Core Java - Interview Questions and Answers for 'Interface' | Search Interview Question

Interview Questions and Answers - Order By Newest

Q31. Will it create any problem if We add elements with key as user defined object into the TreeMap ?

Ans. It won't create any problem if the objects are comparable i.e we have that class implementing Comparable interface.

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Q32. Can we access Interface static method using Interface references ?

Ans. No, only using Interface Name.

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Q33. Can we have default method with same name and signature in the derived Interface as the static method in base Interface and vice versa ?

Ans. Yes , we can do that as static methods are not accessible using references and hence cannot lead to conflict. We cannot do inverse as Default methods cannot be overridden with the static methods in derived interface.

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Q34. What is Closeable ?

Core Java

Ans. A Closeable is an interface which is a source or destination of data that can be closed.

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Not frequently asked as it was introduced with Java 8.

Q35. What is the @FunctionalInterface annotation ?

Core Java

Ans. This is an informative annotation that specify that the interface is a functional interface. A Function Interface has only one abstract method and many default methods. Compiler generates an error if the interface specified with the annotation doesn't abide by the specifications for functional interface.

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Q36. Name few interfaces that extends Collection Interface ?

Ans. http://www.buggybread.com/2015/02/java-collections-interfaces-that.html

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Q37. Name few classes that implement Collection interface ?

Ans. http://www.buggybread.com/2015/02/java-collections-classes-that-implement.html

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Q38. Is it correct to say that Interfaces are abstract data types ?

Ans. No.

Data Type holds data whereas Interface doesn't hold anything. Interface is a contract about how to communicate with the underlying Class.

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Q39. What is the package name for CertStoreParameters class?

Ans. java.lang

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Q40. Which is the Parent Class of CertStoreParameters class?

Ans. Clonable

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Q41. Variables of an interface are intrinsically ...

a. transient
b. final
c. public
d. static

Ans. b,c and d i.e final , public and static

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Q42. Which is the Parent Class of CertStoreParameters class?

Ans. Clonable

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Q43. In the following code , how many methods needs to be implemented in Class B ?

public interface A{
   public void method1();
   public void method2();
   public void method3();
}

abstract class B implements A{
}

Core Java

Ans. As Class B has been declared abstract , we can either implement any of these methods and just declare rest of them abstract.

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Frequently asked.

Q44. What should a class do if its implementing an interface ?

Core Java

Ans. It should either implement all interface methods or declare unimplemented methods as abstract.

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Q45. Does default methods introduce multiple inheritance and the Diamond problem in Java 8 ?

Core Java

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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Q46. Do we need to implement all interface methods if a class is implementing it ?

Core Java

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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Q47. Where do we generally use comparable interface ?

Core Java

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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Q48. Why Collection interface does not extend Cloneable interface ?

Core Java

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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Q49. Which is the root interface of Collection classes in Java ?

Core Java

Ans. java.util.Collection

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Q50. Can we have an inner class within Interface ?

Core Java

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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Q51. What is nested interface?

Core Java

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

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Q52. 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 Java

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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Q53. 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 Java

Ans. 10

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Q54. What are the types of Executor Interfaces ?

Core Java

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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Q55. What are the core interfaces in Hibernate?

Hibernate

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

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Q56. What are the core interfaces of Spring mvc ?

Spring

^{This question is still unanswered. Can you please provide an answer.}

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Q57. Can an interface be defined inside a class just like we can have inner classes inside a class ?

Core Java

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

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Q58. Can an interface implements another interface ?

Core Java

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

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Q59. 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 Java

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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Q60. Why do we use Thread Class as well as Runnable Interface for creating threads in Java ?

Core Java

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