Object 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.moveHorizontal(-140);

3. wall.moveVertical(20);

4. wall.changeSize(120);

5. 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(-120);

4. window.moveVertical(40);

5. window.changeSize(40);

6. window.makeVisible();

How To Draw A HouseA SEQUENCE OF METHOD CALLS

Create the roof:

1. roof = new Triangle();

2. roof.changeSize(60, 180);

3. roof.moveHorizontal(20);

4. roof.moveVertical(-60);

5. roof.makeVisible();

How To Draw A HouseA SEQUENCE OF METHOD CALLS

Create the sun:

1. sun = new Circle();

2. sun.changeColor(“yellow”);

3. sun.moveHorizontal(100);

4. sun.moveVertical(-40);

5. sun.changeSize(80);

6. sun.makeVisible();

How To Draw A HouseA SEQUENCE OF METHOD CALLS

How To Draw A HouseA SEQUENCE OF METHOD CALLS

Final result after a sequence of 22 method calls

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

• The sequence of 22 method calls used to create and manipulate these objects will always be the same.

• It would be convenient to record the 22 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;

private Square window;

private Triangle roof;

private Circle sun;

Java Source CodePicture.java

/** * Constructor for objects of class Picture

*/

public Picture()

{

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

}

Java Source CodePicture.java

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.moveHorizontal(-140);

public void draw(){ wall = new Square(); wall.moveHorizontal(-140); wall.moveVertical(20); wall.changeSize(120); wall.makeVisible(); window = new Square(); window.changeColor("black"); window.moveHorizontal(-120); window.moveVertical(40); window.changeSize(40); window.makeVisible();

Java Source CodePicture.java

Java Source CodePicture.java

roof = new Triangle(); roof.changeSize(60, 180); roof.moveHorizontal(20); roof.moveVertical(-60); roof.makeVisible();

sun = new Circle(); sun.changeColor("yellow"); sun.moveHorizontal(100); sun.moveVertical(-40); sun.changeSize(80); 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, i.e. with the correct number, data types and order of its parameters.

• 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 */

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 pp3-17 (Objects and Classes)