Objects Interaction and Source Code Week 2. OBJECT ORIENTATION BASICS REVIEW.

Objects Interaction and Source Code

Week 2

OBJECT ORIENTATIONBASICS

REVIEW

• Object – Represents an actual thing!

• Class – ‘Blueprint’ for making an object

• Method – Things an object can do (behaviour)

• Parameter – input required by an object method

• Data type – what type of data is allowed

Object Model BasicsFUNDAMENTAL CONCEPTS

• Many objects (instances) can be created from a single class.

• An object has attributes (fields), and those attributes have values assigned to them.

• The class defines what fields an object has, but each object stores its own set of values (state).

• Each class has source code (Java) associated with it that defines its fields and methods.

Object Model BasicsOTHER OBSERVATIONS

OBJECT ORIENTATION

OBJECT INTERACTION

Create the outside wall:

1. wall = new Square();

2. wall.moveVertical(80);

3. wall.changeSize(100);

4. wall.makeVisible();

How To Draw A HouseA SEQUENCE OF METHOD CALLS

Create the window:

1. window = new Square();

2. window.changeColor(“black”);

3. window.moveHorizontal(20);

4. window.moveVertical(100);

5. window.makeVisible();


Create the roof:

1. roof = new Triangle();

2. roof.changeSize(50, 140);

3. roof.moveHorizontal(60);

4. roof.moveVertical(70);

5. roof.makeVisible();


Create the sun:

1. sun = new Circle();

2. sun.changeColor(“yellow”);

3. sun.moveHorizontal(180);

4. sun.moveVertical(-10);

5. sun.changeSize(60);

6. sun.makeVisible();



Final result after a sequence of twenty methods

• A house picture is composed of four different shape objects – representing a wall, window, roof and sun.

• The sequence of twenty method calls used to create and manipulate these objects will always be the same.

• It would be convenient to record the twenty method calls and replay them whenever we wished to draw a picture of the house.

Object InteractionOBSERVATIONS ON DRAWING THE HOUSE

BlueJ DemonstrationA First Look at the Picture class

/**

* This class represents a simple picture.

* You can draw the picture using the draw method…

*/

public class Picture

{ private Square wall; // create a reference to a Square object and call it wall.

private Square window; // create a reference to a Square object and call it window.

private Triangle roof; // create a reference to a Triangle object and call it roof.

private Circle sun; // create a reference to a Circle object and calls it sun.

Java Source CodePicture.java

/** * Constructor for objects of class Picture

*/

public Picture()

{

// nothing to do... instance variables are automatically set to null

}


A constructor is a ‘special method’ that is called when the object is created and is used for object initialisation.

Constructors are defined like other methods but use the name of the

class.

Creating a new object

wall = new Square();

variable of type Square assignment

creates an instance of the class

calls the constructor

Sending messages to objects by invoking their methods

wall.moveVertical(80);

public void draw(){ wall = new Square(); wall.moveVertical(80); wall.changeSize(100); wall.makeVisible(); window = new Square(); window.changeColor("black"); window.moveHorizontal(20); window.moveVertical(100); window.makeVisible();



roof = new Triangle(); roof.changeSize(50, 140); roof.moveHorizontal(60); roof.moveVertical(70); roof.makeVisible();

sun = new Circle(); sun.changeColor("yellow"); sun.moveHorizontal(180); sun.moveVertical(-10); sun.changeSize(60); sun.makeVisible();}

• A method of one class can create objects of other classes, and can invoke those object’s methods.

• An object’s method can only be called with its correct method signature – this will be detailed further in this week’s tutorials.

• Always put comments in your source code to ensure others can understand your program.

Object Interaction & Source CodeIMPORTANT POINTS TO REMEMBER

Comments

• Comments are statements that are put into a program to help others understand what it does

• Comments are marked by:• // this is a single line comment• /* this is a multiline comment */• /** so is this */

Variables• A variable is a location in the computer’s memory that is used

to store data• A variable has a name, a type and a value associated with it

int mark = 10;

mark – its name

int – its type – what we can store in it, in this case integers

value - 10 - what is currently stored in it

The statement can be broken down into two separate statements

int mark; // declaration

mark = 10; // assignment

Variable Declarations

• Variables must be declared before they are used.

• To declare a variable we give it a name and a type i.e. what type of data can be stored in it e.g.

• int mark; • double price;• String name;

Assignment

• To put a value in a variable we use the assignment operator =

• mark = 10;• price = 3.64;• name = “Fred”;

Data types• Storage areas in memory must be of the

correct type• Main primitive data types:

– int whole numbers– e.g. 5, 110, -34, 3760– double numbers with a decimal point– e.g. 2.225, 365.2, 0.234, -8.65, 5.0– char keyboard character e.g. ‘q’, ‘N’, ‘2’, ‘(‘– boolean true or false

• N.B. type String is a class. Example String objects: “Hello”, “Meeting at 18:00”, “”

Java syntax - blocks and semicolons

• Curly brackets { } mark the beginning and end of blocks of code including classes and methods

• Semicolons ; mark the end of statements

COMPILING AND RUNNING JAVA PROGRAMS

Javacompiler

JVM Running program

Edit & debug

Libraries / Java API

Source code

Java byte code

Each Java class definition is held in a separate .java file, and the byte code is held in a .class file.

Java Source Code

Picture.java holds our source code, and Picture.class holds the detail the computer needs to create Picture objects.

Required ReadingObjects First With Java – A Practical Introduction using

BlueJ

Related reading for this lecture

• Chapter 1 pp1-17 (Objects and Classes)

Reading for next week’s lecture

• Chapter 2 pp18-37 (Constructors & Methods)

Objects Interaction and Source Code Week 2. OBJECT ORIENTATION BASICS REVIEW.

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