Interface (Java)

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An interface in the Java programming language is an abstract type that 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). An interface may never contain method definitions.

As interfaces are implicitly abstract, they cannot be directly instantiated except when instantiated by a class which implements the said interface. The class must implement all of the methods described in the interface, or be an abstract class. 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.

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.


[edit] Overview

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 both would 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, because 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.

For example:

  interface Bounceable
      void setBounce();/*Interface methods are by default public and
                                          abstract; methods in an interface end
                                          with a semicolon, not with a curly brace.*/  

[edit] Usage

[edit] Defining an interface

Interfaces are defined with the following syntax (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.[1]

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 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.[2] 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.

[edit] Subinterfaces

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

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

is legal and defines a subinterface. 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.

[edit] 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 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.

[edit] Java interface modifiers

Access Modifier Interface Modifier
public abstract
private strictfp

Note that improper usage of modifiers of interfaces may result in unstable software behavior.

[edit] References

[edit] External links

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