Classes and Objects - Beginning Java 8 Fundamentals - page 230

model Unknown

year 2000

262144

price 0.0

Memory address Object stored at the address

Figure 6-13. Memory state when a Car object is created using the new Car() statement

At this point, there is no way to refer to the newly created Car object from a Java program even though it exists in
memory. The new operator (as used in new Car() ) returns the memory address of the object it creates. In your case, it
will return 262144 . Recall that the memory address of the data (the Car object, in your case) is also called as reference
of that data. From now onwards you will say that the new operator in Java returns a reference to the object it creates
instead of saying that it returns the memory address of the object. Both mean the same thing. However, Java uses the
term “reference,” which has a more generic meaning than “memory address.” In order to access the newly created Car
object, you must store its reference in a reference variable. Recall that a reference variable stores the reference to some
data, which is stored somewhere else. All variables that you declare of reference type are reference variables in Java.
A reference variable in Java can store a null reference, which means that it refers to nothing. Consider the following
snippet of code that performs different things on reference variables:

Car myCar = null; /* #1 */
myCar = new Car(); /* #2 */
Car xyCar = null; /* #3 */
xyCar = myCar; /* $4 */

When the statement labeled #1 is executed, memory is allocated for myCar reference variable, say at memory
address 8192 . The null value is a special value, typically a memory address of zero, which is stored at the memory
address of the myCar variable. Figure 6-14 depicts the memory state for the myCar variable when it is assigned a
null reference.

myCar 8192 null

Figure 6-14. Memory state for the myCar variable, when the “Car myCar = null” statement is executed

The execution of statement labeled #2 is a two-step process. First, it executes the new Car() part of the statement
to create a new Car object. Suppose the new Car object is allocated at memory address of 9216. The new Car()
expression returns the reference of the new object, which is 9216 . In the second step, the reference of the new object is
stored in myCar reference variable. The memory state for the myCar reference variable and the new Car object after the
statement labeled #2 is executed is shown in Figure 6-15 . Note that the memory address of the new Car object ( 9216 )
and the value of the myCar reference variable match at this point. You do not need to worry about the numbers used
in this example for memory addresses; I just made up some numbers to drive home the point of how the memory
addresses are used internally. Java does not let you access the memory address of an object or a variable. Java lets you
access/modify the state of objects through reference variables.