Java — Interfaces
An interface is a reference type in Java. It is similar to class. It is a collection of abstract methods. A class implements an interface, thereby inheriting the abstract methods of the interface.
Along with 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 similar to writing a class. But a class describes the attributes and behaviors of an object. And an interface contains behaviors that a class implements.
Unless the class that implements the interface is abstract, 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 −
/* File name : NameOfInterface.java */ import java.lang.*; // Any number of import statements public interface NameOfInterface < // Any number of final, static fields // Any number of abstract method declarations\ >
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
/* File name : Animal.java */ interface Animal
Implementing Interfaces
When a class implements an interface, you can think of the class as signing a contract, agreeing to perform the specific behaviors of the interface. If a class does not perform all the behaviors of the interface, the class must declare 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.
Example
/* File name : MammalInt.java */ public class MammalInt implements Animal < public void eat() < System.out.println("Mammal eats"); >public void travel() < System.out.println("Mammal travels"); >public int noOfLegs() < return 0; >public static void main(String args[]) < MammalInt m = new MammalInt(); m.eat(); m.travel(); >> This will produce the following result −
Output
Mammal eats Mammal travels
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
// Filename: Sports.java public interface Sports < public void setHomeTeam(String name); public void setVisitingTeam(String name); >// Filename: Football.java public interface Football extends Sports < public void homeTeamScored(int points); public void visitingTeamScored(int points); public void endOfQuarter(int quarter); >// Filename: Hockey.java public interface Hockey extends Sports
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
public interface Hockey extends Sports, Event
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
package java.util; public interface EventListener <>
An interface with no methods in it is referred to as a tagging interface. There are two basic design purposes of tagging interfaces −
Creates a common parent − As with the EventListener interface, which is extended by dozens of other interfaces in the Java API, you can use a tagging interface to create a common parent among a group of interfaces. For example, when an interface extends EventListener, the JVM knows that this particular interface is going to be used in an event delegation scenario.
Adds a data type to a class − This situation is where the term, tagging comes from. A class that implements a tagging interface does not need to define any methods (since the interface does not have any), but the class becomes an interface type through polymorphism.
Java Interface
Another way to achieve abstraction in Java, is with interfaces.
An interface is a completely «abstract class» that is used to group related methods with empty bodies:
Example
// interface interface Animal < public void animalSound(); // interface method (does not have a body) public void run(); // interface method (does not have a body) >To access the interface methods, the interface must be «implemented» (kinda like inherited) by another class with the implements keyword (instead of extends ). The body of the interface method is provided by the «implement» class:
Example
// Interface interface Animal < public void animalSound(); // interface method (does not have a body) public void sleep(); // interface method (does not have a body) >// Pig "implements" the Animal interface class Pig implements Animal < public void animalSound() < // The body of animalSound() is provided here System.out.println("The pig says: wee wee"); >public void sleep() < // The body of sleep() is provided here System.out.println("Zzz"); >> class Main < public static void main(String[] args) < Pig myPig = new Pig(); // Create a Pig object myPig.animalSound(); myPig.sleep(); >> Notes on Interfaces:
- Like abstract classes, interfaces cannot be used to create objects (in the example above, it is not possible to create an «Animal» object in the MyMainClass)
- Interface methods do not have a body — the body is provided by the «implement» class
- On implementation of an interface, you must override all of its methods
- Interface methods are by default abstract and public
- Interface attributes are by default public , static and final
- An interface cannot contain a constructor (as it cannot be used to create objects)
Why And When To Use Interfaces?
1) To achieve security — hide certain details and only show the important details of an object (interface).
2) Java does not support «multiple inheritance» (a class can only inherit from one superclass). However, it can be achieved with interfaces, because the class can implement multiple interfaces. Note: To implement multiple interfaces, separate them with a comma (see example below).
Multiple Interfaces
To implement multiple interfaces, separate them with a comma:
Example
interface FirstInterface < public void myMethod(); // interface method >interface SecondInterface < public void myOtherMethod(); // interface method >class DemoClass implements FirstInterface, SecondInterface < public void myMethod() < System.out.println("Some text.."); >public void myOtherMethod() < System.out.println("Some other text. "); >> class Main < public static void main(String[] args) < DemoClass myObj = new DemoClass(); myObj.myMethod(); myObj.myOtherMethod(); >>