1. 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

2. 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.

3. 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

4. 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

5. 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.

6. 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.

7. 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

8. 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

9. 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.