Interface (Java)

An interface in the Java programming language is an abstract type which is used to specify an interface (in the generic sense of the term) that classes must implement. Interfaces are declared using the interface keyword, and may only contain method signatures and constant declarations (variable declarations which are declared to be both static and final).

As interfaces are abstract, they cannot be directly instantiated. Object references in Java may be specified to be of an interface type; in which case they must either be null, or be bound to an object which implements the interface.

The keyword implements is used to declare that a given class implements an interface. A class which implements an interface must either implement all methods in the interface, or be an abstract class.

One benefit of using interfaces is that they simulate multiple inheritance. All classes in Java (other than java.lang.Object, the root class of the Java type system) must have exactly one base class; multiple inheritance of classes is not allowed. However, a Java class/interface may implement/extend any number of interfaces.

Interfaces

Interfaces are used to encode similarities which classes of various types share, but do not necessarily constitute a class relationship. For instance, a human and a parrot can both whistle, however it would not make sense to represent Humans and Parrots as subclasses of a Whistler class, rather they would most likely be subclasses of an Animal class (likely with intermediate classes), but would both implement the Whistler interface.

Another use of interfaces is being able to use an object without knowing its type of class, but rather only that it implements a certain interface. For instance, if one were annoyed by a whistling noise, one may not know whether it is a human or a parrot, all that could be determined is that a whistler is whistling. In a more practical example, a sorting algorithm may expect an object of type Comparable. Thus, it knows that the object's type can somehow be sorted, but it is irrelevant what the type of the object is. The call whistler.whistle() will call the implemented method whistle of object whistler no matter what class it has, provided it implements Whistler.

Usage

Defining an Interface

Interfaces must be defined using the following formula (compare to Java's class definition).

[visibility] interface InterfaceName [extends other interfaces] {
        constant declarations
        member type declarations
        abstract method declarations
}

The body of the interface contains abstract methods, but since all methods in an interface are, by definition, abstract, the abstract keyword is not required. Since the interface specifies a set of exposed behaviours, all methods are implicitly public.

Thus, a simple interface may be

public interface Predator {
       boolean chasePrey(Prey p);
       void eatPrey(Prey p);
}

The member type declarations in an interface are implicitly static and public, but otherwise they can be any type of class or interface.

Implementing an Interface

The syntax for implementing an interface uses this formula:

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

Classes may implement an interface. For example,

public class Cat implements Predator {

        public boolean chasePrey(Prey p) {
               // programming to chase prey p (specifically for a cat)
        }

        public void eatPrey (Prey p) {
               // programming to eat prey p (specifically for a cat)
        }
}

If a class implements an interface and is not abstract, 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.

Classes can implement multiple interfaces

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

Interfaces are commonly used in the Java language for callbacks. Java does not allow the passing of methods (procedures) as arguments. Therefore, the practice is to define an interface and use it as the argument and use the method signature knowing that the signature will be later implemented.

Multiple inheritance of interfaces

Interfaces can extend several other interfaces, using the same formula are described above. For example

 public interface VenomousPredator extends Predator, Venomous {
         //interface body
 }

is legal. Note how it allows multiple inheritance, unlike classes. Note also that Predator and Venomous may possibly define or inherit methods with the same signature, say kill(Prey prey). When a class implements VenomousPredator it will implement both methods simultaneously.

Examples

/*
* In ancient times, there 
* were Monsters and Greeks
* who fought each other
*/

public abstract class Creature
{
     protected String name;
     //...

     public void attack()
     {
          System.out.println(name + " attacks!");
     }
}

public class Achilles extends Creature
{
     //...
}

public class Mino extends Creature implements Monster
{
     //...

     public void roar()
     {
          System.out.println(name + " roars loudly!");
     }
}

public interface Monster
{
    void roar();
}

public static void main(String[] args)
{
     Creature aCreature;

     //...


     /**
     * You have to cast aCreature to type Monster because
     * aCreature is an instance of Creature, and not all
     * Creatures can Roar.  We know it is a Monster,
     * therefore it can Roar because we check in the if
     * statement below.  Both Creatures can attack, so we
     * attack regardless...just if it's a monster we'd
     * like to roar before attacking.
     */

     if(aCreature instanceof Monster)
          ((Monster)aCreature).roar(); //It's a monster so we can roar

     //Both can attack
     aCreature.attack();
}

Some common Java interfaces are:

• Comparable has the method compareTo, which is used to describe two objects as equal, or to indicate one is greater than the other. Generics allow implementing classes to specify which class instances can be compared to them.
• Serializable is a marker interface with no methods or fields - it has an empty body. It is used to indicate that a class can be serialized. Its Javadoc describes how it should function, although nothing is programmatically enforced.

Java Interface modifiers

Access Modifier	Interface Modifier
public	abstract
private	strictfp
protected

It must be noted that improper usage of modifiers of interfaces may result in unstable software behaviour.

References

External links

• What Is an Interface?
• Interfaces and Packages

Note:- An interface is a keyword which contains only function declaration, not Definition.