Classes and Objects - Beginning Java 8 Fundamentals - page 233

model Unknown

year 2000

myCar 8192 5120 5120

price 0.0

model Civic LX

year 1999

9216

xyCar 10240 9216

price 16000.0

Figure 6-19. Memory state of reference variables myCar and xyCar and two Car objects

Let's make one more change and set the xyCar reference variable to null as shown:

xyCar = null; /* #6 */

Figure 6-20 shows the memory state after statement #6 is executed.

model Unk nown

year 2000

5120

myCar 8192 5120

price 0.0

model Civ ic LX

year 1999

9216

xyCar 10240 null

price 160 00. 0

Figure 6-20. Memory state of reference variables myCar and xyCar and two Car objects after xyCar has been assigned
a null reference

Now the xyCar reference variable stores a null reference and it no longer refers to any Car object. The Car object
with Civic LX model is not being referenced by any reference variable. You cannot access this Car object at all in your
program because you do not have a reference to it. In Java terminology, the Car object with the Civic LX model is
not reachable. When an object in memory is not reachable, it becomes eligible for garbage collection. Note that the
Car object with the Civic LX model is not destroyed (or deallocated) immediately after xyCar is set to null . It stays in
memory until the garbage collector runs and makes sure that it is not reachable. Please refer to the chapter on garbage
collection for more details on how an object's memory is deallocated.
I have covered enough background about variables types and how they work in Java. It is time to discuss the
parameter passing mechanism in Java. In brief, we can state

All parameters in Java are passed by value.