Declaring classes that inherit from another class

Now we will create another abstract class. Specifically, we will create a subclass of the recently created VirtualAnimal abstract class. The following lines show the code for the VirtualMammal abstract class that extends the VirtualAnimal class. Note the abstract class keywords followed by the class name, VirtualMammal, the extends keyword, and VirtualAnimal, that is, the superclass.

The class name that follows the extends keyword indicates the superclass from which the new class inherits in the class definition. The code file for the sample is included in the java_9_oop_chapter_06_01 folder, in the example06_03.java file.

public abstract class VirtualMammal extends VirtualAnimal {
    public boolean isPregnant;

    public VirtualMammal(int age, boolean isPregnant) {
        super(age);
        this.isPregnant = isPregnant;
        System.out.println("VirtualMammal created.");
    }

    public VirtualMammal(int age) {
        this(age,...