Java Interfaces - Java

What is Java Interfaces?

An interface is a reference kind in Java. It is similar to class. It is a set of abstract methods. A class implements an interface, thereby inheriting the abstract methods of the interface.

Alongside abstract methods, an interface may also contain constants, default methods, static methods, and nested types. Method bodies exist only for default methods and static methods.

Writing an interface is much like writing a category. however a category describes the attributes and behaviors of an item. And an interface contains behaviors that a class implements.

Unless the class that implements the interface is summary, all the methods of the interface need to be defined in the class.
An interface is similar to a class in the following ways −

  • An interface can contain any number of methods.
  • An interface is written in a file with a .java extension, with the name of the interface matching the name of the file.
  • The byte code of an interface appears in a .class file.
  • Interfaces appear in packages, and their corresponding bytecode file must be in a directory structure that matches the package name.

However, an interface is different from a class in several ways, including −

  • You cannot instantiate an interface.
  • An interface does not contain any constructors.
  • All of the methods in an interface are abstract.
  • An interface cannot contain instance fields. The only fields that can appear in an interface must be declared both static and final.
  • An interface is not extended by a class; it is implemented by a class.
  • An interface can extend multiple interfaces.

Declaring Interfaces

The interface keyword is used to declare an interface. Here is a simple example to declare an interface −

Example

Following is an example of an interface –

Interfaces have the following properties −

  • An interface is implicitly abstract. You do not need to use the abstract keyword while declaring an interface.
  • Each method in an interface is also implicitly abstract, so the abstract keyword is not needed.
  • Methods in an interface are implicitly public.

Example

Enforcing Interfaces

While a class implements an interface, you can think of the class as signing a contract, agreeing to carry out the unique behaviors of the interface. If a class does not perform all the behaviors of the interface, the class needs to claim itself as abstract.
a class uses the implements keyword to implement an interface. The implements keyword appears in the class declaration following the extends portion of the declaration.

Instance


This will produce the following result −

Output

When overriding methods defined in interfaces, there are several rules to be followed −

  • Checked exceptions should not be declared on implementation methods other than the ones declared by the interface method or subclasses of those declared by the interface method.
  • The signature of the interface method and the same return type or subtype should be maintained when overriding the methods.
  • An implementation class itself can be abstract and if so, interface methods need not be implemented.

When implementation interfaces, there are several rules −

  • A class can implement more than one interface at a time.
  • A class can extend only one class, but implement many interfaces.
  • An interface can extend another interface, in a similar way as a class can extend another class.

Extending Interfaces

An interface can extend another interface in the same way that a class can extend another class. The extends keyword is used to extend an interface, and the child interface inherits the methods of the parent interface.

The following Sports interface is extended by Hockey and Football interfaces.

Example

The Hockey interface has four methods, but it inherits two from Sports; thus, a class that implements Hockey needs to implement all six methods. Similarly, a class that implements Football needs to define the three methods from Football and the two methods from Sports.

Extending Multiple Interfaces

A Java class can only extend one parent class. Multiple inheritance is not allowed. Interfaces are not classes, however, and an interface can extend more than one parent interface.

The extends keyword is used once, and the parent interfaces are declared in a comma-separated list.

For example, if the Hockey interface extended both Sports and Event, it would be declared as −

Example

Tagging Interfaces

The most common use of extending interfaces occurs when the parent interface does not contain any methods. For example, the MouseListener interface in the java.awt.event package extended java.util.EventListener, which is defined as −

Example

An interface with no methods in it is known as a tagging interface. There are two primary design purposes of tagging interfaces −

  • Creates a common parent − as with the EventListener interface, that is extended through dozens of different interfaces in the Java API, you may use a tagging interface to create a not unusual determine amongst a set of interfaces. for example, while an interface extends EventListener, the JVM knows that this particular interface is going for use in an occasion delegation state of affairs.
  • Provides a records kind to a class − this situation is wherein the term, tagging comes from. a category that implements a tagging interface does not want to outline any methods (for the reason that interface does no longer have any), however, the magnificence turns into an interface type thru polymorphism.

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