interface SomeInterface
{
  public final int x = 32;
  public double y;   // No variables allowed

  public double addup( );
}

Implementing an Interface

A class definition must always extend one parent, but it can implement zero or more interfaces:

class SomeClass extends Parent implements SomeInterface
{

   class definition body

}

The body of the class definition is the same as always. However, since it implements an interface the body must have a definition of each of the methods declared in the interface. The class definition can use access modifiers as usual. Here is a class definition that implements three interfaces:

public class BigClass extends Parent implements  InterfaceA, InterfaceB, InterfaceC
{

   class definition body

}

Now BigClass must provide a method definition for every method declared in the three interfaces. Here is another class definition:

public class SmallClass implements InterfaceA
{

   class definition body

}

Any number of classes can implement the same interfaces.