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1. Inheritance

In OOP, one can also create a class that is based on another class.

• The parent class is called the superclass
• The new, child class is called the subclass

In this example, we make a base class Animal to serve as the superclass of a subclass Dog.

We say that a Dog is-a Animal.

For has-a relationships, we make those properties of the class (like has-a name).

class Animal {
    public void speak() {
        printf("I am an animal\n");
    }
    public String getName() {
        return "Animal";
    }
}

|  Added class Animal

Animal animal = new Animal();

|  Added variable animal of type Animal with initial value Animal@44e81672

animal.speak();

I am an animal

"I am an " + animal.getName();

|  Expression value is: "I am an Animal"
|    assigned to temporary variable $5 of type String

I am an Animal

class Dog extends Animal {
    public void speak() {
        printf("woof!\n");
    }    
}

|  Added class Dog

class Cat extends Animal {
    public void speak() {
        printf("meow!\n");
    }    
    
    void poopInBox() {
        printf("wheeew");
    }
}

|  Replaced class Cat
|    Update replaced variable cat, reset to null
|    Update overwrote class Cat

Cat cat = new Cat();

|  Modified variable cat of type Cat with initial value Cat@75412c2f

cat.speak();

meow!

cat.getName();

|  Expression value is: "Animal"
|    assigned to temporary variable $24 of type String

Animal

dog.getName()

|  Expression value is: "Animal"
|    assigned to temporary variable $25 of type String

Animal

Dog dog = new Dog();

|  Modified variable dog of type Dog with initial value Dog@73035e27

dog.speak();

woof!

Animal whatisthis = cat;

|  Modified variable whatisthis of type Animal with initial value Cat@75412c2f

whatisthis.speak();

meow!

cat.poopInBox();

wheeew

whatisthis.poopInBox();

|  Error:
|  cannot find symbol
|    symbol:   method poopInBox()
|  whatisthis.poopInBox();
|  ^------------------^

The Dog object gets the message "speak" and it has a method for that.

This is called Polymorphism.

See Page 19 of your textbook for more details.