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 introduced with the interface keyword, and may only contain function signatures and constant declarations (variable declarations which are declared to be both static and final).

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

The primary capability which interfaces have, and classes lack, is 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 (corresponding to the extends clause in the class definition; classes without an extends clause are defined to inherit from Object); multiple inheritance of classes is not allowed. However, Java classes may implement as many interfaces as the programmer desires (with the implements clause). A Java class which implements an interface, but which fails to implement all the methods specified in the interface, becomes an abstract base class, and must be declared abstract in the class definition.

Uses

Interfaces are used to collect like 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.

Finally, interfaces may be used in place of multiple inheritance of classes.

Usage

Defining an Interface

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

[visibility] interface Interface Name [extends other interfaces] {
        constant 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.

Thus, a simple interface may be

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

Implementing an Interface

The syntax for implementing an interface uses this formula:

... implements interface name[, 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
        }

}

If a class implements an interface and is not abstract, and does not implement a required interface, this will result in a compiler error. 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 { ... }

Creating subinterfaces

Subinterfaces can be created as easily as 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.

Examples

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 an 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.

External links

• What Is an Interface?
• Interfaces and Packages