Review of Object References

An object exists only after it has been constructed. Objects are constructed only as a program is running, when an object constructor is invoked.

Once it exists, an object is accessed by going through a reference. Often, the reference is held in a reference variable such as str. For example, the following first declares a reference variable, then constructs an object and puts the reference to it in str:

String str;             // declare a reference variable
str = "Hello World" ;   // construct the object and 
                        // save its reference

Of course, it is OK to declare an object reference variable and not place a reference in it (it might only sometimes be needed.) And it is OK to use the same reference variable for different objects at different times:

String str; 
str = "Hello World" ;

. . . .

str = "Good-by" ;