Programming Tutorials

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Declaring objects in Java

By: aathishankaran in Java Tutorials on 2007年02月20日 [フレーム]

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. Let us look at the details of this procedure.

In the Box sample program, 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;

Box mybox;
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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