Within an instance method or a constructor,
this is
a reference to the
current object — the object whose
method or constructor is being called. You can refer to any member
of the current object from within an instance method or a constructor
by using
this.
Using this with a Field
The most common reason for using the
this keyword is
because a field is shadowed by a method or
constructor parameter.
For example, the Point class was written like this
public class Point {
public int x = 0;
public int y = 0;
//constructor
public Point(int a, int b) {
x = a;
y = b;
}
}
but it could have been written like this:
public class Point {
public int x = 0;
public int y = 0;
//constructor
public Point(int x, int y) {
this.x = x;
this.y = y;
}
}
Each argument to the constructor shadows one of the object's fields —
inside the constructor
x is a local copy of the constructor's
first argument. To refer to the
Point field
x,
the constructor must use
this.x.
Using this with a Constructor
From within a constructor, you can also use the
this keyword
to call another constructor in the same class. Doing so is called an
explicit constructor invocation. Here's another
Rectangle
class, with a different implementation from the one in the
Objects section.
public class Rectangle {
private int x, y;
private int width, height;
public Rectangle() {
this(0, 0, 0, 0);
}
public Rectangle(int width, int height) {
this(0, 0, width, height);
}
public Rectangle(int x, int y, int width, int height) {
this.x = x;
this.y = y;
this.width = width;
this.height = height;
}
...
}
This class contains a set of constructors. Each constructor initializes
some or all of the rectangle's member variables. The constructors
provide a default value for any member variable whose initial value
is not provided by an argument. For example, the no-argument constructor
calls the four-argument constructor with four 0 values and the two-argument constructor
calls the four-argument constructor with two 0 values.
As before, the compiler determines which constructor to call, based
on the number and the type of arguments.
If present, the invocation of another constructor must be the first
line in the constructor.
