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CHAPTER 8 Inheritance
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The Superclass Can Be Instantiated
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There is practically no difference between a superclass and a subclass class, except that a subclass has access to public and protected members of the superclass This means you can declare an instance of the superclass without having to declare an instance if the subclass Let s modify the previous example to illustrate how this is done This example is practically the same as the previous one The GraduateStudent class inherits the Student class However, we don t declare an instance of the GraduateStudent class in the main() method Instead, we declare an instance of the Student class We then proceed to call the display() method to display the value of the studentID instance variable on the screen
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class Demo { public static void main (String args[]) { Student s = new Student(); sdisplay(); } } class Student { private int studentID; Student (){ studentID = 12345; } protected void display(){ Systemoutprintln("Student ID: " + studentID); } } class GraduateStudent extends Student { }
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One-way Inheritance
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Inheritance is a one-way street That is, a subclass can access public and protected members of the superclass, but the superclass cannot access members of the subclass In fact, the superclass doesn t know the existence of the subclass
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This is important to keep in mind whenever you use inheritance in your program; otherwise, you might use a method member of the superclass to interact with members of the subclass, only to discover that you receive an error when you compile your program
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Calling Constructors
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At least two constructors are involved whenever a subclass inherits a superclass Both of these classes have a constructor As you ll recall from 7, every class has a default constructor that is automatically called when you create an instance of a class This is true even if you don t define a constructor in your class A default constructor is defined for every class as part of the Java language It has no arguments However, if a constructor is declared in a class, then the default constructor doesn t exist Java calls the constructor of the subclass and the superclass when you declare an instance of the subclass That is, both constructors execute Java calls the constructor of the superclass first and then calls the constructor of the subclass Only if a constructor does not explicitly invoke another constructor does Java implicitly invoke the superclass constructor automatically The implicit call is super(), so the superclass must define a constructor with no arguments As you ll see later in this chapter, you use multilevel inheritance in your program This means a superclass is inherited by a subclass, and the subclass is inherited by another subclass Each class has at least one default constructor, and all of them are executed whenever an instance of the lowest subclass is declared Java executes each constructor beginning with the constructor of the superclass, followed by the constructor of the first subclass, and then the constructor of the second subclass This is true only if all classes in the inheritance chain have constructors that have no arguments
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Using the super Keyword
On occasion, you ll need to explicitly reference the members of the superclass from a method member of a subclass For example, the superclass may have overloaded constructors As you ll recall from 7, a class definition can define two or more constructors, each of which has a different method signature One constructor might not have an argument list, whereas another constructor may have an argument list
CHAPTER 8 Inheritance
Typically, the constructor of a superclass is called from within a constructor of a subclass using the following statement:
super ();
This statement tells Java to run the constructor of the superclass that doesn t contain an argument list Here s a similar statement that tells Java to run a constructor of the superclass that has an argument list that consists of one integer:
super (5555);
The super keyword is used to refer to a member of a superclass other than the constructor Let s suppose you want to call the superclass s display() method from within a subclass s display() method Here s how this is written:
superdisplay();
You ll notice that the keyword super is used in the same way a reference to an instance is used to refer to a member of a class within your program The following example illustrates how to use the super keyword within a Java program This example defines the GraduateStudent class and Student class, which you ve seen used in previous examples in this chapter The Student class defines two constructors, both of which initialize the instance variable The first constructor uses a default value for the initialization The second constructor uses the value of the argument list as the initialization value The second constructor is said to overload the first constructor The Student class also defines a display() method member that is used to display the value of the instance variable on the screen The GraduateStudent class defines a constructor and defines a display() method member The GraduateStudent constructor contains one statement that uses the super keyword to call the overloaded constructor of the Student class, passing it the value 5555 The GraduateStudent display member method also has one statement This statement uses the super keyword to tell Java to run the display() member method of the Student class, which displays the value of the instance variable on the screen Here s the value displayed on the screen:
Student ID: 5555 class Demo { public static void main (String args[]) { GraduateStudent gs = new GraduateStudent (); gsdisplay(); } } class Student {
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