When you create a class, you are creating a new data type. You can use
this type to declare objects of that type. However, obtaining objects of a class
is a two-step process. First, you must declare a variable of the class type.
This variable does not define an object. Instead, it is simply a variable that
can refer to an object. Second, you must acquire an actual, physical copy of the
object and assign it to that variable. You can do this using the new operator.
The new operator dynamically allocates (that is, allocates at run time) memory
for an object and returns a reference to it. This reference is, more or less,
the address in memory of the object allocated by new. This reference is then
stored in the variable. Thus, in java, all class objects must be dynamically
allocated. Letâ€™s look at the details of this procedure.
In the preceding sample programs, a line similar to the following is used
to declar an object of type Box.
Box mybox = new Box ();
This statement combines the two steps just described. It can be rewritten like this to show each step more clearly;
mybox = new Box();
This first line declares mybox as a reference to an
object of type Box. After this line executes, mybox contains the value the value
null, which indicates that it does not yet point to an actual object. Any
attempt to use mybox at this point will result in a compile-time error. The next
line allocates an actual object and assigns a reference to it to mybox. After
the second line executes, you can use mybox as if it were Box object. But in
reality, mybox simply holds the
memory address of the actual Box object. The effect of these two lines of code
is depicted .
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