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begins with the keyword abstract The following example shows how to define an abstract class and abstract method member:
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abstract Person { abstract void display(); }
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Notice there isn t a body defined for the display() method member because this version of the display() method member can never be called Therefore, it doesn t make sense to place statements within the body of the method member The following example shows how to use an abstract class and abstract method member in a program This example is very similar to other examples used throughout this chapter in that it defines a Person class, a Student class, and a GraduateStudent class What is different in this program is that the Person class is an abstract class, and the display() method member of that class is an abstract method Another difference is that an instance of the Person class is not declared within the program If we tried to declare an instance of the Person class, we d receive a compiler error Likewise, if the Student class and GraduateStudent class fail to define a display() method member, we d also receive a compiler error
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class Demo { public static void main (String args[]) { Student s = new Student (); GraduateStudent gs = new GraduateStudent (); sdisplay(); gsdisplay(); } } abstract class Person { abstract void display(); } 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:");
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CHAPTER 8 Inheritance
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Systemoutprintln("Student ID: " + studentID); } }
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Although you cannot declare an instance of an abstract class, you can declare a reference to an abstract class and use the reference to point to instances of other appropriate classes For example, the following statement is illegal because the Person class is an abstract class:
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Person p = new Person();
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However, the following statement is legal because you can declare a reference to the Person class even though the Person class is an abstract class:
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Person p;
An abstract class can define nonabstract members A nonabstract member can be accessed by a subclass of the abstract class This is illustrated in the next example, where an instance variable called name is declared in the Person class, and the constructor of the Person class initializes the instance valuable The Student class inherits the Person class and has access to the initialized instance variable
abstract class Person { protected String name; Person(){ name = "Bob Smith"; } abstract void display(); } class Student extends Person { protected int studentID; Student (){ studentID = 12345; } void display(){ Systemoutprintln("Student Class: " + studentID); Systemoutprintln("Student Class: " + name); } }
The final Keyword and Inheritance
The final keyword has two uses in inheritance First, the final keyword is used with a method member of a superclass to prevent a subclass from overriding the
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method member The second use is to use the final keyword with a class to prevent the class from becoming a superclass (that is, to prevent another class from inheriting it) Let s begin exploring the final keyword by using it to prevent a subclass from overriding a superclass s method member Suppose that a superclass defines a method member that displays a warning message on the screen In order to prevent subclasses from overriding this superclass s method member, we designate the method member as final The keyword final tells Java that this is the final definition of the method member You designate a member method as final by preceding the method member definition with the keyword final, as shown here Any attempt by a subclass to override the warningMsg() method member will result in a compiler error
class Person { final void warningMsg(){ Systemoutprintln("Invalid Entry"); } }
A class can inherit any other class However, you can prevent your class from being inherited by preceding the name of the class with the keyword final The keyword final tells Java that no class can inherit this class This means that the only way to access members of the class is to declare an instance of the class within a program and use the instance to access its members The following example shows how to use the final keyword to prevent the Person class from being inherited by other classes:
final class Person { void warningMsg(){ Systemoutprintln("Invalid Entry"); } }
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