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The Java Language
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You should call the file that contains this program BoxDemojava, because the main( ) method is in the class called BoxDemo, not the class called Box When you compile this program, you will find that two class files have been created, one for Box and one for BoxDemo The Java compiler automatically puts each class into its own class file It is not necessary for both the Box and the BoxDemo class to actually be in the same source file You could put each class in its own file, called Boxjava and BoxDemojava, respectively To run this program, you must execute BoxDemoclass When you do, you will see the following output: Volume is 30000 As stated earlier, each object has its own copies of the instance variables This means that if you have two Box objects, each has its own copy of depth, width, and height It is important to understand that changes to the instance variables of one object have no effect on the instance variables of another For example, the following program declares two Box objects:
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// This program declares two Box objects class Box { double width; double height; double depth; } class BoxDemo2 { public static void main(String args[]) { Box mybox1 = new Box(); Box mybox2 = new Box(); double vol; // assign values to mybox1's instance variables mybox1width = 10; mybox1height = 20; mybox1depth = 15; /* assign different values to mybox2's instance variables */ mybox2width = 3; mybox2height = 6; mybox2depth = 9; // compute volume of first box vol = mybox1width * mybox1height * mybox1depth; Systemoutprintln("Volume is " + vol); // compute volume of second box vol = mybox2width * mybox2height * mybox2depth; Systemoutprintln("Volume is " + vol); } }
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Introducing Classes
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The output produced by this program is shown here: Volume is 30000 Volume is 1620 As you can see, mybox1 s data is completely separate from the data contained in mybox2
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Declaring Objects
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As just explained, 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 s look at the details of this procedure In the preceding sample programs, a line similar to the following is used to declare an object of type Box:
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Box mybox = new Box();
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This statement combines the two steps just described It can be rewritten like this to show each step more clearly:
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Box mybox; // declare reference to object mybox = new Box(); // allocate a Box object
The first line declares mybox as a reference to an object of type Box After this line executes, mybox contains 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 a 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 in Figure 6-1
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