it has been deprecated. object.wait() call also takes the current thread into a "Not Runnable" state, just like sleep(), but with a slight change. Wait method is invoked lock on a object, not thread. 

Here is the sequence of operations you can think 

A thread T1 is already running a synchronized block with a lock on object - lets say "lockObject"

Another thread T2 comes to execute the synchronized block and find that its already acquired.

Now T2 calls lockObject.wait() method for waiting on lock to be release by T1 thread.

T1 thread finishes all its synchronized block work.

T1 thread calls lockObject.notifyAll() to notify all waiting threads that its done using the lock.

Since T2 thread is first in the queue of waiting it acquires the lock and starts processing.

What happens when I make a static method as synchronized?

Synchronized static methods have a lock on the class "Class", so when a thread enters a synchronized static method, the class itself gets locked by the thread monitor and no other thread can enter any static synchronized methods on that class. This is unlike instance methods, as multiple threads can access "same synchronized instance methods" at same time for different instances.

Can a thread call a non-synchronized instance method of an Object when a synchronized method is being executed ?

Yes, a Non synchronized method can always be called without any problem. In fact Java does not do any check for a non-synchronized method. The Lock object check is performed only for synchronized methods/blocks. In case the method is not declared synchronized Jave will call even if you are playing with shared data. So you have to be careful while doing such thing. The decision of declaring a method as synchronized has to be based on critical section access. If your method does not access a critical section (shared resource or data structure) it need not be declared synchronized. Below is the example which demonstrates this, The Common class has two methods synchronizedMethod1() and method1() MyThread class is calling both the methods in separate threads,

public class Common {  
  
 public synchronized void synchronizedMethod1() {  
  System.out.println("synchronizedMethod1 called");  
  try {  
   Thread.sleep(1000);    
  } catch (InterruptedException e) {  
   e.printStackTrace();  
  }  
  System.out.println("synchronizedMethod1 done");  
 }
  
 public void method1() {  
  System.out.println("Method 1 called");  
  try {  
   Thread.sleep(1000);  
  } catch (InterruptedException e) {  
   e.printStackTrace();  
  }  
  System.out.println("Method 1 done");  
 }  
}  

public class MyThread extends Thread {  
 private int id = 0;  
 private Common common;  
  
 public MyThread(String name, int no, Common object) {  
  super(name);  
  common = object;  
  id = no;  
 }  
  
 public void run() {  
  System.out.println("Running Thread" + this.getName());  
  try {  
   if (id == 0) {  
    common.synchronizedMethod1();  
   } else {  
   common.method1();  
   }  
  } catch (Exception e) {  
   e.printStackTrace();  
  }  
 }  
  
 public static void main(String[] args) {  
  Common c = new Common();  
  MyThread t1 = new MyThread("MyThread-1", 0, c);  
  MyThread t2 = new MyThread("MyThread-2", 1, c);  
  t1.start();  
  t2.start();  
 }  
}  

Here is the output of the program

Running ThreadMyThread-1  
synchronizedMethod1 called  
Running ThreadMyThread-2  
Method 1 called  
synchronizedMethod1 done  
Method 1 done  

This shows that method1() - is called even though the synchronizedMethod1() was in execution. 

Can two threads call two different synchronized instance methods of an Object?

No. If a object has synchronized instance methods then the Object itself is used a lock object for controlling the synchronization. Therefore all other instance methods need to wait until previous method call is completed. See the below sample code which demonstrate it very clearly. The Class Common has 2 methods called synchronizedMethod1() and synchronizedMethod2() MyThread class is calling both the methods

public class Common {  
 public synchronized void synchronizedMethod1() {  
  System.out.println("synchronizedMethod1 called");  
  try {  
   Thread.sleep(1000);  
  } catch (InterruptedException e) {  
   e.printStackTrace();  
  }  
  System.out.println("synchronizedMethod1 done");  
 }  
  
 public synchronized void synchronizedMethod2() {  
  System.out.println("synchronizedMethod2 called");  
  try {  
   Thread.sleep(1000);  
  } catch (InterruptedException e) {  
   e.printStackTrace();  
  }  
  System.out.println("synchronizedMethod2 done");  
 }  
}  

public class MyThread extends Thread {  
 private int id = 0;  
 private Common common;  
  
 public MyThread(String name, int no, Common object) {  
  super(name);  
  common = object;  
  id = no;  
 }  
  
 public void run() {  
  System.out.println("Running Thread" + this.getName());  
  try {  
   if (id == 0) {  
    common.synchronizedMethod1();  
   } else {  
    common.synchronizedMethod2();  
   }  
  } catch (Exception e) {  
   e.printStackTrace();  
  }  
 }  
  
 public static void main(String[] args) {  
  Common c = new Common();  
  MyThread t1 = new MyThread("MyThread-1", 0, c);  
  MyThread t2 = new MyThread("MyThread-2", 1, c);  
  t1.start();  
  t2.start();  
 }  
}

What is a deadlock?

Deadlock is a situation where two or more threads are blocked forever, waiting for each other. This may occur when two threads, each having a lock on one resource, attempt to acquire a lock on the other's resource. Each thread would wait indefinitely for the other to release the lock, unless one of the user processes is terminated. In terms of Java API, thread deadlock can occur in following conditions:

When two threads call Thread.join() on each other.
When two threads use nested synchronized blocks to lock two objects and the blocks lock the same objects in different order.

What is Starvation? and What is a Livelock?

Starvation and livelock are much less common a problem than deadlock, but are still problems that every designer of concurrent software is likely to encounter.

LiveLock
Livelock occurs when all threads are blocked, or are otherwise unable to proceed due to unavailability of required resources, and the non-existence of any unblocked thread to make those resources available. In terms of Java API, thread livelock can occur in following conditions:
When all the threads in a program execute Object.wait(0) on an object with zero parameter. The program is live-locked and cannot proceed until one or more threads call Object.notify() or Object.notifyAll() on the relevant objects. Because all the threads are blocked, neither call can be made.
When all the threads in a program are stuck in infinite loops.

Starvation
Starvation describes a situation where a thread is unable to gain regular access to shared resources and is unable to make progress. This happens when shared resources are made unavailable for long periods by "greedy" threads. For example, suppose an object provides a synchronized method that often takes a long time to return. If one thread invokes this method frequently, other threads that also need frequent synchronized access to the same object will often be blocked. Starvation occurs when one thread cannot access the CPU because one or more other threads are monopolizing the CPU. In Java, thread starvation can be caused by setting thread priorities inappropriately. A lower-priority thread can be starved by higher-priority threads if the higher-priority threads do not yield control of the CPU from time to time.

How to find a deadlock has occurred in Java? How to detect a Deadlock in Java?

Earlier versions of Java had no mechanism to handle/detect deadlock. Since JDK 1.5 there are some powerful methods added in the java.lang.management package to diagnose and detect deadlocks. The java.lang.management.ThreadMXBean interface is management interface for the thread system of the Java virtual machine. It has two methods which can leveraged to detect deadlock in a Java application.

findMonitorDeadlockedThreads() - This method can be used to detect cycles of threads that are in deadlock waiting to acquire object monitors. It returns an array of thread IDs that are deadlocked waiting on monitor.
findDeadlockedThreads() - It returns an array of thread IDs that are deadlocked waiting on monitor or ownable synchronizers.

What is immutable object? How does it help in writing concurrent application?

An object is considered immutable if its state cannot change after it is constructed. Maximum reliance on immutable objects is widely accepted as a sound strategy for creating simple, reliable code. Immutable objects are particularly useful in concurrent applications. Since they cannot change state, they cannot be corrupted by thread interference or observed in an inconsistent state. Examples of immutable objects from the JDK include String and Integer. Immutable objects greatly simplify your multi threaded program, since they are

Simple to construct, test, and use.
Automatically thread-safe and have no synchronization issues.
To create a object immutable You need to make the class final and all its member final so that once objects gets crated no one can modify its state. You can achieve same functionality by making member as non final but private and not modifying them except in constructor.

How will you take thread dump in Java? How will you analyze Thread dump?

A Thread Dump is a complete list of active threads. A java thread dump is a way of finding out what each thread in the JVM is doing at a particular point of time. This is especially useful when your java application seems to have some performance issues. Thread dump will help you to find out which thread is causing this. There are several ways to take thread dumps from a JVM. It is highly recommended to take more than 1 thread dump and analyze the results based on it. Follow below steps to take thread dump of a java process

Step 1 

On UNIX, Linux and Mac OSX Environment run below command:

ps -el | grep java 

Step 2: 

Use jstack command to print the Java stack traces for a given Java process PID

jstack [PID] 

More details of jstack command can be found here : JSTACK Command Manual

What is a thread leak? What does it mean in Java?

Thread leak is when a application does not release references to a thread object properly. Due to this some Threads do not get garbage collected and the number of unused threads grow with time. Thread leak can often cause serious issues on a Java application since over a period of time too many threads will be created but not released and may cause applications to respond slow or hang.

How can I trace whether the application has a thread leak?

If an application has thread leak then with time it will have too many unused threads. Try to find out what type of threads is leaking out. This can be done using following ways
Give unique and descriptive names to the threads created in application. - Add log entry in all thread at various entry and exit points in threads.
Change debugging config levels (debug, info, error etc) and analyze log messages.
When you find the class that is leaking out threads check how new threads are instantiated and how they're closed.
Make sure the thread is Guaranteed to close properly by doing following - Handling all Exceptions properly.
Make sure the thread is Guaranteed to close properly by doing following
Handling all Exceptions properly.
releasing all resources (e.g. connections, files etc) before it closes.

What is thread pool? Why should we use thread pools?

A thread pool is a collection of threads on which task can be scheduled. Instead of creating a new thread for each task, you can have one of the threads from the thread pool pulled out of the pool and assigned to the task. When the thread is finished with the task, it adds itself back to the pool and waits for another assignment. One common type of thread pool is the fixed thread pool. This type of pool always has a specified number of threads running; if a thread is somehow terminated while it is still in use, it is automatically replaced with a new thread. Below are key reasons to use a Thread Pool
Using thread pools minimizes the JVM overhead due to thread creation. Thread objects use a significant amount of memory, and in a large-scale application, allocating and de-allocating many thread objects creates a significant memory management overhead.
You have control over the maximum number of tasks that are being processed in parallel (= number of threads in the pool).
Most of the executor implementations in java.util.concurrent use thread pools, which consist of worker threads. This kind of thread exists separately from the Runnable and Callable tasks it executes and is often used to execute multiple tasks.

Can we synchronize the run method? If yes then what will be the behavior?

Yes, the run method of a runnable class can be synchronized. If you make run method synchronized then the lock on runnable object will be occupied before executing the run method. In case we start multiple threads using the same runnable object in the constructor of the Thread then it would work. But until the 1st thread ends the 2nd thread cannot start and until the 2nd thread ends the next cannot start as all the threads depend on lock on same object

Can we synchronize the constructor of a Java Class?

As per Java Language Specification, constructors cannot be synchronized because other threads cannot see the object being created before the thread creating it has finished it. There is no practical need of a Java Objects constructor to be synchronized, since it would lock the object being constructed, which is normally not available to other threads until all constructors of the object finish

This is not an exhaustive list of questions and I am sure I have missed many important questions from Multi-threading area. Can you think of a question which should be included in this list? Please feel free to share any question/suggestion in the comments section.

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Collections Framework Interview Questions

Java Collections Framework contains most commonly asked Java interview questions. A good understanding of Collections framework is required to understand and leverage many powerful features of Java technology.

JAVA INTERVIEW QUESTION

                                             1. Java OOPS Interview Questions
                                             2. Core Java Interview Questions
                                             3. Java Serialization Interview Question
                                             4. Collection Framework Interview Question
                                             5. Java Multi-Threading Interview Question

Java Collections framework is fundamental utility tools provided by Java that are not only topic of interview but also heavily used in java programming on all types of programs including web based and desktop applications.

Your knowledge of Java will be considered incomplete without a good working experience of collections API, therefore I highly recommend writing more code and experimenting with it as much as possible.

Here are few important practical questions that can be asked in a Java interview. 

What is Java Collections API?

Java Collections framework API is a unified architecture for representing and manipulating collections. The API contains Interfaces, Implementations & Algorithm to help java programmer in everyday programming. In nutshell, this API does 6 things at high level

    Reduces programming efforts. - Increases program speed and quality.

    Allows interoperability among unrelated APIs.

    Reduces effort to learn and to use new APIs.

    Reduces effort to design new APIs.

    Encourages & Fosters software reuse.

To be specific, There are six collection java interfaces. The most basic interface is Collection. Three interfaces extend Collection: Set, List, and SortedSet. The other two collection interfaces, Map and SortedMap, do not extend Collection, as they represent mappings rather than true collections.

What is an Iterator?

Some of the collection classes provide traversal of their contents via a java.util.Iterator interface. This interface allows you to walk through a collection of objects, operating on each object in turn. Remember when using Iterators that they contain a snapshot of the collection at the time the Iterator was obtained; generally it is not advisable to modify the collection itself while traversing an Iterator.

What is the difference between java.util.Iterator and java.util.ListIterator?

Iterator : Enables you to traverse through a collection in the forward direction only, for obtaining or removing elements 

ListIterator : extends Iterator, and allows bidirectional traversal of list and also allows the modification of elements.

What is HashMap and Map?

Map is Interface which is part of Java collections framework. This is to store Key Value pair, and Hashmap is class that implements that using hashing technique.

Difference between HashMap and HashTable? Compare Hashtable vs HashMap?

Both Hashtable & HashMap provide key-value access to data. The Hashtable is one of the original collection classes in Java (also called as legacy classes). HashMap is part of the new Collections Framework, added with Java 2, v1.2. There are several differences between HashMap and Hashtable in Java as listed below

    The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls. (HashMap allows null values as key and value whereas Hashtable doesn’t allow nulls).

    HashMap does not guarantee that the order of the map will remain constant over time. But one of HashMap's subclasses is LinkedHashMap, so in the event that you'd want predictable iteration order (which is insertion order by default), you can easily swap out the HashMap for a LinkedHashMap. This wouldn't be as easy if you were using Hashtable.

    HashMap is non synchronized whereas Hashtable is synchronized.

    Iterator in the HashMap is fail-fast while the enumerator for the Hashtable isn't. So this could be a design consideration.

What does synchronized means in Hashtable context?

Synchronized means only one thread can modify a hash table at one point of time. Any thread before performing an update on a hashtable will have to acquire a lock on the object while others will wait for lock to be released.

What is fail-fast property?

At high level - Fail-fast is a property of a system or software with respect to its response to failures. A fail-fast system is designed to immediately report any failure or condition that is likely to lead to failure. Fail-fast systems are usually designed to stop normal operation rather than attempt to continue a possibly-flawed process. When a problem occurs, a fail-fast system fails immediately and visibly. Failing fast is a non-intuitive technique: "failing immediately and visibly" sounds like it would make your software more fragile, but it actually makes it more robust. Bugs are easier to find and fix, so fewer go into production. In Java, Fail-fast term can be related to context of iterators. If an iterator has been created on a collection object and some other thread tries to modify the collection object "structurally", a concurrent modification exception will be thrown. It is possible for other threads though to invoke "set" method since it doesn't modify the collection "structurally". However, if prior to calling "set", the collection has been modified structurally, "IllegalArgumentException" will be thrown.

Why doesn't Collection extend Cloneable and Serializable?

From Sun FAQ Page: Many Collection implementations (including all of the ones provided by the JDK) will have a public clone method, but it would be mistake to require it of all Collections. For example, what does it mean to clone a Collection that's backed by a terabyte SQL database? Should the method call cause the company to requisition a new disk farm? Similar arguments hold for serializable. If the client doesn't know the actual type of a Collection, it's much more flexible and less error prone to have the client decide what type of Collection is desired, create an empty Collection of this type, and use the addAll method to copy the elements of the original collection into the new one. Note on Some Important Terms

    Synchronized means only one thread can modify a hash table at one point of time. Basically, it means that any thread before performing an update on a hashtable will have to acquire a lock on the object while others will wait for lock to be released.

    Fail-fast is relevant from the context of iterators. If an iterator has been created on a collection object and some other thread tries to modify the collection object "structurally”, a concurrent modification exception will be thrown. It is possible for other threads though to invoke "set" method since it doesn’t modify the collection "structurally”. However, if prior to calling "set", the collection has been modified structurally, "IllegalArgumentException" will be thrown.

How can we make Hashmap synchronized?

HashMap can be synchronized by Map m = Collections.synchronizedMap(hashMap);

Where will you use Hashtable and where will you use HashMap?

There are multiple aspects to this decision: 1. The basic difference between a Hashtable and an HashMap is that, Hashtable is synchronized while HashMap is not. Thus whenever there is a possibility of multiple threads accessing the same instance, one should use Hashtable. While if not multiple threads are going to access the same instance then use HashMap. Non synchronized data structure will give better performance than the synchronized one. 2. If there is a possibility in future that - there can be a scenario when you may require to retain the order of objects in the Collection with key-value pair then HashMap can be a good choice. As one of HashMap's subclasses is LinkedHashMap, so in the event that you'd want predictable iteration order (which is insertion order by default), you can easily swap out the HashMap for a LinkedHashMap. This wouldn't be as easy if you were using Hashtable. Also if you have multiple thread accessing you HashMap then Collections.synchronizedMap() method can be leveraged. Overall HashMap gives you more flexibility in terms of possible future changes.

Difference between Vector and ArrayList? What is the Vector class?

Vector & ArrayList both classes are implemented using dynamically resizable arrays, providing fast random access and fast traversal. ArrayList and Vector class both implement the List interface. Both the classes are member of Java collection framework, therefore from an API perspective, these two classes are very similar. However, there are still some major differences between the two. Below are some key differences

    Vector is a legacy class which has been retrofitted to implement the List interface since Java 2 platform v1.2

    Vector is synchronized whereas ArrayList is not. Even though Vector class is synchronized, still when you want programs to run in multithreading environment using ArrayList with Collections.synchronizedList() is recommended over Vector.

    ArrayList has no default size while vector has a default size of 10.

    The Enumerations returned by Vector's elements method are not fail-fast. Whereas ArraayList does not have any method returning Enumerations.

What is the Difference between Enumeration and Iterator interface?

Enumeration and Iterator are the interface available in java.util package. The functionality of Enumeration interface is duplicated by the Iterator interface. New implementations should consider using Iterator in preference to Enumeration. Iterators differ from enumerations in following ways:

    Enumeration contains 2 methods namely hasMoreElements() & nextElement() whereas Iterator contains three methods namely hasNext(), next(),remove().

    Iterator adds an optional remove operation, and has shorter method names. Using remove() we can delete the objects but Enumeration interface does not support this feature.

    Enumeration interface is used by legacy classes. Vector.elements() & Hashtable.elements() method returns Enumeration. Iterator is returned by all Java Collections Framework classes. java.util.Collection.iterator() method returns an instance of Iterator.

Why Java Vector class is considered obsolete or unofficially deprecated? or Why should I always use ArrayList over Vector?

You should use ArrayList over Vector because you should default to non-synchronized access. Vector synchronizes each individual method. That's almost never what you want to do. Generally you want to synchronize a whole sequence of operations. Synchronizing individual operations is both less safe (if you iterate over a Vector, for instance, you still need to take out a lock to avoid anyone else changing the collection at the same time) but also slower (why take out a lock repeatedly when once will be enough)? Of course, it also has the overhead of locking even when you don't need to. It's a very flawed approach to have synchronized access as default. You can always decorate a collection using Collections.synchronizedList - the fact that Vector combines both the "resized array" collection implementation with the "synchronize every operation" bit is another example of poor design; the decoration approach gives cleaner separation of concerns. Vector also has a few legacy methods around enumeration and element retrieval which are different than the List interface, and developers (especially those who learned Java before 1.2) can tend to use them if they are in the code. Although Enumerations are faster, they don't check if the collection was modified during iteration, which can cause issues, and given that Vector might be chosen for its syncronization - with the attendant access from multiple threads, this makes it a particularly pernicious problem. Usage of these methods also couples a lot of code to Vector, such that it won't be easy to replace it with a different List implementation. Despite all above reasons Sun may never officially deprecate Vector class. (Read details Deprecate Hashtable and Vector)

What is an enumeration?

An enumeration is an interface containing methods for accessing the underlying data structure from which the enumeration is obtained. It is a construct which collection classes return when you request a collection of all the objects stored in the collection. It allows sequential access to all the elements stored in the collection.

What is the difference between Enumeration and Iterator?

The functionality of Enumeration interface is duplicated by the Iterator interface. Iterator has a remove() method while Enumeration doesn't. Enumeration acts as Read-only interface, because it has the methods only to traverse and fetch the objects, where as using Iterator we can manipulate the objects also like adding and removing the objects. So Enumeration is used when ever we want to make Collection objects as Read-only.

Where will you use Vector and where will you use ArrayList?

The basic difference between a Vector and an ArrayList is that, vector is synchronized while ArrayList is not. Thus whenever there is a possibility of multiple threads accessing the same instance, one should use Vector. While if not multiple threads are going to access the same instance then use ArrayList. Non synchronized data structure will give better performance than the synchronized one.

What is the importance of hashCode() and equals() methods? How they are used in Java?

The java.lang.Object has two methods defined in it. They are - public boolean equals(Object obj) public int hashCode(). These two methods are used heavily when objects are stored in collections. There is a contract between these two methods which should be kept in mind while overriding any of these methods. The Java API documentation describes it in detail. The hashCode() method returns a hash code value for the object. This method is supported for the benefit of hashtables such as those provided by java.util.Hashtable or java.util.HashMap. The general contract of hashCode is: Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, provided no information used in equals comparisons on the object is modified. This integer need not remain consistent from one execution of an application to another execution of the same application. If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result. It is not required that if two objects are unequal according to the equals(java.lang.Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hashtables. As much as is reasonably practical, the hashCode method defined by class Object does return distinct integers for distinct objects. The equals(Object obj) method indicates whether some other object is "equal to" this one. The equals method implements an equivalence relation on non-null object references: It is reflexive: for any non-null reference value x, x.equals(x) should return true. It is symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true. It is transitive: for any non-null reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true. It is consistent: for any non-null reference values x and y, multiple invocations of x.equals(y) consistently return true or consistently return false, provided no information used in equals comparisons on the objects is modified. For any non-null reference value x, x.equals(null) should return false. The equals method for class Object implements the most discriminating possible equivalence relation on objects; that is, for any non-null reference values x and y, this method returns true if and only if x and y refer to the same object (x == y has the value true). Note that it is generally necessary to override the hashCode method whenever this method is overridden, so as to maintain the general contract for the hashCode method, which states that equal objects must have equal hash codes. A practical Example of hashcode() & equals(): This can be applied to classes that need to be stored in Set collections. Sets use equals() to enforce non-duplicates, and HashSet uses hashCode() as a first-cut test for equality. Technically hashCode() isn't necessary then since equals() will always be used in the end, but providing a meaningful hashCode() will improve performance for very large sets or objects that take a long time to compare using equals().

What is the difference between Sorting performance of Arrays.sort() vs Collections.sort() ? Which one is faster? Which one to use and when?

Many developers are concerned about the performance difference between java.util.Array.sort() java.util.Collections.sort() methods. Both methods have same algorithm the only difference is type of input to them. Collections.sort() has a input as List so it does a translation of List to array and vice versa which is an additional step while sorting. So this should be used when you are trying to sort a list. Arrays.sort is for arrays so the sorting is done directly on the array. So clearly it should be used when you have a array available with you and you want to sort it.

What is java.util.concurrent BlockingQueue? How it can be used?

Java has implementation of BlockingQueue available since Java 1.5. Blocking Queue interface extends collection interface, which provides you power of collections inside a queue. Blocking Queue is a type of Queue that additionally supports operations that wait for the queue to become non-empty when retrieving an element, and wait for space to become available in the queue when storing an element. A typical usage example would be based on a producer-consumer scenario. Note that a BlockingQueue can safely be used with multiple producers and multiple consumers. An ArrayBlockingQueue is a implementation of blocking queue with an array used to store the queued objects. The head of the queue is that element that has been on the queue the longest time. The tail of the queue is that element that has been on the queue the shortest time. New elements are inserted at the tail of the queue, and the queue retrieval operations obtain elements at the head of the queue. ArrayBlockingQueue requires you to specify the capacity of queue at the object construction time itself. Once created, the capacity cannot be increased. This is a classic "bounded buffer" (fixed size buffer), in which a fixed-sized array holds elements inserted by producers and extracted by consumers. Attempts to put an element to a full queue will result in the put operation blocking; attempts to retrieve an element from an empty queue will be blocked.

Set & List interface extend Collection, so Why doesn't Map interface extend Collection?

Though the Map interface is part of collections framework, it does not extend collection interface. This is by design, and the answer to this questions is best described in Sun's FAQ Page: This was by design. We feel that mappings are not collections and collections are not mappings. Thus, it makes little sense for Map to extend the Collection interface (or vice versa). If a Map is a Collection, what are the elements? The only reasonable answer is "Key-value pairs", but this provides a very limited (and not particularly useful) Map abstraction. You can't ask what value a given key maps to, nor can you delete the entry for a given key without knowing what value it maps to. Collection could be made to extend Map, but this raises the question: what are the keys? There's no really satisfactory answer, and forcing one leads to an unnatural interface. Maps can be viewed as Collections (of keys, values, or pairs), and this fact is reflected in the three "Collection view operations" on Maps (keySet, entrySet, and values). While it is, in principle, possible to view a List as a Map mapping indices to elements, this has the nasty property that deleting an element from the List changes the Key associated with every element before the deleted element. That's why we don't have a map view operation on Lists.

Which implementation of the List interface provides for the fastest insertion of a new element into the middle of the list?

a. Vector b. ArrayList c. LinkedList ArrayList and Vector both use an array to store the elements of the list. When an element is inserted into the middle of the list the elements that follow the insertion point must be shifted to make room for the new element. The LinkedList is implemented using a doubly linked list; an insertion requires only the updating of the links at the point of insertion. Therefore, the LinkedList allows for fast insertions and deletions.

What is the difference between ArrayList and LinkedList? (ArrayList vs LinkedList.)

java.util.ArrayList and java.util.LinkedList are two Collections classes used for storing lists of object references Here are some key differences:

    ArrayList uses primitive object array for storing objects whereas LinkedList is made up of a chain of nodes. Each node stores an element and the pointer to the next node. A singly linked list only has pointers to next. A doubly linked list has a pointer to the next and the previous element. This makes walking the list backward easier.

    ArrayList implements the RandomAccess interface, and LinkedList does not. The commonly used ArrayList implementation uses primitive Object array for internal storage. Therefore an ArrayList is much faster than a LinkedList for random access, that is, when accessing arbitrary list elements using the get method. Note that the get method is implemented for LinkedLists, but it requires a sequential scan from the front or back of the list. This scan is very slow. For a LinkedList, there's no fast way to access the Nth element of the list.

    Adding and deleting at the start and middle of the ArrayList is slow, because all the later elements have to be copied forward or backward. (Using System.arrayCopy()) Whereas Linked lists are faster for inserts and deletes anywhere in the list, since all you do is update a few next and previous pointers of a node.

    Each element of a linked list (especially a doubly linked list) uses a bit more memory than its equivalent in array list, due to the need for next and previous pointers.

    ArrayList may also have a performance issue when the internal array fills up. The arrayList has to create a new array and copy all the elements there. The ArrayList has a growth algorithm of (n*3)/2+1, meaning that each time the buffer is too small it will create a new one of size (n*3)/2+1 where n is the number of elements of the current buffer. Hence if we can guess the number of elements that we are going to have, then it makes sense to create a arraylist with that capacity during object creation (using construtor new ArrayList(capacity)). Whereas LinkedLists should not have such capacity issues.

Where will you use ArrayList and Where will you use LinkedList? Or Which one to use when (ArrayList / LinkedList).

Below is a snippet from SUN's site. The Java SDK contains 2 implementations of the List interface - ArrayList and LinkedList. If you frequently add elements to the beginning of the List or iterate over the List to delete elements from its interior, you should consider using LinkedList. These operations require constant-time in a LinkedList and linear-time in an ArrayList. But you pay a big price in performance. Positional access requires linear-time in a LinkedList and constant-time in an ArrayList.

What is performance of various Java collection implementations/algorithms? What is Big 'O' notation for each of them ?

Each java collection implementation class have different performance for different methods, which makes them suitable for different programming needs.

Performance of Map interface implementations

Hashtable

An instance of Hashtable has two parameters that affect its performance: initial capacity and load factor. The capacity is the number of buckets in the hash table, and the initial capacity is simply the capacity at the time the hash table is created. Note that the hash table is open: in the case of a "hash collision", a single bucket stores multiple entries, which must be searched sequentially. The load factor is a measure of how full the hash table is allowed to get before its capacity is automatically increased. The initial capacity and load factor parameters are merely hints to the implementation. The exact details as to when and whether the rehash method is invoked are implementation-dependent.

HashMap

This implementation provides constant-time [ Big O Notation is O(1) ] performance for the basic operations (get and put), assuming the hash function disperses the elements properly among the buckets. Iteration over collection views requires time proportional to the "capacity" of the HashMap instance (the number of buckets) plus its size (the number of key-value mappings). Thus, it's very important not to set the initial capacity too high (or the load factor too low) if iteration performance is important.

TreeMap

The TreeMap implementation provides guaranteed log(n) [ Big O Notation is O(log N) ] time cost for the containsKey, get, put and remove operations.

LinkedHashMap

A linked hash map has two parameters that affect its performance: initial capacity and load factor. They are defined precisely as for HashMap. Note, however, that the penalty for choosing an excessively high value for initial capacity is less severe for this class than for HashMap, as iteration times for this class are unaffected by capacity.

Performance of Set interface implementations

HashSet

The HashSet class offers constant-time [ Big O Notation is O(1) ] performance for the basic operations (add, remove, contains and size), assuming the hash function disperses the elements properly among the buckets. Iterating over this set requires time proportional to the sum of the HashSet instance's size (the number of elements) plus the "capacity" of the backing HashMap instance (the number of buckets). Thus, it's very important not to set the initial capacity too high (or the load factor too low) if iteration performance is important.

TreeSet

The TreeSet implementation provides guaranteed log(n) time cost for the basic operations (add, remove and contains).

LinkedHashSet

A linked hash set has two parameters that affect its performance: initial capacity and load factor. They are defined precisely as for HashSet. Note, however, that the penalty for choosing an excessively high value for initial capacity is less severe for this class than for HashSet, as iteration times for this class are unaffected by capacity.

Performance of List interface implementations

LinkedList

Performance of get and remove methods is linear time [ Big O Notation is O(n) ] - Performance of add and Iterator.remove methods is constant-time [ Big O Notation is O(1) ]

ArrayList

The size, isEmpty, get, set, iterator, and listIterator operations run in constant time. [ Big O Notation is O(1) ] - The add operation runs in amortized constant time [ Big O Notation is O(1) ] , but in worst case (since the array must be resized and copied) adding n elements requires linear time [ Big O Notation is O(n) ] - Performance of remove method is linear time [ Big O Notation is O(n) ] - All of the other operations run in linear time [ Big O Notation is O(n) ]. The constant factor is low compared to that for the LinkedList implementation.

How set implantation maintains uniqueness of the objects?

Set implantation internally use Map to store object. Basically it stores the objects to key of the Map. Set implementations does not have it's own logic to evaluate uniqueness. This is how here We have re used the code. And this is how HashSet stores the object using hashing technique, as you know HashMap's key is being stored using hashing technique.

Can you think of a questions which is not part of this post? Please don't forget to share it with me in comments section & I will try to include it in the list.

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Core Java Interview Questions

Recently I was looking for a new role in my career and I came across some important question in the interviews. This is list of some Java fundamental questions and answers, which are commonly asked in a Core Java interview for Experienced Developers. As a senior and matured Java Programmer you must know the answers to these questions to demonstrate basic understanding of Java language and depth of knowledge. Below is a list of some tricky core java interview questions that may give you an edge in your next Java interview.

JAVA INTERVIEW QUESTION

                                             1. Java OOPS Interview Questions
                                             2. Core Java Interview Questions
                                             3. Java Serialization Interview Question
                                             4. Collection Framework Interview Question
                                             5. Java Multi-Threading Interview Question

What is immutable object in Java? Can you change values of a immutable object?

A Java object is considered immutable when its state cannot change after it is created. Use of immutable objects is widely accepted as a sound strategy for creating simple, reliable code. Immutable objects are particularly useful in concurrent applications. Since they cannot change state, they cannot be corrupted by thread interference or observed in an inconsistent state. java.lang.String and java.lang.Integer classes are the Examples of immutable objects from the Java Development Kit. Immutable objects simplify your program due to following characteristics :

• Immutable objects are simple to use test and construct.
• Immutable objects are automatically thread-safe.
• Immutable objects do not require a copy constructor.
• Immutable objects do not require an implementation of clone.
• Immutable objects allow hashCode to use lazy initialization, and to cache its return value.
• Immutable objects do not need to be copied defensively when used as a field.
• Immutable objects are good Map keys and Set elements (Since state of these objects must not change while stored in a collection).
• Immutable objects have their class invariant established once upon construction, and it never needs to be checked again.
• Immutable objects always have "failure atomicity" (a term used by Joshua Bloch) : if an immutable object throws an exception, it's never left in an undesirable or indeterminate state.

How to create a immutable object in Java? Does all property of immutable object needs to be final?

To create a object immutable You need to make the class final and all its member final so that once objects gets crated no one can modify its state. You can achieve same functionality by making member as non final but private and not modifying them except in constructor. Also its NOT necessary to have all the properties final since you can achieve same functionality by making member as non final but private and not modifying them except in constructor.

What is difference between String, StringBuffer and StringBuilder? When to use them?

The main difference between the three most commonly used String classes as follows.

StringBuffer and StringBuilder objects are mutable whereas String class objects are immutable.

StringBuffer class implementation is synchronized while StringBuilder class is not synchronized.

Concatenation operator "+" is internally implemented by Java using either StringBuffer or StringBuilder.

Criteria to choose among String, StringBuffer and StringBuilder

If the Object value will not change in a scenario use String Class because a String object is immutable.

If the Object value can change and will only be modified from a single thread, use a StringBuilder because StringBuilder is unsynchronized(means faster).

If the Object value may change, and can be modified by multiple threads, use a StringBuffer because StringBuffer is thread safe(synchronized).

Why String class is final or immutable?

It is very useful to have strings implemented as final or immutable objects. Below are some advantages of String Immutability in Java

• Immutable objects are thread-safe. Two threads can both work on an immutable object at the same time without any possibility of conflict.
• Security: the system can pass on sensitive bits of read-only information without worrying that it will be altered
• You can share duplicates by pointing them to a single instance.
• You can create substrings without copying. You just create a pointer into an existing base String guaranteed never to change. Immutability is the secret that makes Java substring implementation very fast.
• Immutable objects are good fit for becoming Hashtable keys. If you change the value of any object that is used as a hash table key without removing it and re-adding it you will lose the object mapping.
• Since String is immutable, inside each String is a char[] exactly the correct length. Unlike a StringBuilder there is no need for padding to allow for growth.
• If String were not final, you could create a subclass and have two strings that look alike when "seen as Strings", but that are actually different.

Is Java Pass by Reference or Pass by Value?

The Java Spec says that everything in Java is pass-by-value. There is no such thing as "pass-by-reference" in Java. The difficult thing can be to understand that Java passes "objects as references" passed by value. This can certainly get confusing.

What is OutOfMemoryError in java? How to deal with java.lang.OutOfMemeryError error?

This Error is thrown when the Java Virtual Machine cannot allocate an object because it is out of memory, and no more memory could be made available by the garbage collector. Note: Its an Error (extends java.lang.Error) not Exception. Two important types of OutOfMemoryError are often encountered

java.lang.OutOfMemoryError: Java heap space

The quick solution is to add these flags to JVM command line when Java runtime is started:

-Xms 1024m -Xmx 1024m 

java.lang.OutOfMemoryError: PermGen space

The solution is to add these flags to JVM command line when Java runtime is started:

-XX+CMSClassUnloadingEnabled-XX+CMSPermGenSweepingEnabled

Long Term Solution: Increasing the Start/Max Heap size or changing Garbage Collection options may not always be a long term solution for your Out Of Memory Error problem. Best approach is to understand the memory needs of your program and ensure it uses memory wisely and does not have leaks. You can use a Java memory profiler to determine what methods in your program are allocating large number of objects and then determine if there is a way to make sure they are no longer referenced, or to not allocate them in the first place.

What is the use of the finally block? Is finally block in Java guaranteed to be called? When finally block is NOT called?

Finally is the block of code that executes always. The code in finally block will execute even if an exception is occurred. Finally block is NOT called in following conditions

If the JVM exits while the try or catch code is being executed, then the finally block may not execute. This may happen due to System.exit() call.

if the thread executing the try or catch code is interrupted or killed, the finally block may not execute even though the application as a whole continues.

If a exception is thrown in finally block and not handled then remaining code in finally block may not be executed.

What is Marker interface? How is it used in Java?

The marker interface is a design pattern, used with languages that provide run-time type information about objects. It provides a way to associate metadata with a class where the language does not have explicit support for such metadata. To use this pattern, a class implements a marker interface, and code that interact with instances of that class test for the existence of the interface. Whereas a typical interface specifies methods that an implementing class must support, a marker interface does not do so. The mere presence of such an interface indicates specific behavior on the part of the implementing class. There can be some hybrid interfaces, which both act as markers and specify required methods, are possible but may prove confusing if improperly used. Java utilizes this pattern very well and the example interfaces are

java.io.Serializable - Serializability of a class is enabled by the class implementing the java.io.Serializable interface. The Java Classes that do not implement Serializable interface will not be able to serialize or deserializ their state. All subtypes of a serializable class are themselves serializable. The serialization interface has no methods or fields and serves only to identify the semantics of being serializable.

java.lang.Cloneable - A class implements the Cloneable interface to indicate to the Object.clone() method that it is legal for that method to make a field-for-field copy of instances of that class. Invoking Object's clone method on an instance that does not implement the Cloneable interface results in the exception CloneNotSupportedException being thrown.

java.util.EvenListener - A tagging interface that all event listener interfaces must extend.

The "instanceof" keyword in java can be used to test if an object is of a specified type. So this keyword in combination with Marker interface can be used to take different actions based on type of interface an object implements.

Why main() in java is declared as public static void main? What if the main method is declared as private?

Public - main method is called by JVM to run the method which is outside the scope of project therefore the access specifier has to be public to permit call from anywhere outside the application static - When the JVM makes are call to the main method there is not object existing for the class being called therefore it has to have static method to allow invocation from class. void - Java is platform independent language therefore if it will return some value then the value may mean different to different platforms so unlike C it can not assume a behavior of returning value to the operating system. If main method is declared as private then - Program will compile properly but at run-time it will give "Main method not public." error. 

What is the difference between an Interface and an Abstract class?

An abstract class can have instance methods that implement a default behavior. An Interface can only declare constants and instance methods, but cannot implement default behavior and all methods are implicitly abstract. An interface has all public members and no implementation. An abstract class is a class which may have the usual flavors of class members (private, protected, etc.), but has some abstract methods.

What is the purpose of garbage collection in Java, and when is it used?

The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources can be reclaimed and reused. A Java object is subject to garbage collection when it becomes unreachable to the program in which it is used.

Describe synchronization in respect to multithreading.

With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchonization, it is possible for one thread to modify a shared variable while another thread is in the process of using or updating same shared variable. This usually leads to significant errors. 

 

Explain different way of using thread?

The thread could be implemented by using runnable interface or by inheriting from the Thread class. The former is more advantageous, because you can place the class in object hierarchy. But remember when you use Runnable stil you need a Thread object to create a thread. 

What are pass by reference and passby value?

Pass By Reference means the passing the address itself rather than passing the value. Passby Value means passing a copy of the value to be passed. 

 

Difference between HashMap and HashTable?

The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls. (HashMap allows null values as key and value whereas Hashtable doesnt allow). HashMap does not guarantee that the order of the map will remain constant over time. HashMap is unsynchronized and Hashtable is synchronized.

What is an Iterator?

Some of the collection classes provide traversal of their contents via a java.util.Iterator interface. This interface allows you to walk through a collection of objects, operating on each object in turn. Remember when using Iterators that they contain a snapshot of the collection at the time the Iterator was obtained; generally it is not advisable to modify the collection itself while traversing an Iterator.

What is an abstract class?

Abstract class must be extended/subclassed (to be useful). It serves as a template. A class that is abstract may not be instantiated (ie, you may not call its constructor), abstract class may contain static data. Any class with an abstract method is automatically abstract itself, and must be declared as such.

A class may be declared abstract even if it has no abstract methods. This prevents it from being instantiated.

What is static in java?

Static means one per class, not one for each object no matter how many instance of a class might exist. This means that you can use them without creating an instance of a class. Static methods are attached to a class, not an object. A static method in a superclass can be shadowed by another static method in a subclass, as long as the original method was not declared final. However, you can't override a static method with a nonstatic method. In other words, you can't change a static method into an instance method in a subclass.

What is final?

A final class can't be extended ie., final class may not be subclassed. A final method can't be overridden when its class is inherited. You can't change value of a final variable (is a constant).

What are Checked and UnChecked Exception?

checked exception is some subclass of Exception (or Exception itself), excluding class RuntimeException and its subclasses.

Making an exception checked forces client programmers to deal with the possibility that the exception will be thrown. eg, IOException thrown by java.io.FileInputStream's read() method·

Unchecked exceptions are RuntimeException and any of its subclasses. Class Error and its subclasses also are unchecked. With an unchecked exception, however, the compiler doesn't force client programmers either to catch the

exception or declare it in a throws clause. In fact, client programmers may not even know that the exception could be thrown. eg, StringIndexOutOfBoundsException thrown by String's charAt() method· Checked exceptions must be caught at compile time. Runtime exceptions do not need to be. Errors often cannot be.

Can you think of a questions which is not part of this post? Please don't forget to share it with me in comments section & I will try to include it in the list.

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