Using Objects - Java Concepts - page 73

It is very important that you remember that the box variable does not contain the
object. It refers to the object. You can have two object variables refer to the same
object:
Rectangle box2 = box;
Now you can access the same Rectangle object both as box and as box2 , as
shown in Figure 17 .
Multiple object variables can contain references to the same object.
However, number variables actually store numbers. When you define
int luckyNumber = 13;
then the luckyNumber variable holds the number 13, not a reference to the number
(see Figure 18 ).
You can see the difference between number variables and object variables when you
make a copy of a variable. When you copy a primitive type value, the original and the
copy of the number are independent values. But when you copy an object reference,
both the original and the copy are references to the same object.
Number variables store numbers. Object variables store references.
Consider the following code, which copies a number and then changes the copy (see
Figure 19 ):
int luckyNumber = 13;
int luckyNumber2 = luckyNumber;
luckyNumber2 = 12;
Now the variable luckyNumber contains the value 13, and luckyNumber2
contains 12.
Now consider the seemingly analogous code with Rectangle objects.
Rectangle box = new Rectangle(5, 10, 20, 30);