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Core Java - Interview Questions and Answers for 'Interface' - 68 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
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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 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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 Q3. 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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Very frequently asked in phone and walk in interviews.
  Q4. 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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 Q5. Why do we need Thread class even in case we execute thread using runnable interface ?Core Java
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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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 Q6. Which of the following doesn't extend Collection interface ?

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

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 Q7. 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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 Q8. Can we declare interface methods as private ?
Anonymous
Ans. No.

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 Q9. 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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 Q10. Difference between serializable and externalizable interface ?
Anonymous
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
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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 Q12. Can we use static method definitions in Interfaces ?Core Java
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Ans. Yes, Effective Java 8.

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 Q13. How is Abstraction implemented in Java ?Core Java
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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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 Q14. What is the use of runnable interface if we can always create a new thread using Thread class ?Core Java
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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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 Q15. 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.
Anonymous
Ans. Static Final Variables and Instance Method Declarations.

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 Q16. 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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 Q17. Is runnable a Marker interface ?Core Java
Anonymous
Ans. No , it has run method declared.

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  Q18. what is the difference between collections class vs collections interface ?Core Java
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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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 Q19. Whats the purpose of marker interfaces ?
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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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 Q20. What will happen if class implement two interface having common method?
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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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 Q21. 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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  Q22. What is Comparable Interface?Core Java
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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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 Q23. Explain Set Interface?Core Java
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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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 Q24. What is Externalizable interface?Core Java
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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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 Q25. What is an enumeration?Core Java
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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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 Q26. How TreeMap orders the elements if the Key is a String ?
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Ans. As String implements Comparable, It refers to the String compareTo method to identify the order relationship among those elements.

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 Q27. Can we add heterogeneous elements into TreeMap ?
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Ans. No, Sorted collections don't allow addition of heterogeneous elements as they are not comparable.

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 Q28. Will it create any problem if We add elements with key as user defined object into the TreeMap ?
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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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 Q29. Can we access Interface static method using Interface references ?
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Ans. No, only using Interface Name.

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 Q30. Can we have default method with same name and signature in the derived Interface as the static method in base Interface and vice versa ?
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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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 Q31. What is Closeable ?Core Java
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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.
 Q32. What is the @FunctionalInterface annotation ?Core Java
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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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 Q33. Name few interfaces that extends Collection Interface ?
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Ans. http://www.buggybread.com/2015/02/java-collections-interfaces-that.html

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 Q34. Name few classes that implement Collection interface ?
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Ans. http://www.buggybread.com/2015/02/java-collections-classes-that-implement.html

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 Q35. Is it correct to say that Interfaces are abstract data types ?
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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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 Q36. What is the package name for CertStoreParameters class?
Anonymous
Ans. java.lang

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 Q37. Which is the Parent Class of CertStoreParameters class?
Anonymous
Ans. Clonable

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 Q38. Variables of an interface are intrinsically ...

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

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 Q39. Which is the Parent Class of CertStoreParameters class?
Anonymous
Ans. Clonable

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 Q40. 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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