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Core Java - Interview Questions and Answers for 'Interfaces' - 69 question(s) found - Order By Newest

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Very Frequently asked. Have been asked in HCL Technologies very frequently ( based on 3 feedback ). Among first few questions in many interviews.
  Q1. Differences between abstract class and interface ?Core Java
Ans. Abstract classes can have both abstract methods ( method declarations ) as well as concrete methods ( inherited to the derived classes ) whereas Interfaces can only have abstract methods ( method declarations ).

A class can extend single abstract class whereas it can implement multiple interfaces.

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 Q2. What are the different uses of Interfaces in Java ?Core Java
Ans. 1. Regulation / Enforcement / Policy / Contract , It's primary use

2. Use as a collection of utility methods(since java 8 through default methods)

3. Collecting constants together, As Interface are lighter, it makes sense to use them instead of classes if no getters and setters required.

4. Creation of Custom Annotations

5. Special services like marker interface

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 Q3. Why can't we have diamond problem with interfaces ?Core Java
Ans. Interfaces don't have member elements and method definitions that could cause diamond problem. With Java 8, Interfaces have default method definitions. This could have created diamond problem but Java introduced a compile time check for "duplicate default methods" in case same method is derived from multiple interfaces and no definition is overridden by the class.

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Very frequently asked in phone and walk in interviews.
  Q4. What are Marker Interfaces ? Name few Java marker interfaces ?Core Java
Ans. An interface without any method declaration is called as marker interface. there are 3 in-built interfaces in JVM i.e. serializable, clonable, remote

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 Q5. Can we declare interface methods as private ?
Ans. With Java 9 we can declare interface methods as private

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 Q6. Why do we need Thread class even in case we execute thread using runnable interface ?Core Java
Ans. Thread class holds the definition of start method ( This is the method that starts execution of new thread and then calls run method within the scope of new thread ). Interfaces don't hold any definition and so does runnable. So it makes it necessary the usage of Thread class , whatever implementation you choose.

When your class extends the thread class, it carries the definition of start method from parent Thread class onto itself and hence new yourClass.start() helps starting a new thread and then executing run method in that new thread scope.

When you implement runnable interface , you are just making it sure to the JVM that you have implemented the required method ( run() ) which the Thread start method will look for upon executing start method.

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 Q7. Which of the following doesn't extend Collection interface ?

a. Set
b. List
c. Map
d. Queue
Core Java
Ans. Map

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 Q8. If an Abstract class has only abstract methods, What's the difference between such a class and an interface ?Core Java
Ans. Such a class still can have member elements which can be inherited and hence facilitate code reuse. Moreover Abstract class can have non final static elements whereas interfaces are only allowed to have static final elements.

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 Q9. What is an Externalizable interface ?
Ans. Externalizable interface is used to write the state of an object into a byte stream in compressed format.

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 Q10. Difference between serializable and externalizable interface ?
Ans. Serializable is a marker interface whereas externalizable is not.

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 Q11. What are the design considerations while making a choice between using interface and abstract class ?Core Java
Ans. Keep it as a Abstract Class if its a "Is a" Relationsship and should do subset/all of the functionality. Keep it as Interface if its a "Should Do" relationship.

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  Q12. what is the difference between collections class vs collections interface ?Core Java
Ans. Collections class is a utility class having static methods for doing operations on objects of classes which implement the Collection interface. For example, Collections has methods for finding the max element in a Collection.

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 Q13. What is Externalizable interface?Core Java
Ans. Externalizable is an interface which contains two methods readExternal and writeExternal. These methods give you a control over the serialization mechanism.

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 Q14. Can we use static method definitions in Interfaces ?Core Java
Ans. Yes, Effective Java 8.

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 Q15. How is Abstraction implemented in Java ?Core Java
Ans. Abstraction is provided in Java by following ways -

Coding to the ( Interfaces / Abstract Classes ) or contracts

By Encapsulating details within classes and exposing the minimal Door ( few public methods )

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 Q16. What is the use of runnable interface if we can always create a new thread using Thread class ?Core Java
Ans. It's a choice to be made whether to use first approach ( Thread class ) or second approach ( runnable interface ) by the programmer. The second facility has been given for cases where your class is already extending some parent class and hence cannot extend another class ( for Thread ) as Java doesn't allow multiple inheritance.

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 Q17. Interface can only have ...

a. Member elements and Methods.
b. Static Variables and Static Methods.
c. Static Final Variables and Instance Method Declarations.
d. Member Elements , Instance Methods, Static variables and Static Methods.
Ans. Static Final Variables and Instance Method Declarations.

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 Q18. What was the driving force to introduce default methods in Interfaces wef from Java 8 ?Core Java
Ans. Earlier any class implementing an interface was supposed to implement all methods declared in an interface. There was no place for optionally implementing all or subset of methods.Though we have abstract classes wherein we could have provided such a mechanism by declaring some methods as abstract while providing definition for some. But as Abstract classes have a body and are comparatively heavier than interfaces and interfaces associate closely to the concept of providing interfacing than abstract classes, Java might have though of providing optional implementation for default methods. This way same interface can be reused in variety of ways rather than making copies of an interface to suit different needs.

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 Q19. How Marker interfaces works internally ?Core Java
 This question was recently asked at 'GGK Technologies'.This question is still unanswered. Can you please provide an answer.


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 Q20. What will happen if there are no interfaces in Java ?Core Java
Ans. Abstract classes can take care of that to a certain extent. Though they are little heavier than Interfaces but An abstract class with all abstract methods and no instance variables will be able to help with everything that currently an interface does.

The only problem is that a class can only extend one class whereas it can implements multiple interfaces and that is the reason Interfaces were introduced in Java, i.e to get over the problem of multiple inheritance.

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 Q21. Write a program to show thread usage in Java by implementing runnable interface
Ans. public class MyClass {
   static class MyThreadClass implements Runnable{

      public void start() {
         Thread t = new Thread(this,"threadName");
         t.start();
      }
      
      @Override
      public void run() {
         System.out.println("Hello");
         try {
            Thread.sleep(1000);
            System.out.println("Hello Again");
         } catch (InterruptedException e) {
            e.printStackTrace();
         }
         
      }
      
   }
   
   public static void main(String[] args){
      MyThreadClass myThreadClass = new MyThreadClass();
      myThreadClass.start();
      
      MyThreadClass myThreadClass2 = new MyThreadClass();
      myThreadClass2.start();
   }
}

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 Q22. Is runnable a Marker interface ?Core Java
Ans. No , it has run method declared.

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 Q23. Whats the purpose of marker interfaces ?
Ans. They just tell the compiler that the objects of the classes implementing the interfaces with no defined methods need to be treated differently.

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 Q24. What will happen if class implement two interface having common method?
Ans. That would not be a problem as both are specifying the contract that implement class has to follow.
If class C implement interface A & interface B then Class C thing I need to implement print() because of interface A then again Class think I need to implement print() again because of interface B, it sees that there is already a method called test() implemented so it's satisfied.

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 Q25. What is a cloneable interface and what all methods does it contain?Core Java
Ans. Cloneable is a declaration that the class implementing it allows cloning or bitwise copy of it's object state. It is not having any method because it is a MARKER interface.

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  Q26. What is Comparable Interface?Core Java
Ans. It is used to sort collections and arrays of objects using the collections.sort() and java.utils. The objects of the class implementing the Comparable interface can be ordered.

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 Q27. Explain Set Interface?Core Java
Ans. It is a collection of element which cannot contain duplicate elements. The Set interface contains only methods inherited from Collection and adds the restriction that duplicate elements are prohibited.

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 Q28. What is an enumeration?Core Java
Ans. An enumeration is an interface containing methods for accessing the underlying data structure from which the enumeration is obtained. It allows sequential access to all the elements stored in the collection.

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 Q29. How TreeMap orders the elements if the Key is a String ?
Ans. As String implements Comparable, It refers to the String compareTo method to identify the order relationship among those elements.

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 Q30. Can we add heterogeneous elements into TreeMap ?
Ans. No, Sorted collections don't allow addition of heterogeneous elements as they are not comparable.

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