INTRODUCTION TO OBJECTS AND INPUT/OUTPUT - Java Programming: From Problem Analysis to Program Design - page 117

would be stored in str . However, the address of the allocated memory space will be
different from that in the first statement. To be specific, suppose that the address of the
allocated memory space is 3850 . Figure 3-4 illustrates the result.

3850

3

str 3850 Hello there!

FIGURE 3-4 Variable str , its value, and the object str

This is an important property of reference variables of the String type and String
objects, and must be recognized and understood. Furthermore, it is especially important
to understand this property when we start comparing strings.
To simplify Figure 3-4, we usually use the format shown in Figure 3-5.

str Hello there!

FIGURE 3-5 Variable str and the object str

In Figure 3-5, the arrow originating in the box str means that str contains an address.
The arrow pointing to the memory space containing the value "Hello there!" means
that the variable str contains the address of the object containing the value "Hellothere!" .
We will use this arrow notation to help explain various examples.
You might ask: What happened to memory space 2500 and the string "Java Programming"
stored in it? If no other String variable refers to it, then sometime during program
execution, the Java system reclaims this memory space for later use. This is called
garbage collection.

If you do not want to depend on the system to choose when to perform garbage collection,
then you can include the statement:

System.gc();

in your program to instruct the computer to run the garbage collector (immediately). In
general, it is not necessary to do so.

We can now summarize the discussion of the preceding sections. You can declare two types
of variables in Java: primitive type variables and reference variables, as shown in Figure 3-6.