Java Interface
In this chapter you will learn:
What is a Java Interface
interface specifies what a class must do, but not how it does it.
An interface in Java is like a contract.
It defines certain rules through Java methods and the class which
implements that interface must follow the rules by implementing the methods.
To implement an interface, a class must create the complete set of methods defined
by the interface.
Defining an Interface
An interface is defined much like a class. This is the general form of an interface:
access interface name {
return-type method-name1(parameter-list);
return-type method-name2(parameter-list);
//j a v a2 s. c om
type final-varname1 = value;
type final-varname2 = value;
// ...
return-type method-nameN(parameter-list);
type final-varnameN = value;
}
Variables can be declared inside of interface declarations. They are implicitly final and static. Variables must also be initialized with a constant value. All methods and variables are implicitly public if the interface, itself, is declared as public.
Here is an example of an interface definition.
interface MyInterface{
void callback(int param);
}
Implementing Interfaces
To implement an interface, include the implements clause in a class definition, and then create the methods defined by the interface.
The general form of a class that includes the implements clause looks like this:
access-level class classname [extends superclass] [implements interface [,interface...]] {
// class-body
}
Here is a small example class that implements the interface shown earlier.
interface MyInterface {
void callback(int param);
}/*from j a v a 2s .c o m*/
class Client implements MyInterface{
// Implement Callback's interface
public void callback(int p) {
System.out.println("callback called with " + p);
}
}
callback() is declared using the public access specifier.
When you implement an interface method, it must be declared as public.
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What you will learn in the next chapter: