Inheritance Inheritance allows a programmer to derive a new class from an existing one The existing class is called the super class, or parent class, or base class The derived class is called the subclass or child class. As the name implies, the child inherits characteristics of the parent That is, the child class inherits (can use, has access to) the methods and variables defined in the parent class

Inheritance To tailor a parent class, the programmer can add new variables or methods, or can modify the inherited ones What’s the point of inheritance? Software reuse. It’s somewhat like using a class/client, but works best when there is a “parent-child” relationship.

Inheritance Inheritance relationships often are shown graphically in a class diagram, with an arrow with an open arrowhead pointing to the parent class Vehicle Car Inheritance should create an is-a relationship, meaning the child is a more specific version of the parent

Deriving Subclasses { // code } In Java, we use the reserved word extends to establish an inheritance relationship public class Car extends Vehicle { // code } Demo: Book, Dictionary, InheritanceClient

Notice that in the examples, the parent class, Book, has a variable called pages. This makes sense, because all books have pages. Also, notice that the child class, Dictionary, has a variable called definitions. A dictionary is-a type of book, so it has pages, and has access to that variable, but only a dictionary has definitions. So, when you design child/parent classes, it is important where you place certain variables. It would be a mistake to place pages in Dictionary, or definitions in Book.

Visibility Modifiers Visibility modifiers determine which class members are inherited and which are not Variables and methods declared with public visibility are inherited Variables and methods declared with private visibility are not The problem with the Book class is that the pages variable is public. This violates the concept of encapsulation. Encapsulation: ALL variables should be private, and if a child class (or a client) wants to use that variable, then there should be a method they can call to access it.

The super Reference Constructors are not inherited, even though they have public visibility Yet we often want to use the parent's constructor to set up the "parent's part" of the object The super reference can be used to refer to the parent class, and often is used to invoke the parent's constructor Super can be used to call other methods in the parent class as well Demo: Book, Dictionary, InheritanceClient

A child’s constructor is responsible for calling the parent’s constructor. If the child’s constructor does not do this, it is an error. The first line of a child’s constructor should use the super reference to call the parent’s constructor The super reference can also be used to reference other variables and methods defined in the parent’s class

Assignment Think of an “is-a” relationship in real life, such as dictionarybook, in which the parent contains a common piece of information (such as pages), and the child contains a more specific piece of information (such as definitions). Now, create a parent class, as well as a child class that represents this relationship. In the child class, use the super reference to call the parent’s constructor from the child’s constructor. Finally, create a client program that tests the child and parent classes.