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public void doThis(){
System.out.println(this);}//end area() The bottom line
In this program, the instance variable in the object referred to by obj2 contains the value 22. The instance variables in the other three objectsinstantiated from the same class contain different values.
The bottom line is that the following statement in the main method causes the value 22 to be displayed on the standard output device. Along theway, the this keyword is used to cause the println method to get and display the value stored in a specific object, and to ignore three otherobjects that were instantiated from the same class.
obj2.doThis(); C. 33
The this keyword
The key to an understanding of this program lies in an understanding of the single statement that appears in the method named doThis , as shown in the following fragment.
public void doThis(){
new Helper().helpMe(this);}//end area() The keyword named this has several uses in Java, some of which are explicit, and some of which take place behind the scenes.
What do you need to know about the this keyword?
One of the uses of the keyword this is passing the implicit parameter in its entirety to another method.
That is exactly what this program does. But what is the implicit parameter named this anyway?
Every object holds a reference to itself
This implicit reference can be accessed using the keyword this in a non-static (instance) method belonging to the object. (The implicit reference named this cannot be accessed from within a static method for reasons that won't be discussed here.)
Calling an instance method
An instance method can only be called by referring to a specific object and joining that object's reference to the name of the instance method using aperiod as the joining operator. This is illustrated in the following statement, which calls the method named doThis on a reference to an object of the class named Worker .
new Worker().doThis(); An anonymous object
The above statement creates an anonymous object of the class named Worker . (An anonymous object is an object whose reference is not assigned to a named reference variable.)
The code to the left of the period returns a reference to the new object. Then the code calls the instance method named doThis on the reference to the object.
Which object is this object?
When the code in the instance method named doThis refers to the keyword this , it is a reference to the specific object on which the doThis method was called. The statement in the following fragment passes a reference to that specific instance of the Worker class to a method named helpMe in a new object of the Helper class.
public void doThis(){
new Helper().helpMe(this);}//end area() A little help here please
The helpMe method is shown in the following fragment.
class Helper{
public void helpMe(Worker param){System.out.println(
param.getData());}//end helpMe()
}//end class Helper Using the incoming reference
The code in the helpMe method uses the incoming reference to the object of the Worker class to call the getData method on that object.
Thus code in the helpMe method is able to call a method in the object containing the method that called the helpMe method in the first place.
A callback scenario
When a method in one object calls a method in another object, passing this as a parameter, that makes it possible for the method receiving the parameter tomake a callback to the object containing the method that passed this as a parameter.
The getData method returns a String representation of the int instance variable named data with a value of 33 that is contained in the object ofthe Worker class.
Display the value
The code in the helpMe method causes that string to be displayed on the computer screen.
And the main point is ...
Any number of objects can be instantiated from a given class. A given instance method can be called on any of those objects. When the code in such aninstance method refers to this , it is referring to the specific object on which it was called, and is not referring to any of the many other objects thatmay have been instantiated from the same class.
A. Compiler Error
A wakeup call
The purpose of this question is simply to give you a wakeup call. The declaration for the method named getData indicates that the method returns a reference to an object of the class String . However, the code in the method attempts to return an int . The program produces the following compiler error under JDK 1.3.
found : int
required: java.lang.Stringreturn data; -end-
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