Object%20Orientation:%20Objects%20and%20Classes

1
Chapter 1

• Object Orientation Objects and Classes

2
What is Programming?

• Phases of Programming
• Design
• Implementation
• Testing
• Repeating

3
Polyas Four Step Process

• Four steps to any problem solving activity
• Understand the problem
• Devise a solution
• Test the solution
• Rework the solution

4
Solving Computer Problems

• Need a tool to solve the problem
• Use a computer language to do this
• Different levels of languages
• Machine Language
• Assembly Language
• High Level Language
• Some are talking about 4th Level Languages

5
Algorithms

• What is an algorithm?
• A finite set of steps that specify a sequence of
  operations to be carried out in order to solve a
  specific problem.

6
Properties of Algorithms

1. Finiteness
2. Absence of Ambiguity
3. Definition of Sequence
4. Feasibility
5. Input
6. Output

7
What is a Computer?

• Made up of many independent parts all working
  together
• Memory Unit
• Arithmetic/Logic Unit (ALU)
• Control Unit
• Input Devices
• Output Devices
• Auxiliary Storage Devices

8
Memory Unit

• Can be thought of to look like a giant grid
• Each square in the grid is a memory location
• We can access each location in the grid, if we
  choose to

9
Objects and Classes

• When you create a program using an
  object-oriented language, you are
• Creating a model of the problem
• The model is built up from objects in the problem
  domain
• Those objects must be represented in your program

10
Object vs. Class

• The difference between an object and a class is
  subtle.
• We will examine a student class.
• If we wanted to identify one particular student,
  we would need to ask some questions
• What color is his hair?
• How tall is he?
• What is his QCA?

11
Object vs. Class

• In order to answer these questions we need to
  know what student we are talking about.
• This particular student object is an instance of
  the Student class.
• In this example, the Student is a class and the
  instance of the Student is a student object.

12
Object vs. Class

• A class is a generic example a blue print if you
  will.
• An object is one particular instance of a class.
• From one set of blue prints we can make multiple
  instances.
• All the instances will be unique but based on the
  same set of blue prints

13
Convention

• Names of Classes start with a Capital Letters.
• Names of objects start with lower case letters.
• This case convention is fairly well followed
  throughout the Java programming community.

14
BlueJ Example

• Lets take a look at an example in BlueJ.
• All the examples in the notes assume that you
  have BlueJ installed in C\BlueJ
• The Shape Example

15
Calling Methods

• When we right clicked on a object on the
  workbench in BlueJ, we got a popup menu with a
  bunch of function names listed.
• In Java, these functions are called methods
• When we chose one of these methods for the object
  to perform, we called or invoked the method
• When the method requires information from the
  programmer, the method is said to have a parameter

16
Method Signatures

• A method signature provides information about the
  method.
• It tells us the name of the method
• Whether or not the method requires a parameter
• What type of information is returned from the
  method

17
Data Types

• The values a parameter can take
• Examples
• int whole numbers
• String a section of text
• Pitfall forgetting the double quotes around a
  string. Youll get a message like Error
  undefined variable

18
Java Data Types

• Java supports two kinds of data types
• Built-in data types
• Object data types
• Built-in data types are types like integers,
  characters, etc.
• Object data types are like Strings, Arrays, etc.

19
Built-in Types

• Java has a limited amount of built-in types.
• We will be most concerned with
• int - For integer numbers
• double - For floating point numbers
• boolean - For true/false values
• char For character data

20
Object Types

• There are many objects types.
• Some are provided for us
• The others we will create ourselves.
• There are some object types we can use with no
  extra work
• String For representing strings of characters
• System For some basic output

21
State

• All of the attributes that define an object is
  referred to as the objects state
• Java refers to the individual attributes as
  fields

22
Multiple Instances

• Object-oriented programming (OOP) allows for
  creating more than one object from the same
  class.
• The internal state of the object allows the
  computer to tell the different objects apart.

23
What is an object?

• All objects of the same class have the same
  fields (and methods).
• The values stored in those fields can be
  different.
• Simply being an object of a given class
  guarantees that the objects will have the same
  fields

24
Object Interaction

• Objects do not normally operate by themselves
• Objects can create other objects and invoke those
  objects methods
• Most Java programs have objects that will do just
  this
• Question How do we do this?

25
Source Code

• Like most programming languages, Java is written
  using plain text files or source code files
• The source code tells the program how objects are
  created and how the objects will interact
• After a source code file is written, it must be
  compiled so that the Java Virtual Machine (JVM)
  can interpret it

26
Return Values

• Some methods not only take parameters, but give
  something back to you.
• These methods are said to have a return value
• You can tell this in the signature
• String getName()
• This methods returns a String
• void moveRight()
• This method returns void (nothing)

27
Homework

• 1.2, 1.5, 1.6, 1.9, 1.11, 1.18, 1.26

28
Quiz

• Explain the difference between an object and a
  class.