visual basic 2008 barcode generator Inheritance in Java

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CHAPTER 8 Inheritance
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For example, you d reference the instance of the Person class when calling the Person class s display() method member Likewise, you d reference the instance of the Student class when calling its display() method member These calls are made at compile time Dynamic method dispatch enables you to use the same reference to call different versions of an overridden method member These calls are made at run time Let s discuss how this works In 7, you learned how to assign an instance of a class to a reference and then use the reference to call members of the instance Typically, programmers declare a reference, declare an instance, and assign the instance to the reference all in one statement, as shown here:
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Person p = new Person();
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Alternatively, a reference can be declared in one statement, and the assignment of the instance to the reference happens in another statement, as illustrated here:
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Person p; p = new Person();
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Java uses dynamic method dispatch when you declare a reference and then assign the reference with the reference of an instance whose method member you want to call Throughout the program, you can switch references and thus call different versions of the overridden method member The following example illustrates how to call method members this way by using dynamic method dispatch:
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class Demo { public static void main (String args[]) { Person temp; Person p = new Person (); Student s = new Student (); GraduateStudent gs = new GraduateStudent (); temp = p; tempdisplay(); temp = s; tempdisplay(); temp = gs; tempdisplay(); } } class Person { protected String name; Person(){ name = "Bob Smith";
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} void display(){ Systemoutprintln("Person Class: " + name); } } class Student extends Person { protected int studentID; Student (){ studentID = 12345; } void display(){ Systemoutprintln("Student Class: " + studentID); } } class GraduateStudent extends Student { void display(){ Systemoutprintln("Graduate Student Class:"); Systemoutprintln("Name:" + name); Systemoutprintln("Student ID: " + studentID); } }
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As you can see, the example defines three classes the Person class, the Student class, and the GraduateStudent class All three are the same classes used in the previous example in this chapter The Person class defines a display() method member, and the other two classes override the display() method member Dynamic method dispatch occurs within the main() method, where the first statement declares a reference of the Person class called temp The temp reference is later assigned references to instances of the Person class, the Student class, and the GraduateStudent class Remember that a reference points to the first member address of the instance When a reference to an instance is assigned to the temp reference, the temp reference also points to the same instance This is illustrated in the following statement of the preceding program:
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temp = p;
Prior to this assignment, the program declares p as a reference to an instance of the Person class and assigns it an instance Likewise, the program declares temp as a reference to an instance of the Person class However, temp is not assigned an instance of the class Instead, temp is a reference to nothing that is until the program assigns it the reference p, at which time both the reference temp and p point to the same instance
CHAPTER 8 Inheritance
Once the temp reference is assigned another reference, the temp reference is used to access members of the instance, which in this program is calling the display() method member of the instance
Abstract Classes
So far in this chapter, you have seen how a subclass can override a method member of its superclass However, overriding the method member is optional for a subclass There will be occasions when the superclass requires that a subclass override a method member Let s say that you want any class that inherits the Person class to define a display() method in order to display a person s name Each subclass has it own way of displaying a person s name For example, the Student class might use the student s first name, whereas the GraduateStudent class might use the graduate student s first name You could define a display() method member in the Person class with statements that display a person s name, but none of the subclasses will use this method member because each overrides the method with its own way of displaying this information to the user Another option is not to define the display() method member in the Person class and leave each subclass to define its own method member The problem with this option is that the programmer writing a subclass has the option not to define such a method member The third and preferred alternative is to define a general display() method member in the Person class and let each subclass provide its own details within the display() method member in order to retrieve a person s name A general method member is a method definition that contains a method name, and argument list, and return value, but no method body Each subclass overrides this method member and provides its own method body A subclass must override this method member It cannot simply use the method member of the superclass The superclass that defines a general method member is called an abstract class, and the general method is called an abstract method member You cannot declare an instance of an abstract class All abstract classes must be superclasses You define an abstract class similarly to how you define any class, except you precede the class name with the keyword abstract Within the abstract class definition, there must be at least one abstract method member You declare an abstract method member similarly to how you define a method member, except the definition
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