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.
Box myBox; // declare reference to object
myBox = new Box(); // allocate a Box objectThese two steps is combined and can be rewritten as:
Box myBox = new Box();As just explained, the new operator dynamically allocates memory for an object. It has this
general form:
classVar = new classname ( );It is important to understand that new allocates memory for an object during run time. The advantage of this approach is that your program can create as many or as few objects as it needs during the execution of your program. However, since memory is finite, it is possible that new will not be able to allocate memory for an object because insufficient memory exists. If this happens, a run-time exception will occur.
Object reference variables act differently than you might expect when an assignment takes place. For example,
Box b1 = new Box();
Box b2 = b1;You might think that b2 is being assigned a reference to a copy of the object referred to by
b1. That is, you might think that b1 and b2 refer to separate and distinct objects. However,
this would be wrong. Instead, after this fragment executes, b1 and b2 will both refer to the
same object.
The assignment of b1 to b2 did not allocate any memory or copy any part of the original object. It simply makes b2 refer to the same object as does b1. Thus, any changes made to the object through b2 will affect the object to which b1 is referring, since they are the same object.