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Core Java - Interview Questions and Answers for 'Interface' - 72 question(s) found - Order By Rating

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 Q1. What will happen if there are no interfaces in Java ?Core Java2018-12-11 16:16:37

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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   Like         Discuss         Correct / Improve     interfaces      expert


 Q2. Which Java version supports Interface Default methods ?Core Java2018-07-22 11:46:24

Ans. Java 1.8 or Java 8

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   Like         Discuss         Correct / Improve     default methods   interface default methods   java 8  java 8 features


 Q3. Does Java generate .class file for interfaces ? Core Java2018-07-20 18:20:53

Ans. Yes

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 Q4. Can we declare constructor inside an interface ? Why ?Core Java2018-07-20 18:19:59

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 Q5. Can we have interface as a replacement for utility class ? Core Java2018-07-07 20:43:47

Ans. Yes, With Java 8 we can use Interfaces as collection of utility methods through the use of default methods.

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   Like         Discuss         Correct / Improve     java 8  interfaces  utility class   utility classes


 Q6. How do you define a functional interface?Core Java2018-01-22 17:32:13

Ans. Create interface with the only one non-overriding abstract method and annotate it with @FunctionalInterface

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 Q7. What is an Interface ?Core Java2018-01-20 12:18:59

 This question was recently asked at 'Adella'.This question is still unanswered. Can you please provide an answer.


  Sample Code for interface

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 Q8. Why do we use Thread Class as well as Runnable Interface for creating threads in Java ?Core Java2017-08-19 19:34:44

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   Like         Discuss         Correct / Improve     Threads  Thread class  runnable interface


 Q9. What are the advantage of Abstract classes over interfaces with respect to Java 7 ? and What changed in Java 8 to help facilitate that in Java 8 ?Core Java2017-08-17 19:28:33

Ans. Abstract Classes provide default implementations of methods that are inherited by the classes that extend them, which was not the case for Interfaces. This changed in Java 8, where default implementations are provided for methods.

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 Q10. Write a program to show thread usage in Java by implementing runnable interface2017-08-03 12:16:07

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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 Q11. Why can't we have diamond problem with interfaces ?Core Java2017-07-30 08:15:33

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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   Like         Discuss         Correct / Improve     diamond problem  interfaces  java 8  default methods


 Q12. Can an interface implements another interface ?Core Java2017-06-23 08:14:33

Ans. No. No interface provides any implementation and hence the declaration isn't viable.

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 Q13. Can an interface be defined inside a class just like we can have inner classes inside a class ?Core Java2017-06-23 08:13:43

Ans. Yes , we can have an interface that is part of a class.

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   Like         Discuss         Correct / Improve     inner classes   inner interface  nested classes


 Q14. If an Abstract class has only abstract methods, What's the difference between such a class and an interface ?Core Java2017-05-07 07:26:29

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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   Like         Discuss         Correct / Improve     abstract class   interfaces     Asked in 2 Companies      Basic


 Q15. What are the core interfaces of Spring mvc ?Spring2017-03-09 13:13:56

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 Q16. What are the core interfaces in Hibernate?Hibernate2017-03-09 11:37:41

Ans. Session,SessionFactory,Configuration,Transaction,query and Criteria

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 Q17. What are the types of Executor Interfaces ?Core Java2017-03-09 11:37:29

Ans. Executor - Simple Interface that supports launching new tasks.

ExecutorService - Subinterface of Executor, which adds features that help manage the lifecycle, both of the individual tasks and of the Executor itself.

ScheduledExecutorService - Subinterface of ExecutorService, supports future and-or periodic execution of tasks.

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 Q18. How Marker interfaces works internally ?Core Java2017-02-23 10:38:31

 This question was recently asked at 'GGK Technologies'.This question is still unanswered. Can you please provide an answer.


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   Like         Discuss         Correct / Improve     marker interfaces  interfaces     Asked in 1 Companies      expert        rare


 Q19. What will be the output of following code ?

Base Interface

public interface BaseInterface {
   int i = 4;
}

Derived Interfaces

public interface DerivedInterface1 extends BaseInterface{
   int i = 5;
}

public interface DerivedInterface2 extends BaseInterface{
   int i=6;
}

Implementing Class

public class Class implements DerivedInterface1,DerivedInterface2 {
int i=10;   

public static void main(String[] args){
      System.out.println(new Class().i);
   }
}

What will be the output upon executing main and Why ?
Core Java2017-01-26 13:43:11

Ans. 10

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 Q20. What will be the output of following code ?

Base Interface

public interface BaseInterface {
   int i = 4;
}

Derived Interfaces

public interface DerivedInterface1 extends BaseInterface{
   int i = 5;
}

public interface DerivedInterface2 extends BaseInterface{
   int i=6;
}

Implementing Class

public class Class implements DerivedInterface1,DerivedInterface2 {
   public static void main(String[] args){
      System.out.println(BaseInterface.i);
   }
}

What will be the output upon executing main and Why ?
Core Java2017-01-26 13:42:29

Ans. It will print 4 because member elements of an interface are implicitly static and hence the concept of overriding doesn't work.


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 Q21. What is nested interface?Core Java2017-01-12 14:14:44

Ans. Interface that is declared inside the interface or class, is known as nested interface. It is static by default.

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 Q22. Can we have an inner class within Interface ?Core Java2016-10-24 11:18:24

Ans. Yes, we can define an inner class within interface.Inside the interface, the inner class is implicitly public static.

So the following is legit in Java

public interface BuggyBreadInterface {
   void doSomething();

   public class BuggyBreadClass implements BuggyBreadInterface{
      void doSomething(){
         System.out.println("Do something");
      }
   }
}

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 Q23. Which is the root interface of Collection classes in Java ?Core Java2016-10-12 08:26:03

Ans. java.util.Collection

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   Like         Discuss         Correct / Improve     collection interface  collections


 Q24. Why Collection interface does not extend Cloneable interface ?Core Java2016-09-15 09:40:06

Ans. Here is the list of classes that implements Collections Interface - http://www.buggybread.com/2015/02/java-collections-classes-that-implement.html

Having Collection interface to extend Cloneable interface would mean necessarily implement clone method by all implementing classes. As not all collection classes allow duplicate elements, it makes no sense to clone elements for them.

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   Like         Discuss         Correct / Improve     cloneable  collections  collection interface  cloning


 Q25. Where do we generally use comparable interface ?Core Java2016-08-09 10:02:01

Ans. Implementing Comparable interface means that the elements of the class are comparable i.e the class provides the implementation of compareTo method that would help comparing the elements.

This is usually required if we are planning to sort elements of a collection, If compareTo method is not defined , the sorting class / method could never understand a way to compare its elements in order to sort them.

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 Q26. Do we need to implement all interface methods if a class is implementing it ?Core Java2016-08-04 14:29:39

Ans. Yes, a Class is supposed to define all abstract methods declared in the interface. With Java 8 , Interfaces can have default methods which need not be implemented by the implementing class.

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 Q27. Does default methods introduce multiple inheritance and the Diamond problem in Java 8 ?Core Java2016-07-20 17:16:28

Ans. Default methods results in multiple inheritance of behavior and not of state. In case we try to implement multiple interfaces with default method having same name and signature, and don't override it in implementation class, it will throw an error.

For example -

interface MyInterface {
public void default myMethod(){
}
}

interface MyInterface2 {
public void default myMethod(){
}
}

class MyClass implements MyInterface,MyInterface2 {
}

This code will compilation error "Duplicate Default Method"

if we specify the definition of myMethod() in myClass, compiler won't complain and there is no conflict and MyClass can use overridden definition. But if we don't override myMethod() in MyClass, Java would be in conflict as to what definition should be carried to MyClass and hence throws compilation error.

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   Like         Discuss         Correct / Improve     default methods  java 8  multiple inheritance  object oriented programming (oops)  oops concepts  diamond problem   interfaces


 Q28. What are the different uses of Interfaces in Java ?Core Java2016-07-20 15:14:43

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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 Q29. What was the driving force to introduce default methods in Interfaces wef from Java 8 ?Core Java2016-07-14 14:36:49

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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Very frequently asked in phone and walk in interviews.
  Q30. What are Marker Interfaces ? Name few Java marker interfaces ?Core Java
Anonymous
Ans. These are the interfaces which have no declared methods.Serializable and cloneable are marker interfaces.

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 Q31. Is runnable a Marker interface ?Core Java
Anonymous
Ans. No , it has run method declared.

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 Q32. Can we declare interface methods as private ?
Anonymous
Ans. With Java 9 we can declare interface methods as private

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 Q33. What is an Externalizable interface ?
Anonymous
Ans. Externalizable interface is used to write the state of an object into a byte stream in compressed format.

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 Q34. Difference between serializable and externalizable interface ?
Anonymous
Ans. Serializable is a marker interface whereas externalizable is not.

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 Q35. What are the design considerations while making a choice between using interface and abstract class ?Core Java
Anonymous
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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  Q36. what is the difference between collections class vs collections interface ?Core Java
Admin
info@buggybread.com
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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 Q37. Whats the purpose of marker interfaces ?
Admin
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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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 Q38. What will happen if class implement two interface having common method?
Admin
info@buggybread.com
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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 Q39. What is a cloneable interface and what all methods does it contain?Core Java
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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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  Q40. What is Comparable Interface?Core Java
Admin
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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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