2/19/08

To Notes

CSC 212 -- 2/19/08

Midterm Exam

Go over the Midterm Exam.

Project Proj5

Discuss Project Proj5 (Inheritance).

Inheritance

A class B inherits a class A if B has all of the instance variables and methods of A.
B may add additional instance variables and methods of its own for extra functionality.
B is a "fancy" version of a.
Use the "is-a" test to see if inheritance makes sense:
If B is-a A (an object of B is an object of A), then it makes sense for B to inherit A.
The class header for B in Java:
Here is a table of some of the names for the classes A and B:

Class A Class B

Base Class Inherited Class

Base Class Derived Class

Class Subclass

Superclass Class

Parent Class Child Class

Class A	Class B
Base Class	Inherited Class
Base Class	Derived Class
Class	Subclass
Superclass	Class
Parent Class	Child Class

Examples of Inheritance Hierarchies

A class is a subclass if it is indented.

Person
    Employee
        Executive

Vehicle
    Automobile
        StationWagon
    DeliveryVehicle
        Truck
        Van

CelestialObject
    FixedObject
        Star
        Nebula
            Galaxy
            DustCloud
    OrbitingObject
        Planet
        Moon    
        Asteroid
        Comet

Animal
    Mammal
        Cat
            DomesticCat
            Lion
            Tiger
        Rodent
            Mole
            Mouse
            Rat 
    Insect
        Ant
        Fly
        Wasp

Classes in an inheritance hierarchy satisfy an is-a relationship. For example:
Classes in a composition relationship satisfy a has-a relationship. For example:

The Employee Example

See the Employee Example:
The UML Diagram for the Employee Example.

Overloaded vs. Overridden Methods

The signature of a method is the ordered list of the types of its parameters
Example: The signature of the constructor header
is (String, char, int).
Two methods are overloaded if they have the same name but different signatures.

Here are two overloaded methods:

System.out.println(45);

System.out.println("abc");

A method in the child class overrides (replaces) a method in the base class if the methods have the same name and the same signatures.
Here are two overridden methods in the Employee Example:

The super Keyword

super is used to invoke methods in the superclass or base class.
1. To invoke a constructor from the base class:
  This call must be the first line of the derived class.
2. To invoke a method from the base class:
  This call can be anywhere in any method of the derived class.

Protected Instance Variables

Here are the Java accessibility modifiers

Modifier Accessibility UML Symbol

public Entire Project +

private Current Class -

protected Current Class or
Derived Classes
(Direct or Indirect) #

Modifier	Accessibility	UML Symbol
public	Entire Project	+
private	Current Class	-
protected	Current Class or Derived Classes (Direct or Indirect)	#

The variable salary is protected in the Employee class of the Employee Project.

More Examples

Box Example UML Diagram
Student Example UML Diagram

Two-dimensional Arrays

An n by m two-dimensional array is a rectangular grid of values having height n and width m.

Here is a 3x2 two-dimentional array a:

            Index 2
           0   1   2
         +---+---+---+
Index  0 | 4 | 5 | 2 |
  1      +---+---+---+
       1 | 9 | 8 | 3 |
         +---+---+---+

The value of a[1][2] is 3.

Here are three ways to define this array:

// Method 1.
int[ ][ ] a;
a = new int[2][3];
a[0][0] = 4;   a[0][1] = 5;   a[0][2] = 2;   
a[1][0] = 9;   a[1][1] = 8;   a[2][2] = 3;   

// Method 2.
int[ ][ ] a = new int[2][3];
a[0][0] = 4;   a[0][1] = 5;   a[0][2] = 2;   
a[1][0] = 9;   a[1][1] = 8;   a[2][2] = 3;   

// Method 3.
int[ ][ ] a = { {4, 5, 2}, {9, 8, 3} };

Composition

Composition refers to using a class as a member of larger class.

Here is an example of composition:

public class Student
{
    private String name;
    private char gender;
    private Address address;
    private String[ ] classes;
}

An Address object is contained inside of the Student class.

The Address class is defined like this:

public class Address
{
    private String poBoxNumber;
    private String apartment;
    private String street;
    private int buildingNumber;
    private String city;
    private String state;
    private int zipCode;
}

A stack that contains Person objects is an example of composition.

Project Proj6

Look at Project Proj6.