Example Interface

A class can implement several interfaces like this:

class SomeClass extends SomeParent implements interfaceA, interfaceB, interfaceC
{

}

Now the class must implement or inherit the methods listed in each interface.

Here is an example interface definition. The constants don't have to be separated from the methods (as here), but doing so makes the interface easier to read.

interface MyInterface
{
  public static final int CONSTANT = 32;      // a constant
  public static final double PI = 3.14159;    // a constant

  public void methodA( int x );               // a method header
  public double methodB();                    // a method header
}

A method in an interface is public by default. The constants in an interface are public static final by default. Recall that final means that the value cannot change as the program is running.

Making use of the defaults, the above interface is equivalent to the following:

interface MyInterface
{
  int CONSTANT = 32;      // a constant (public static final, by default)
  double PI = 3.14159;    // a constant (public static final, by default)

  void methodA( int x );  // a method header (public, by default)
  double methodB();       // a method header (public, by default)
}

The second interface (above) is the preferred way to define an interface. The defaults are assumed and not explicitly coded.

A class that implements an interface must implement each method in the interface or inherit it from a parent.
Methods from the interface must be declared public in the class.
Constants from the interface can be used as if they had been defined in the class.
Constants should not be redefined in the class.

Of course, if you truely need to use pi in a program, use Math.PI (discussed in chapter 18.)

Answer:

Example Interface

QUESTION 4: