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Object-Oriented Software Engineering

CS288

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Java OO Fundamentals

Contents

• Classes and Objects

• Making new objects

• Method declarations

• Encapsulating fields

• Manipulating object field values

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Classes and Objects

Class: Template that defines how to make an object.Object: Is an instance of a class.

Car

int numDoorsDouble engineSizeString make

void start ( ) { code }void stop ( ) { code }

Car bigFlashJag

numDoors = 5engineSize = 3.5lmake = Jaguar

start ( )stop ( )

Car ecoCar

numDoors = 2engineSize = 0.5lmake = Smart

start ( )stop ( )

Class

Object

Object

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Building Objects

Computer MemoryClass Definition(acts like template for

new objects)

Object1 Object2 Object3 Object4

Each object has copies of fields and methods defined by class template, but initialised differently each time

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Very Simple Class Example

public class SimpleClass {

public SimpleClass(String newFieldVal) { uselessField = newFieldVal; }

private String uselessField;

public static void main(String[ ] args) { /* Code Goes Here */ }

}Main method executed first.

This is where objects can first be setup.

Constructor, defines how new objectsare initialised.

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Syntax for defining new object

SimpleClass ob1 = new SimpleClass("Fluffy");

object type object name

new keyword

class constructor to set up object semicolon terminatesALL statements(just like C++)

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Very Simple Class Example

public class SimpleClass {

public SimpleClass(String newFieldVal) { uselessField = newFieldVal; }

private String uselessField;

public static void main(String[ ] args) { SimpleClass ob1 = new SimpleClass("Fluffy"); SimpleClass ob2 = new SimpleClass(“I like chocolate"); SimpleClass ob3 = new SimpleClass(“Why cant I go home"); SimpleClass ob4 = new SimpleClass("Flobber Worms Taste Nice"); SimpleClass ob5 = new SimpleClass("Guildford"); }

}

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Executing the Code

Five new objects are created in memory, each with their owncopy of ‘uselessField’, and each copy has a different value.

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Setter and Getter Methods

public class SimpleClass { /* declare class field */ /* define class constructor */

public String getUselessField () { return uselessField; }

public void setUselessField (String newUselessField) { uselessField = newUselessField; } /* main method */}

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Getter method for field

public String getUselessField () { return uselessField; }

public keywordallows this method to be called from other classes

type of object returned by method

name and parameters of method

body of method,what it does

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Setter method for field

public void setUselessField (String newUselessField) { uselessField = newUselessField; }

has String object parameterdoes not return value

changes value of field

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Extending main method

public static void main(String[ ] args) { SimpleClass ob1 = new SimpleClass("Fluffy"); SimpleClass ob2 = new SimpleClass("I like chocolate"); SimpleClass ob3 = new SimpleClass("Why cant I go home"); SimpleClass ob4 = new SimpleClass("Flobber Worms Taste Nice"); SimpleClass ob5 = new SimpleClass("Guildford"); ob1.setUselessField ("I wish I'd done C instead"); String aStrVar = ob2.getUselessField (); ob1.setUselessField (aStrVar); }

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Execution of main method public static void main(String[] args) { SimpleClass ob1 = new SimpleClass("Fluffy"); SimpleClass ob2 = new SimpleClass("I like chocolate"); SimpleClass ob3 = new SimpleClass("Why cant I go home"); SimpleClass ob4 = new SimpleClass("Flobber Worms Taste Nice"); SimpleClass ob5 = new SimpleClass("Guildford"); ob1.setUselessField ("I wish I'd done C instead"); String aStrVar = ob2.getUselessField (); ob1.setUselessField (aStrVar); }

When execution gets to hereobjects have these values

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Execution of main method public static void main(String[] args) { SimpleClass ob1 = new SimpleClass("Fluffy"); SimpleClass ob2 = new SimpleClass("I like chocolate"); SimpleClass ob3 = new SimpleClass("Why cant I go home"); SimpleClass ob4 = new SimpleClass("Flobber Worms Taste Nice"); SimpleClass ob5 = new SimpleClass("Guildford"); ob1.setUselessField ("I wish I'd done C instead"); String aStrVar = ob2.getUselessField (); ob1.setUselessField (aStrVar); }

When execution gets to hereobjects have these values

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Execution of main method public static void main(String[] args) { SimpleClass ob1 = new SimpleClass("Fluffy"); SimpleClass ob2 = new SimpleClass("I like chocolate"); SimpleClass ob3 = new SimpleClass("Why cant I go home"); SimpleClass ob4 = new SimpleClass("Flobber Worms Taste Nice"); SimpleClass ob5 = new SimpleClass("Guildford"); ob1.setUselessField ("I wish I'd done C instead"); String aStrVar = ob2.getUselessField (); ob1.setUselessField (aStrVar); }

When execution finishes hereobjects have these values

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Backup Method for Field Value

Modify SimpleClass to include new field and methods:

private String previousVal;

public void setUselessField (String newUselessField) { previousVal = uselessField; uselessField = newUselessField; }

public void restoreField () { uselessField = previousVal; }

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Experiment with main method

public static void main (String[ ] args) { SimpleClass ob1 = new SimpleClass ("Fluffy"); ob1.setUselessField ("I wish I'd done C instead"); String aStrVar = "Eat Cabbage"; ob1.setUselessField (aStrVar); ob1.restoreField (); } When execution gets to

hereobjects have these values

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Experiment with main method

public static void main (String[ ] args) { SimpleClass ob1 = new SimpleClass ("Fluffy"); ob1.setUselessField ("I wish I'd done C instead"); String aStrVar = "Eat Cabbage"; ob1.setUselessField (aStrVar); ob1.restoreField (); } When execution gets to

hereobjects have these values

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Experiment with main method

public static void main (String[ ] args) { SimpleClass ob1 = new SimpleClass ("Fluffy"); ob1.setUselessField ("I wish I'd done C instead"); String aStrVar = "Eat Cabbage"; ob1.setUselessField (aStrVar); ob1.restoreField (); }

When execution gets to hereobjects have these values

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Experiment with main method

public static void main (String[ ] args) { SimpleClass ob1 = new SimpleClass ("Fluffy"); ob1.setUselessField ("I wish I'd done C instead"); String aStrVar = "Eat Cabbage"; ob1.setUselessField (aStrVar); ob1.restoreField (); }

When execution gets to hereobjects have these values

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Is there a problem with the restore method?

public static void main (String[ ] args) { SimpleClass ob1 = new SimpleClass ("Fluffy"); SimpleClass ob2 = new SimpleClass("I like chocolate"); ob1.setUselessField ("I wish I'd done C instead"); ob2.restoreField (); }

When execution gets to hereobjects have these values

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Summing Up

• Every object has copies of the class fieldsand methods.

• We have seen example field values for multiple objects.

• Seen examples of methods for accessing and changing field values.

• We have seen how objects are created and manipulated during execution of the main method.