interface

■ interface is one of the only three statements that can appear with no modifiers. The others are class and constructors. Methods cannot appear with no modifiers (they need a return type or void). Nor can variables (they need a type).

■ You cannot put constructors in an interface.

■ You must label all methods in an interface to be abstract or make them abstract-like. (As opposed to a class, which can be labeled abstract without actually having any abstract methods.)

■ The actual abstract modifier on an interface's methods is optional. i.e.

public abstract void method1( ); and

public void method1( ); both create abstract methods.

■ All variables in an interface must be final. They can be static final. Meaning they must be constants.

■ An interface can extend another interface. You cannot "implement" another interface. i.e. The class Demo below must implement all four methods of One and Two or be abstract itself:

interface One {

public abstract void method1( );

public void method2( );

}

interface Two extends One {

public abstract void method3( );

public void method4( );

}

public class Demo implements Two {

public void method1( ) { }

public void method2( ) { }

public void method3( ) { }

public void method4( ) { }

}

■ There is no “base” interface, serving like Object does for classes.