The interface keyword is used to declare 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\
}
/* File name : Animal.java */
interface Animal {
public void eat();
public void travel();
}
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.
/* 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();
}
}
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.
// 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 {
public void homeGoalScored();
public void visitingGoalScored();
public void endOfPeriod(int period);
public void overtimePeriod(int ot);
}
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.
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.
public interface Hockey extends Sports, Event