Classes - Building Java Programs: A Back to Basics Approach - page 511

During the second method call p2 's translate method is executed, so the lines in
the body of the translate method change p2.x and p2.y :

p2.translate(1, 7);

x x 3 5 y 10

p2

public void translate(int dx, int dy) {

x += dx;
y += dy;

}

Mutators and Accessors
The translate method is an example of a mutator.

Mutator
An instance method that modifies the object's internal state.

Generally, a mutator assigns a new value to one of the object's fields. Going back
to the radio example, the mutators would be the switches and knobs that turn the
radio on and off or change the station or volume.
It is a common convention for a mutator method's name to begin with “set,” as in
setID or setTitle . Usually, a mutator method has a void return type. Mutators
often accept parameters that specify the new state of the object or the amount by
which to modify the object's current state.
Accessors form a second important category of instance methods.

Accessor
An instance method that provides information about the state of an object
without modifying it.

Generally, an accessor returns the value of one of the object's fields. Using our
radio analogy, an accessor might return the current station or volume. Examples of
accessor methods you have seen in previous chapters include the length and
substring methods of String objects and the exists method of File objects.
Our client program computes the distance of two Point s from the origin, (0, 0).
Since this is a common operation related to the data in a Point , let's give each Point
object an accessor called distanceFromOrigin that computes and returns that
Point object's distance from the origin. The method accepts no parameters and
returns the distance as a double .
The distance from the origin is computed using the Pythagorean Theorem, taking
the square root of the sum of the squares of the x and y values. As we did when we