visual basic 2008 barcode generator When to Use Inheritance in Java

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When to Use Inheritance
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Programmers follow a simple rule to determine when inheritance is appropriate for a program The rule is called is a, and it requires an object to have a relationship with another object before it can inherit the object The is a rule asks the question, Is object A an object B If so, then object A can inherit object B If not, then object A should not inherit object B Technically, there is nothing prohibiting one class from inheriting another class, but each inheritance should pass the is a rule Let s apply the is a rule to the student example discussed in the previous section of this chapter The three objects in this example are student, graduate student, and undergraduate student First, apply the is a rule to the graduate student object You do this by asking the question, Is a graduate student a student Sure is! Therefore, the graduate student can inherit the student object Next, apply the is a rule to the undergraduate student object by asking, Is an undergraduate student a student That s also true Therefore, the undergraduate student can inherit the student object Now let s look at a slightly different example Suppose you want to have the graduate student object inherit the undergraduate student object Will this pass the is a test In order to answer this question, you need to ask, Is a graduate student an undergraduate student The answer is no Therefore, you know that the graduate student should not inherit the undergraduate student
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CHAPTER 8 Inheritance
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You cause a class to inherit another class by using the keyword extends in the class definition Think of this as telling Java that the second class is extending the class definition of the first class The class that is being inherited is called a superclass Some programmers also call this a parent The class doing the inheriting is called a subclass or a child The keyword extends is placed in the class definition of the subclass Here is the form for using the extends keyword Class B is inheriting some or all the attributes and behaviors of class A, depending on the access specifier (see 7)
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class B extends class A { }
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The following example illustrates how to inherit a class This example contains two class definitions First is the Student class definition and the second is the GraduateStudent class definition The GraduateStudent class is a subclass and inherits the Student class by using the extends keyword, which is placed to the right of the class name
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class Student { } class GraduateStudent extends Student { }
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Accessing Members of an Inherited Class
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As you ll recall from 7, members of a class can be accessed based on its access specifier There are three access specifiers: public, private, and protected A member of any class can access a member designated as public A member designated as private can only be accessed by a method member of its own class A member designated as protected can be accessed by method members of its own class and by method members of subclasses that inherit the superclass This may seem confusing, so let s take a look at an example so you can see how this works This example declares two classes other than the Java application class
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called demo These are the Student class and the GraduateStudent class The GraduateStudent class inherits the Student class
class Demo { public static void main (String args[]) { GraduateStudent gs = new GraduateStudent(); gsdisplay(); } } class Student { private int studentID; Student (){ studentID = 12345; } protected void display(){ Systemoutprintln("Student ID: " + studentID); } } class GraduateStudent extends Student { }
There are two members of the Student class besides the constructor These are an instance variable called studentID and a display() method member that displays the value of the instance variable on the screen The constructor (see 7) is used to initialize the instance variable The GraduateStudent class doesn t have any members, but it can access public and protected members of the Student class because the GraduateStudent class inherits the Student class This means that an instance of the GraduateStudent class can access the display() member method of the Student class, just as if the display() method member was a member of the GraduateStudent class This is illustrated in statements within the main() method of the application The first statement declares an instance of the GraduateStudent class The second statement uses the instance to call the display() method member Looking at the main() method, you d think that the display() method is a member of the GraduateStudent class, when in fact the display() method is a member of the Student class
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