Interface In Java Interview Questions
Commonly Asked Interview Questions about Java Interfaces
An interface in Java is a collection of method signatures without any method bodies, that can be implemented by a class to provide specific functionalities. It allows for abstraction and encapsulation by defining a set of methods that must be implemented by its implementing class, promoting code reusability. An interface can also extend other interfaces, allowing for multiple inheritance and promoting modularity in code. It is commonly used to define contracts and specifications for classes in object-oriented programming.
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1 - What is an interface in Java?
An interface in Java is an abstract type that is used to specify a set of method declarations without any implementation. It acts as a contract between the class and the outside world, defining the behavior that a class should implement. It only contains method signatures, constant variables, and default methods, but no method implementations.
2) What is the difference between an interface and a class?
A class can have both method declarations and method implementations, whereas an interface can only have method declarations and constants. Classes can be instantiated and used to create objects, while interfaces cannot be instantiated. A class can implement multiple interfaces, but it can only inherit from one superclass.
3) Why do we use interfaces in Java?
Interfaces in Java are used to achieve abstraction. They allow us to separate the declaration and implementation of classes, making the code more flexible and reusable. They also enable multiple inheritance in Java, which is not possible with classes.
4) How do you define an interface?
To define an interface, we use the keyword “interface” followed by the name of the interface. Inside the interface, we specify the method declarations and constant variables that should be implemented by the class.
5) Can we have a constructor in an interface?
No, we cannot have a constructor in an interface. Interfaces are not meant to be instantiated, so there is no need for a constructor. Also, interface methods do not have a body, and hence, there is no need for initialization.
6) Can interfaces have variables?
Yes, interfaces can have variables, but they must be declared as constant variables using the “final” keyword. By default, variables in an interface are public, static, and final.
7) What is a default method in an interface?
A default method in an interface is a method that has a default implementation. It was introduced in Java 8 to allow interfaces to have method implementations without breaking backward compatibility.
8) How do you implement an interface?
To implement an interface, we use the “implements” keyword in the class declaration, followed by the name of the interface. The class must then provide implementations for all the methods declared in the interface.
9) Can an interface extend another interface?
Yes, an interface can extend another interface using the “extends” keyword. This allows us to inherit methods from multiple interfaces, promoting code reusability.
10) Can an interface extend a class?
No, an interface cannot extend a class. Java does not support multiple inheritance for classes, and allowing interfaces to extend classes would violate this rule.
11) Can an interface have a private method?
No, an interface cannot have a private method. All methods in an interface are automatically public, and private methods are not allowed in an interface.
12) Why do we use marker interfaces?
Marker interfaces are used to mark a class as a special type. They do not have any methods, and their sole purpose is to inform the compiler or runtime environment about the behavior of the marked class.
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13) Can an interface have a static method?
Yes, an interface can have static methods since Java 8. These methods do not require an instance of the interface to be called and are primarily used as utility methods.
14) What is the difference between a normal class and an abstract class?
Abstract classes can have both abstract and non-abstract methods, while interfaces can only have abstract methods. Also, a class can extend only one abstract class, but it can implement multiple interfaces.
15) Can we extend an interface in Java?
No, we cannot extend an interface in Java. Interfaces are meant to define a set of behaviors, and extending an interface would result in modifying its behavior, which is not allowed in Java.
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