Inheritance

Inheritance

1. Makes code extensible in any OOP language

   • To extend a class, create a new class.
     • Parent Class
     • Child Class
       • Also known as a subclass
       • Can add new methods and new fields to subclass
       • A subclass can override existing methods to change previous behaviors
       • A subclass can be extended, however to make it inextensible, declare it as final
       • Use "extend" keyword in class declaration to extend a class.
         • EX: public class Volvo extends Car { }
           • In this example, the class Volvo extends parent class Car.
             • Characteristics
               • The "Is-A" Relationship
                 • The type of relationship between a superclass (parent class) and subclass (child class)
                   • EX: public class Volvo extends Car { public int numberOfWheels = 4 public void drive() }
                     • A subclass inherits all public methods and field of its superclass, however, new methods and fields can be added to the subclass to increase functionality and behavior.
                   • In the example, Volvo is a subclass of the superclass Car. A Volvo "is a" car however a Car is not necessarily always a Volvo. So the "is-a" relationship only goes one way!
               • Accessibility
                 • From within a subclass, you can access its superclass's public and protected methods and fields, but not the superclass's private methods.
                 • If the subclass and the superclass are in the same package, you can also access the superclass's default methods and fields.
               • Method Overriding
                 • Used to change the behavior of a method in a parent class.
                 • Method overriding happens when a method with the same signature as a method in the parent class is written in a subclass.
                   • To override a method, write the new method in the subclass, without having to change anything in the parent class.
                   • If the subclass and the superclass are in the same package, you can also override a method with the default access level.
                 • Cannot override a final method.
               • Calling the Superclass's Constructors
                 • To explicitly call a parents constructors from a subclass's constructor, use the "super" keyword. (Must be the first statement in the constructor.)
                   • "Super" keyword is perfect if you want another constructor in the superclass to be invoked.
               • Calling the Superclass's Hidden Members
                 • The "super" keyword can be used to call a hidden member or an overridden method in a superclass.
                   • "super.memberName" returns the specific member in the parent class.
                 • You can access any member in the superclass that is visible from the subclass using the "super" keyword.
               • Type Casting
                 • Can cast an instance of a subclass to its parent class.
                   • Upcasting: Casting an object to a parent class
                   • To upcast a child object, assign the object to a reference variable type parent.
               • instanceof Keyword
                 • "instanceof" keyword used to test if an object is of a specified type.
                   • Used in an if statement: if (objectReference instanceof type)
                   • Applying instanceOf on a null reference variable returns false.
           • A class can only extend one class in Java