INHERITANCE AND POLYMORPHISM - Introduction to Java Programming: Comprehensive Version - page 419

You could rewrite the printCircle() method in the Circle class as follows:

public void printCircle() {
System.out.println( "The circle is created " +
super .getDateCreated() + " and the radius is " + radius);

}

It is not necessary to put super before getDateCreated() in this case, however, because
getDateCreated is a method in the GeometricObject class and is inherited by the Circle
class. Nevertheless, in some cases, as shown in the next section, the keyword super is needed.

11.4 What is the output of running the class C in (a)? What problem arises in compiling the
program in (b)?

✓ ✓

Check

Point

class A {
public A() {
System.out.println(
"A's no-arg constructor is invoked" );

class A {
public A( int x) {
}
}

}
} class B extends A {

public B() {
}
}

class B extends A {
}

public class C {
public static void main(String[] args) {
B b = new B();
}
}

public class C {
public static void main(String[] args) {
B b = new B();
}
}

(a) (b)

11.5

How does a subclass invoke its superclass's constructor?

11.6

True or false? When invoking a constructor from a subclass, its superclass's no-arg
constructor is always invoked.

11.4 Overriding Methods
To override a method, the method must be defined in the subclass using the same sig-
nature and the same return type as in its superclass.

Key
Point

A subclass inherits methods from a superclass. Sometimes it is necessary for the subclass to modify
the implementation of a method defined in the superclass. This is referred to as method overriding .
The toString method in the GeometricObject class (lines 46-49 in Listing 11.1)
returns the string representation of a geometric object. This method can be overridden to
return the string representation of a circle. To override it, add the following new method in the
Circle class in Listing 11.2.

method overriding

1 public class CircleFromSimpleGeometricObject
2 extends SimpleGeometricObject {

3 // Other methods are omitted

4
5

// Override the toString method defined in the superclass

6 public String toString() {

toString in superclass

7 return super .toString() + "\nradius is " + radius;

8 }
9 }