Classes - Building Java Programs: A Back to Basics Approach - page 521

After constructing the Point objects, we make the following method calls:

p1.translate(11, 6);
p2.translate(1, 7);

Essentially, the behavior of these two method calls is the following:

• Set this to refer to the same object as p1 , and execute the translate method

with parameters (11, 6) .

• Set this to refer to the same object as p2 , and execute the translate method

with parameters (1, 7) .

During the first call, this refers to the same object as p1 . Therefore, the method
call adjusts the (x, y) coordinates of p1 's object:

p1 x 3 x 18 y 8 p2 x 3 x 4 y 3

<methods> <methods>

this

During the second method call this refers to the same object as p2 , so the lines in
the body of the translate method change the x and y fields of p2 's object:

p1 x 3 x 18 y 8 p2 x 3 x 5 y 10

<methods> <methods>

this

One common usage of the keyword this is to deal with shadowed variables. As
described earlier, shadowing occurs when a field is obscured by another variable with
the same name. Shadowing can happen when a field has the same name as a para-
meter or local variable in a class. For example, the following is a legal header for our
Point method, even though our fields are also called x and y :

public Point(int x, int y) {

As explained at the beginning of this section, Java would normally interpret the
expression x to mean this.x . However, if a parameter or local variable called x
exists, the program will use that one instead if you just write x, because the field x is
shadowed by the parameter/variable. If you write this.x , though, the program will
always use the field x .