Implementing Interfaces in Java classes

The syntax for implementing an interface makes use of this pattern:

... implements InterfaceName[, another interface, another, ...] ...

Classes may implement an interface. For example:

    public class Lion implements Predator

        public boolean chasePrey(Prey p)
               // programming to  chase prey p (specifically for a lion)

        public void eatPrey(Prey p)
               // programming to eat prey p (specifically for a lion)

If a class implements an interface and does not implement all its methods, it must be marked as abstract. If a class is abstract, one of its subclasses is expected to implement its unimplemented methods. Although if any of the abstract class' subclasses does not implement all interface methods, the subclass itself must be marked again as abstract.

Classes can implement multiple interfaces:

     public class Frog implements Predator, Prey { ... }

Interfaces can share common class methods:

     class Animal implements LikesFood, LikesWater
 boolean likes() {return true;}

However a given class cannot implement the same or a similar interface multiple times:

     class Animal implements Shares, Shares ...

Interfaces are commonly used in the Java language for callbacks, as Java does not allow multiple inheritance of classes, nor does it allow the passing of methods (procedures) as arguments. Therefore, in order to pass a method as a parameter to a target method, current practice is to define and pass a reference to an interface as a means of supplying the signature and address of the parameter method to the target method rather than defining multiple variants of the target method to accommodate each possible calling class.

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