Computer Sciences 313e - Inheritance

• To create a subclass, we use the extends keyword
• Subclasses inherit the variables and methods of the parent class, and use them as if they were declared in the subclass.

Example:

class Animal
{
    float weight;
    int numberOfLegs;

    void eat() {...}
    void drink() {...}
    void move() {...}
}

class Dog extends Animal
{
    String breed;
    // inherits weight, numberOfLegs

    void bark() {...}
    // inherits eat(), drink(), move()
}

Dog instances can be used anywhere Animal instances can be used.

Example:
Animal tbone;
tbone = new Dog();
tbone.move(); <--- A Dog instance is also an instance of Animal, so the move() method can be invoked

Overriding Methods in the Parent Class

• A subclass can redefine a method that it inherited from its superclass.
• The subclass method overrides the superclass method, and essentially replaces its implementation for instances of the subclass.<>

Example Overriding the move() method from Animal in the Dog class:

class Dog extends Animal
{
    String breed;
    // inherits weight, numberOfLegs

    void bark() {...}
    void move() { // a special version of the move() method just for Dog objects }
    // inherits eat(), drink()
}

Keyword Super

• Super refers to members of the superclass.
• Common use: an overriding method in the subclass will call the superclass method by the same name.

Example

class Dog extends Animal
{
    String breed;
   ...
    void move()
   {
       ...
       super.move(); // Call implementation of move() in superclass Animal
    }
}

• The statement super() calls the superclass constructor.
  

Example

class Person
{
    Person(String name)
    { ... }
}

class Doctor extends Person
{
    Doctor(String name, String specialization)
    {
        super(name); // Call the Person constructor and pass in name
        ... // do some other stuff
    }
}

• Using super in this way avoids code duplication and thus makes our program easier to read.
• Since the Person class does not contain a default no argument constructor, we must call super() explicitly. Otherwise the compiler will complain that it cannot find a default constructor in the parent class.