• Inheritance allows you to create a class from an existing class.

• This is one of the most important concepts in Object-Oriented Programming and allows you to re-use code rather than maintain multiple copies of the same code.

• The existing class is called the parent, super, or base class.

• The new class is called the child, sub, or derived class.

• The child class just gets (inherits) anything defined in the parent class, including anything that the super class has inherited.

• You do NOT recode anything that is already in the parent.

• Both attributes (instance variables) and behaviors (instance methods) are inherited.

• The child class can add its own instance variables and methods, as needed.

• The idea is that each child class is a specialized version of its parent.

• In Java, use the keyword extends to create a child class.

• If the parent is not expicitly specified, it is the Object class; Object is the parent of all classes in Java.

• Inheritance is a very powerful mechanism for reusing code and is an important feature of all object-oriented programming languages.

Hierarchies

• A class hierarchy refers to a set of classes that are related by inheritence.

• A relationship that can be describe as X hasa Y commonly indicates that X is a class and Y is a data member or member function of the class. (You already know this.)

• A relationship that can be describe as A isa B commonly indicates that A and B are both classes, and B is a super class of A.

• If a class farther down in the hierarchy needs different behavior from its parent, you can override the method.

• This means to write a method in the child with the exact signature of the method in the parent class.

• The child-class method can either completely replace or augment the corresponding method in the superclass.

• Any member functions that are not overidden in the derived class will be inherited from the super class.

• Note that this is different than overloading a method.

• Overloading means to have a method in the same class with the same name and a different signature (either the number or class of arguments is different).