Classes - Building Java Programs: A Back to Basics Approach - page 508

and any parameters that the method accepts. Here's the start of a Point class with a
translate method, with the header declared but the body blank:

public class Point {
int x;
int y;

public void translate(int dx, int dy) {
...

}

}

When we declare a translate method in the Point class, we are saying that
each Point object has its own copy of that method. Each Point object also has its
own x and y values. A Point object would look like the following:

x x 3 7 y 2

public void translate(int dx, int dy) {
...
}

Whenever an instance method is called, it is called on a particular object. So, when
we're writing the body of the translate method, we'll think of that code from the
perspective of the particular Point object that receives the message: “The client has
given me a dx and dy and wants me to change my x and y values by those amounts.”
Essentially, we need to write code to match the following pseudocode:

public void translate(int dx, int dy) {
add dx to this Point object's x value.
add dy to this Point object's y value.

}

It's helpful to know at this point that an object's instance methods can refer to its
fields. In previous chapters we've talked about scope, the range in which a variable
can be seen and used. The scope of a variable is the set of braces in which it is
declared. The same rule applies to fields: Since they are declared directly inside a
class, their scope is the entire class.
This rule means that the translate method can directly refer to the fields x and y .
For example, the statement x += 3 ; would increase the Point object's x value by 3 .
Here is a working translate method that adjusts the Point object's location:

public void translate(int dx, int dy) {
x += dx;
y += dy;

}