Interface Interview Questions In Java


Interface Interview Questions In Java

Top Java Interview Questions for Interfaces

An interface is a collection of abstract methods that provides a way to specify the behavior of a class without implementing it. It is an important aspect of object-oriented programming in Java and is commonly used to achieve abstraction and multiple inheritance. During an interface interview in Java, candidates may be asked questions related to the definition and purpose of interfaces, their differences from classes, how to declare and implement an interface, and how it can be used in real-world scenarios. Additionally, they may be asked about the role of default and static methods in interfaces, the concept of functional interfaces, and how interfaces are related to the concept of polymorphism in Java.

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intterivew questions

1 - What is the Java Interface?

Java Interface is a blueprint of a class that contains only abstract methods (methods without body) and constant variables. It is used to achieve abstraction and provide a mechanism for multiple inheritance in Java. Interfaces are declared using the interface keyword.

2) How is a class different from an interface in Java?

A class can contain both abstract and non-abstract methods, while an interface can only have abstract methods. A class can also have constructors, whereas an interface cannot. Another difference is that a class can extend only one other class, while an interface can extend multiple interfaces.

3) Can an interface extend another interface?

Yes, an interface in Java can extend another interface using the extends keyword. This allows for the creation of a hierarchy of interfaces.

4) Can an interface have fields?

Yes, an interface can have constant variables, which are by default static and final. These variables cannot be changed by implementing classes.

5) Can an interface have private methods?

No, an interface cannot have private methods, as all methods in an interface are by default public and abstract. However, Java 9 introduced the concept of private methods in interfaces using the private keyword.

6) What is a default method in Java interface?

A default method in an interface is a method with a body that can be inherited by implementing classes. This allows for the addition of new methods to an interface without breaking the existing code.

7) Can a class implement multiple interfaces in Java?

Yes, a class can implement multiple interfaces in Java, but it can only extend one class. This allows for achieving multiple inheritance in Java.

8) What is the purpose of the marker interface in Java?

A marker interface in Java is an empty interface with no methods. Its purpose is to specify a special behavior to a class that implements it. Examples of marker interfaces are Serializable and Cloneable.

9) How can we implement an interface in a class?

To implement an interface in a class, the keyword implements must be used after the name of the class, followed by the interface name. The class must then provide an implementation for all the abstract methods in the interface.

10) Can an interface be instantiated?

No, an interface cannot be instantiated as it is an abstract type. However, an interface reference variable can be used to reference an object of a class that implements the interface.

11) Can interfaces have private constructors?

No, interfaces cannot have constructors as they do not have instance variables to initialize. Also, an interface is meant to be implemented by other classes, so having a constructor is not necessary.

12) What is the purpose of the static keyword in interfaces?

The static keyword is used in interfaces to create static methods that can be accessed without creating an instance of the class. These methods can be called using the interface name.

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13) What is multiple inheritance and does Java support it?

Multiple inheritance is a feature that allows a class to inherit from more than one superclass. Java does not support multiple inheritance through classes, but it can be achieved using interfaces.

14) Can an abstract class implement an interface in Java?

Yes, an abstract class can implement one or more interfaces, just like any other class. This allows the abstract class to provide an implementation for the methods declared in the interface.

15) Can an interface extend a class in Java?

No, an interface cannot extend a class in Java. It can only extend another interface. This is because a class can only have one superclass, but interfaces can have multiple superinterfaces.

16) What is the difference between abstract class and interface in Java?

The main difference between an abstract class and an interface in Java is that an abstract class can have both abstract and non-abstract methods, while an interface can only have abstract methods. Another key difference is that an abstract class can have instance variables, while an interface cannot.

17) Can an interface have a constructor?

No, an interface cannot have a constructor. Interfaces are meant to declare methods and constants, and constructors are used to initialize instance variables, which are not present in interfaces.

18) What is the purpose of the @FunctionalInterface annotation in Java?

The @FunctionalInterface annotation is used to indicate that an interface is a functional interface. A functional interface is an interface that contains only one abstract method and is used in lambda expressions and method references.

19) How can we prevent a class from inheriting from an interface in Java?

To prevent a class from inheriting from an interface, the final keyword can be used before the interface declaration. This will make the interface unable to be extended by any class.

20) Can an interface have a private static method in Java?

Yes, Java 9 allows interfaces to have private static methods, which can only be accessed within the interface. These methods are useful for code reuse within the interface.


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