INTRODUCTION TO OBJECTS AND INPUT/OUTPUT - Java Programming: From Problem Analysis to Program Design - page 115

2500

str 2500 Java Programming

FIGURE 3-2 Variable str and the data it points to

3

For the String variable str , the statement in Line 4 causes the system to allocate memory
space starting at, say, location 2500 , stores the string (literal) "Java Programming" in this
memory space, and then stores the address 2500 in the memory space of str .
The next obvious question is: How does this happen? In reality, for the most part, the effect
of the statement in Line 4 is the same as the effect of the following statement:

str = new String("Java Programming"); //Line 5

In Java, new is an operator. It causes the system to allocate memory space of a specific
type, store specific data in that memory space, and return the address of the memory
space. Therefore, the statement in Line 4 causes the system to allocate memory space
large enough to store the string (literal) "Java Programming" , stores this string in that
memory space, and returns the address of the allocated memory space. The assignment
operator stores the address of that memory space into the variable str .

As noted, for the most part the effects of the statements in Lines 4 and 5 are the same.
In both the cases, the String variable str will point to a memory location that contains
the string "Java Programming" . Note that in the statement in Line 5, the operator
new is used explicitly, while the statement in Line 4 does not explicitly use the operator
new . In reality, when the statement in Line 4 executes, it first looks if the program
has already created the string "Java Programming" . If this is the case, then the
String variable str will point to that memory location. However, when the statement
in Line 5 executes, the system will allocate a memory space, store the string "Java
Programming" into that memory space, and then store the address of that memory
space into str . This is a key difference and plays an important role when strings and
String variables are compared, which we will explain in Chapter 4.

String is not a primitive data type. In Java terminology, the data type String is defined
by the class String . In this and subsequent chapters, you will encounter some other
classes provided by the Java system. In Chapter 8, you will learn how to create your own
classes.
In Java, variables such as str are called reference variables. More formally, reference
variables are variables that store the address of a memory space. In Java, any variable
declared using a class (such as the variable str ) is a reference variable. Because str is a
reference variable declared using the class String , we say that str is a reference
variable of the String type.