OBJECTS AND CLASSES - Introduction to Java Programming: Comprehensive Version - page 357

simply by using the ClassName.staticVariable reference. A hidden instance variable
can be accessed by using the keyword this , as shown in Figure 9.21a.

public class F {
private int i = 5 ;
private static double k = 0 ;

Suppose that f1 and f2 are two objects of F.

Invoking f1.setI( 10 ) is to execute
this .i = 10 , where this refers f1

public void setI( int i) {
this .i = i; Invoking f2.setI( 45 ) is to execute

this .i = 45 , where this refers f2 }

public static void setK( double k) {
F.k = k;

Invoking F.setK( 33 ) is to execute
F.k = 33 . setK is a static method

}

// Other methods omitted

}

(a) (b)

F IGURE 9.21

The keyword this refers to the calling object that invokes the method.

The this keyword gives us a way to reference the object that invokes an instance method.
To invoke f1.setI(10) , this.i = i is executed, which assigns the value of parameter i to
the data field i of this calling object f1 . The keyword this refers to the object that invokes the
instance method setI , as shown in Figure 9.21b. The line F.k = k means that the value in param-
eter k is assigned to the static data field k of the class, which is shared by all the objects of the class.

9.14.2 Using this to Invoke a Constructor
The this keyword can be used to invoke another constructor of the same class. For example,
you can rewrite the Circle class as follows:

public class Circle {
private double radius;
public Circle( double radius) {
this . radius = radius;

}

The this keyword is used to reference the hidden
data field radius of the object being constructed.

public Circle() {
this ( 1.0 );

}

The this keyword is used to invoke another
constructor.

...

}

The line this(1.0) in the second constructor invokes the first constructor with a double
value argument.

Note
Java requires that the this(arg-list) statement appear first in the constructor
before any other executable statements.

Tip
If a class has multiple constructors, it is better to implement them using this(arg-list)
as much as possible. In general, a constructor with no or fewer arguments can invoke a
constructor with more arguments using this(arg-list) . This syntax often simpli-
fies coding and makes the class easier to read and to maintain.