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CHAPTER 7 Classes
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Previously in this chapter, you learned that an instance variable is data that is associated with a class, such as a student s ID number for the class that emulates a student An instance variable is declared similarly to how you declare a variable ( 2), with three exceptions You ll recall that a variable is declared by specifying a data type followed by the name of the variable in a statement The following statement declares a student ID The variable is an int data type and is called student
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int student;
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The same statement is used to declare an instance variable However, an instance variable must be declared within the body of the class and should be preceded by an access specifier Remember that Java designates an instance variable as available to classes within the same package as this class if you don t use an access specifier The following example shows how to declare an instance variable Instance variables are usually designated as private, thus limiting the access to these variables to only method members of the same class
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class Demo { public static void main (String args[]) { } } class myClass { private int student; }
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The third difference between declaring a variable and declaring an instance variable is in the way both are initialized You ll recall that initialization is the process of assigning a variable an initial value You can initialize a variable when the variable is declared, as shown in this next statement:
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int student = 12345;
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However, some programmers initialize an instance variable by using the class constructor (see the next section) Keep in mind that changes to an instance variable of one instance don t have any effect on the instance variable of a different instance This means that if we declare two instances called student of the myClass class, each instance has its own student instance variable A change to one instance variable doesn t change the other
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Constructor
A constructor is automatically called whenever an instance of the class is created Technically, the constructor is called before the new operator completes its operation when declaring an instance of the class (see the section Declaring an Instance of a Class ) Programmers use the constructor to initialize instance variables as well as to perform other tasks when an instance of the class is created A constructor is defined nearly the same way as how you define a method member of a class, with three exceptions First, the name of the constructor must be the same name as the class Second, the constructor has an implicit return value, which is the class type That is, you are not permitted to define a return value for a constructor Third, the constructor must be designated with any access specifier or no access specifier The following example illustrates how to define a constructor This example defines the myClass class that contains an instance variable called student and a method member called myClass() This method member is the constructor for the class because it has the same name as the class The constructor is designated as having public access The constructor in this example initializes the instance variable student with a student ID number
class Demo { public static void main (String args[]) { } } class myClass { private int student; myClass (){ student = 12345; } }
You don t have to define a constructor for your program unless there is a need to execute statements when an instance is created If you don t define a constructor, Java uses a default constructor You can define multiple constructors for your program, with each one having a different argument list For example, a constructor without an argument list might initialize instance variables with default values Another constructor with an argument list might initialize instance variables with values passed to the constructor when the instance is declared
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