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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• Static Declarations

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

Car bigFlashJag

numDoors = 5engineSize = 3.5lmake = Jaguarstart ( )stop ( )

Car ecoCar

numDoors = 2engineSize = 0.5lmake = Smartstart ( )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.Slide shows view of data from NetBeans debugging console.

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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); }

Example shows use of setting and getting methods for uselessField

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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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Static Declarations

• Not all methods and fields belong to Objects• Static methods and fields stay inside the class• There is only ever one copy of a static method or field• Static methods and fields are `shared’ between all

the objects of that class

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Static Declarations

• When we declare a new variable with a statement we create a reference to the thing we want the variable to name

• E.g SimpleClass ob1 = new SimpleClass("Fluffy");SimpleClass ob1 = ob2;

Object:SimpleClass("Fluffy")

ob1 ob2

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Static Declarations

• Static fields stay with the class not the object• E.g change declaration in SimpleClass

private static String uselessField = "oooo";• Then in main method create two objects:

SimpleClass ob1 = new SimpleClass("Fluffy");SimpleClass ob1 = new SimpleClass("Fluffy");

Object:SimpleClass("Fluffy")field:

ob1 ob2

Class: SimpleClassfield: uselessField

Object:SimpleClass("Fluffy")field:

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Static Declarations

Changing the value of the field within one object now affects all the otherobjects of that class:

SimpleClass ob1 = new SimpleClass("Fluffy"); SimpleClass ob2 = new SimpleClass("Fluffy"); ob1.setUselessField ("I wish I'd done C instead"); String aStrVar = ob2.getUselessField ();

NetBeans Debugger Output

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Static Methods

• Static methods are more complex than static fields• This is NOT allowed:

public class SimpleClass { private String uselessField = "oooo"; private String initialVal = ""; public static void setUselessField(String newUselessField) { uselessField = newUselessField; } /* and so on */

If we attempt to compile this we get error:SimpleClass.java: non-static variable uselessField cannot be referenced from a static context uselessField = newUselessField;1 errorBUILD FAILED (total time: 0 seconds)

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Static Methods

• Static methods are more complex than static fields• E.G. this is NOT allowed:

public class SimpleClass { private String uselessField = "oooo"; private String initialVal = ""; public static void setUselessField(String newUselessField) { uselessField = newUselessField; } /* and so on */

Static methods may only refer to static fields.Whereas here uselessField is NOT declared static.

WHY NOT?

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Static Methods

• Static methods are more complex than static fields• E.G. this is NOT allowed:

public class SimpleClass { private static String uselessField = "oooo"; private String initialVal = ""; public static void setUselessField(String newUselessField) { uselessField = newUselessField; } /* and so on */

Now uselessField is declared static. Code compiles without error.

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Static Methods

• Can reference static methods and fields directly from the class,do not need to create an object first.

• E.G. public static void main(String[ ] args) {

SimpleClass ob1 = new SimpleClass("Fluffy"); SimpleClass ob2 = new SimpleClass("Fluffy"); SimpleClass.setUselessField ("I wish I'd done C instead"); }

To use the static method here we can access it directly fromthe class.(Notice that the main method is always declared static. WHY?)

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Static Methods

Consider adding a new method to store previous value used for uselessField:

Consider adding a new method to store previous value used for uselessField, which needs a new field to hold that value in:

private String lastValueField;

public void storeLastValue () {

lastValueField = uselessField;

}

Then could use this whenever set value of uselessField so that we can track the last value

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Static Methods

Then could use this whenever set value of uselessField so that we can track the last value.

public static void setUselessField(String newUselessField) { uselessField = newUselessField; /* this keeps track of the last value used for uselessField */ storeLastValue(); }

Problem: Wont Compile, get error:

non-static method storeLastValue() cannot be referenced from a static context: storeLastValue();

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Static Methods

Problem:

• We must have setUselessField declared static for some reason.

• For some other reason we must have that storeLastValue is not declared static.

• But we cant access storeLastValue in method body setUselessField of as it is not static.

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Static Methods

Solution:

public static void setUselessField(String newUselessField, SimpleClass obj) { uselessField = newUselessField; obj.storeLastValue(); } public void storeLastValue ( ) { lastValueField = uselessField; }

Adds new argument

Now refer to method via objectsname.

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`this’ keyword

New Problem, the constructor method.

public SimpleClass( ) { setUselessField("Some String", ????????); initialVal = uselessField; }

What value goes here?

Solution: keyword `this’`this’ is a reference to the current object.The object whose method or constructor is being called.

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`this’ keyword

public SimpleClass( ) { setUselessField("Some String", this); initialVal = uselessField; } public void resetField() { setUselessField(initialVal, this); } public SimpleClass(String newUslessField) { setUselessField(newUslessField, this); }

Reference with `this’ must then be introduced into othermethods that call setUselessField:

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`this’ keyword`this’ can also be used to make methods more readable.When a method argument is to be used to update an existingfield, then it is often the case that the argument is given the same name as the field name. This leads to a conflict thatcan be resolved by using `this’.

For example instead of: public static void setUselessField(String newUselessField, SimpleClass obj) { uselessField = newUselessField; obj.storeLastValue(); }

we can have: public static void setUselessField(String uselessField, SimpleClass obj) { this.uselessField = uselessField; obj.storeLastValue(); }

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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.• Seen how static methods and fields are only created once

within the class and are not copied to new objects.