Inheritance

Inheritance

1. inheritance: the process of extending a class in OOP

   • the original class is called the parent class, base class, or superclass
   • the new class is called the child class, subclass, or derived class
   • you extend a class by using the extends keywork in a class declaration, after the class name and before the parent class
     • all Java classes automatically extend the java.lang.Object class
     • a class can only extend one class

2. the is-a relationship

   • when you create a new class by inheritance, the subclass and the superclass have an "is-a" relationship
     • a subclass is a special type of its superclass
       • the is-a relationship does not go the other way
     • a subclass inherits all public methods and fields of its superclass
     • it is legal to assign an instance of a subclass to a reference variable of the parent type

3. 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

4. method overriding

   • you can change the behavior of a method in the parent class
     • happens when you write in a subclass a method that has the same signature as a method in the parent class
       • simply write the new method in the subclass
       • you can override the superclass's public and protected methods, and if the subclass and superclass are in the same package, you can override a method with the default access level
     • if only the name is the same but the list of arguments is not, then it is method overloading
   • you cannot override a final method

5. calling the superclass's constructors

   • when you instantiate a child class by invoking one of its constructors, it calls the no-argument constructor of the direct parent class
     • you can explicitly call the parent's constructor from a subclass's constructor by using the super keyword, but it must be the first statement in the constructor
       • the super keyword can also be used to call a hidden member or an overridden method in a superclass

6. type casting

   • you can only cast an instance of a subclass to its parent class - called upcasting
   • downcasting to a sublcass is only allowed if the parent class reference is already pointing to an instance of the subclass

7. final classes

   • you can prevent others from extending your class by making it final using the keyword final in the class declaration

8. the instanceof keyword

   • can be used to test if an object is of a specified type