7.2 Java1636 java oop member classes  (Page 9/15)

Accessing the object of the class C, and the this keyword

The syntax used with the keyword this is somewhat different for member classes and contained objects than is the case fortop-level classes. For example, continuing with the method named cShow , the code in Listing 17 shows five different ways to access the object instantiated from the member class named C in order to get and display thename of the class file that represents the member class named C.

The screen output

All five statements in Listing 17 display the name of the same class file, as shown in Figure 10 .

Obviously in this situation, the last three statements in Listing 17 are overly complex. There is no particular problem writing code in the methodnamed cShow to gain access to the object to which the method belongs. It isn't even necessary to use this to refer to that object, although the use of the hidden reference this may make the code more readable.

Accessing the containing object of the class B

However, things get a little more complicated when you need to gain access to acontaining object, such as the containing object instantiated from the class named B.

The two statements shown in Listing 18 gain access to the containing object of the class named B. Each statement gets and displays thename of the class file that represents the member class named B. (Note the use of the keyword this in these statements.)

The screen output

The output produced by the code in Listing 18 is shown in Figure 11 . Once again, both statements get and display the name of the same class file.

Accessing the containing object of the class named A

Finally, the code in Listing 19 gains access to the containing object of the class named A. (Once again, note the use of the this keyword in the statement in Listing 19 .)

The code in Listing 19 produces the output shown in Figure 12 .

(Since the class named A is a top-level class, the name of the class file is the same as the name of the class, with no $ characters inserted by thecompiler.)

Investigate independent objects of classes A and B

Recall that when the object of the member class named C was instantiated, theconstructor for the class instantiated independent objects of the enclosing classes named A and B, and saved those object's references in instancevariables of the class named C.

(See Listing 11 , noting the parameter values of 10 and 20 passed to the constructors for A and B. Recall that the constructors for A and B savethose parameter values in private instance variables named aVar and bVar .)

Display variable values and class file names

The code in Listing 20 displays the values stored in the private instance variables belonging to thoseobjects. The code in Listing 20 also displays the names of the class files representing the classes from which those objects were instantiated.

Screen output

The code in Listing 20 produces the output shown in Figure 13 .

There should be no surprises in the output shown in Figure 13 . The values of the instance variables match the parameter values passed to theconstructors in Listing 11 when the objects were instantiated. The class file names match what you already know to be true from previous discussionsearlier in this module.