Chapter 10. Mutable Objects

As you learned in the previous chapter, an object is a collection of data that provides a set of methods. For example, a String is a collection of characters that provides methods like charAt and substring.

This chapter explores two new types of objects: Point and Rectangle. You’ll see how to write methods that take objects as parameters and produce objects as return values. You will also take a first look at the source code for the Java library.

Point Objects

In math, 2D points are often written in parentheses with a comma separating the coordinates. For example, $(0, 0)$ indicates the origin, and $(x, y)$ indicates the point x units to the right and y units up from the origin.

The java.awt package provides a class named Point that represents a location in a Cartesian plane. In order to use the Point class, you have to import it:

import java.awt.Point;

Then, to create a new point, you use the new operator:

Point blank;
blank = new Point(3, 4);

The first line declares that blank has type Point. The second line creates the new Point with the coordinates $x = 3$ and $y = 4$ . The result of the new operator is a reference to the object. Figure 10-1 shows the result.