Defining Classes II - Absolute Java - page 288

references address of where the object is stored. This memory address is called a reference (to the

object). 1 This is diagrammed in Display 5.12 .
Variables of a primitive type and variables of a class type are different for a reason.
A value of a primitive type, such as the type int , always requires the same amount of
memory to store one value. There is a maximum value of type int , so values of type
int have a limit on their size. However, an object of a class type, such as an object
of the class String , might be of any size. The memory location for a variable of type
String is of a fixed size, so it cannot store an arbitrarily long string. It can, however,
store the address of any string because there is a limit to the size of an address.
Because variables of a class type contain a reference (memory address), two variables
may contain the same reference, and in such a situation, both variables name the same
object. Any change to the object named by one of these variables will produce a change
to the object named by the other variable, because they are the same object. For example,
consider the following code. (The class ToyClass is defined in Display 5.11 , but the
meaning of the code should be obvious and you should not need to look up the definition.)

ToyClass variable1 = new ToyClass("Joe", 42);
ToyClass variable2;
variable2 = variable1; //Now both variables name the same object.
variable2.set("Josephine", 1);
System.out.println(variable1); //Invokes variable1' s toString
//method

The output is

Josephine 1
The object named by variable1 has been changed without ever using the name
variable1 . This is diagrammed in Display 5.13 .

Variables of a Class Type Hold References

A variable of a primitive type stores a value of that type. However, a variable of a class
type does not store an object of that class. A variable of a class type stores the reference
(memory address) of where the object is located in the computer's memory. This causes
some operations, such as = and == , to behave quite differently for variables of a class type
than they do for variables of a primitive type.

Reference Types

A type whose variables contain references are called reference types . In Java, class types
are reference types, but primitive types are not reference types.

1 Readers familiar with languages that use pointers will recognize a reference as another name for a pointer.
However, Java does not use the term pointer , but instead uses the term reference . Moreover, these references are
handled automatically. There are no programmer-accessible pointer (reference) operations for dereferencing or
other pointer operations. The details are all handled automatically in Java.