Interfaces and Polymorphism - Java Concepts - page 536

Unlike a class, an interface type provides no implementation.

An interface type is similar to a class, but there are several important differences:

ȗ All methods in an interface type are abstract; that is, they have a name,

parameters, and a return type, but they don't have an implementation.

ȗ All methods in an interface type are automatically public.

ȗ An interface type does not have instance fields.

Now we can use the interface type Measurable to declare the variables x and
maximum .

public class DataSet
{
. . .
public void add( Measurable x)
{
sum = sum + x .getMeasure() ;
if (count == 0 || maximum .getMeasure() <
x .getMeasure() )
maximum = x;
count++;
}
public Measurable getMaximum()
{
return maximum;
}
private double sum;
private Measurable maximum;
private int count;
}

Use the implements keyword to indicate that a class implements an interface
type.

This DataSet class is usable for analyzing objects of any class that implements the
Measurable interface. A class implements an interface type if it declares the