CS 141 Topics

CS141: Topics for Week 12: Nov 9, 11 & 13, 2015

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Lab

Lab 9: Inheritance

Chapter 9 Inheritance

Inheritance, "Is a" relationship
superclass (or base or parent class)
subclass (or derived or child class)
super()
Object class
extends
override
Polymorphism
dynamic binding
Abstract (vs concrete) class
Interfaces
protected access
package access
inheritance hierarchy
overriding methods (e.g. equals)
instanceof operator
comparable interface

Topics: Chp 9 - Inheritance and Interfaces

What is inheritance and why use it?

Subclasses vs superclasses - See Hierarchy
A class can have only one direct superclass. A class can have multiple subclasses.
Don't overuse subclasses.
Subclasses:
- How are they defined (using extends)
- Overriding methods
- What variables and methods in the super class can be accessed by the subclass?
  - public vs protected: Protected modifier allows access by all subclasses and all classes in the same package (folder).
  - private variables in the superclass are not accessible from the subclass except using setter/getters.
  - Using super() in the first line of constructor
  - Using super.method_name() anywhere in a subclass method.

Polymorphism:

    Shape[] shapes = new Shape[3];
    shapes[0] = new Circle(2);
    shapes[1] = new Rectangle(2,3);
    shapes[2] = new Square(5);
    for (int i=0; i < shapes.length; i++) {
	     System.out.println("The area for " + shapes[i] + " is " + shapes[i].calcArea());
     }

Which toString() method is called? Which calcArea() is called? The one in superclass (Shape) or in the subclass (Circle, Rectangle, Square)?

Abstract class:
- Declare as
```
public abstract class Shape { 
	    public abstract double calcArea(); 
	    ... 
```
  Must define method in the subclass. Can't create a Shape object; can only create a subclass.
Using the keyword final so that one can't override a method in the subclass.
The universal super class: Object
All Java classes are subclasses of the Object class and inherit the methods: toString(), equals(), hashCode(). These methods should generally be overriden.
instanceof operator can be used to check what type of object you have. For example, if you need to cast an object. Should be used sparingly. Don't use as a way to implement different behavior, i.e. to bypass polymorphism.
Interface - using the implements keyword.
- An interface (typically) has no variables, only methods which are not implemented. Any variables must be static and final.
- An interface is used to force a class to define a set of methods so as to force consistency across a set a classes.
- Examples: The Comparable interface
- A class can have only one superclass but can implement multiple interfaces.

In Class Practice Activities

We will look at a much simpler version of the Shape class used in the lab.