Casting a shadow
The existence of the local parameter named data casts a shadow on the instance variable having the same name, making it inaccessible by using its namealone.
(A local variable having the same name as an instance variable casts a similar shadow on the instance variable.)
In this shadowing circumstance, when the code in the constructor refers simply to the name data , it is referring to the local parameter having that name. In order for the code in the constructor to refer to the instance variablehaving the name data, it must refer to it as this.data .
In other words ...
In other words, this.data is a reference to an instance variable named data belonging to the object being constructed by the constructor (this object) .
Not always necessary
You could always use this syntax to refer to an instance variable of the object being constructed if you wanted to. However, the use of this syntax isnecessary only when a local parameter or variable has the same name as the instance variable and casts a shadow on the instance variable. When this is notthe case, you can refer to the instance variable simply by referring to its name without the keyword this .
Finally, the main point ...
Now consider the main point of this program. The following fragment shows the main method of the controlling class for the application.
public class Ap112{
public static void main(String args[]){Worker obj1 = new Worker(11);
Worker obj2 = new Worker(22);Worker obj3 = new Worker(33);
Worker obj4 = new Worker(44);obj2.doThis();
}//end main()}//end class Ap112 Four different objects of type Worker
The code in the above fragment instantiates four different objects from the class named Worker , passing a different value to the constructor for each object. Thus, individual instance variable in each ofthe four objects contain the int values 11, 22, 33, and 44 respectively.
Call an instance method on one object
Then the code in the main method calls the instance method named doThis on only one of the objects, which is the one referred to by the reference variable named obj2 .
An overridden toString method of the Worker class is eventually called to return a String representation of the value stored in the instance variable named data for the purpose of displaying that value on the standard output device.
Overridden toString method
The next fragment shows the overridden toString method for the Worker class. As you can see, this overridden method constructs and returns a reference to a String representation of the int value stored in the instance variable named data . Thus, depending on which object the toString method is called on, different string values will be returned by the overridden method.
public String toString(){
return "" + data;}//end toString()
}// end class Worker Passing reference to this object to println method
The next fragment shows the doThis instance method belonging to each object instantiated from the Worker class. When this method is called on a specific object instantiated from the Worker class, it uses the this keyword to pass that specific object's reference to the println method. The println method uses that reference to call the toString method on that specific object. This, in turn causes a String representation of the value of the instance variable named data belonging to that specific object to be displayed.