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

Inheritance should create an is-a relationship, meaning the child is a more specific version of the

parent

Vehicle

Car

Deriving Subclasses• In Java, we use the reserved word extends to

establish an inheritance relationship

• Demo: Book, Dictionary, InheritanceClient

public class Car extends Vehicle

{

// code

}

• 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

• 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

dictionarybook, 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.