Interface Interview Questions

Why use Java interface?

There are mainly three reasons to use interface. They are given below:

  • It is used to achieve fully abstraction.

  • By interface, we can support the functionality of multiple inheritance.

  • It can be used to achieve loose coupling.

In other words, Interface fields are public, static and final bydefault, and methods are public and abstract.

interface in java

What is Interface in Java?
  • An interface in java is a blueprint of a class. It has static constants and abstract methods only.

  • The interface in java is a mechanism to achieve fully abstraction. There can be only abstract methods in the java interface not method body. It is used to achieve fully abstraction and multiple inheritance in Java.

  • Java Interface also represents IS-A relationship.

  • It cannot be instantiated just like abstract class.

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What is the Understanding relationship between classes and interfaces?

As shown in the figure given below, a class extends another class, an interface extends another interface but a class implements an interface.

Relationship between classes and interfaces in java

Give the Simple example of Java interface.

In this example, Printable interface have only one method, its implementation is provided in the A class.

What is the Multiple inheritance in Java by interface?

If a class implements multiple interfaces, or an interface extends multiple interfaces i.e. known as multiple inheritance.

Multiple inheritance in Java by interface

Multiple inheritance is not supported through class in java but it is possible by interface, why?

As we have explained in the inheritance chapter, multiple inheritance is not supported in case of class. But it is supported in case of interface because there is no ambiguity as implementation is provided by the implementation class.

Example :

As you can see in the above example, Printable and Showable interface have same methods but its implementation is provided by class TestTnterface1, so there is no ambiguity.

What is Interface inheritance ?

A class implements interface but one interface extends another interface.

What is marker or tagged interface?

An interface that have no member is known as marker or tagged interface. For example: Serializable, Cloneable, Remote etc. They are used to provide some essential information to the JVM so that JVM may perform some useful operation.

What is Nested Interface in Java?

An interface can have another interface i.e. known as nested interface. We will learn it in detail in the nested classes chapter.

Example :

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