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 signature and constant declarations (variable declarations that are declared to be both static and final). An interface may never contain method definitions.
Interfaces cannot be instantiated. A class that implements an interface 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 that implements the interface.
One benefit of using interfaces is that they simulate multiple inheritance. All classes in Java must have exactly one base class, the only exception being java.lang.Object (the root class of the Java type system); multiple inheritance of classes is not allowed. A Java class may implement, and an interface may extend, any number of interfaces; however an interface may not implement an interface.
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Interfaces are used to encode similarities which the 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(); // Note the semicolon: // Interface methods are public and abstract. // Think of them as prototypes only; no implementations are allowed. }
Interfaces are defined with the following syntax (compare to Java's class definition):
[visibility] interface InterfaceName [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. Since the interface specifies a set of exposed behaviors, 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, final 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 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 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.
Interfaces can extend several other interfaces, using the same formula as described below. 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.
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.| Access Modifier | Interface Modifier |
|---|---|
public |
abstract |
strictfp |
Note that improper usage of modifiers of interfaces may result in unstable software behavior.