OOP in Java : © W. Milner 2005 : Slide 1 Java and OOP Part 2 – Classes and objects.

OOP in Java : © W. Milner 2005 : Slide 1

Java and OOP

Part 2 – Classes and objects


Objects

OOP programs make and use objects An object has data members (fields) An object has methods The program can tell an object to carry out

one of its methods


Classes

A class is a type of object Objects belong to classes An object instantiates a class Most classes have several objects - instances A class is defined in a source code file with the

same name as the class


The Product Class

Models a stock control item 2 data members, barcode and stockLevel Defined in file name Product.java Compiled to Product.class Cannot run it – no main method

public class Product{public int barcode;public int stockLevel;}


Using the Product class

Classes start with capital letters

Everything else does not This instantiates 2

Product objects ( new ) and sets their data

members This can be run – but no

output

public class First{public static void main(String[] args){Product p1 = new Product();p1.barcode=3;p1.stockLevel=20;Product p2 = new Product();p2.barcode=4;p2.stockLevel=60;}

}


Adding a display method

Check out how the display method is defined

barcode means the barcode field of the object executing this method

NB barcode is not a variable

public class Product{public void display(){

System.out.println("Barcode = "+barcode);System.out.println("Stocklevel = "+stockLevel);System.out.println("=========================");

}

public int barcode;public int stockLevel;

}


Using the display method

We tell products p1 and p2 to do their display methods.

Try this code out

public class First{public static void main(String[] args){Product p1 = new Product();p1.barcode=3;p1.stockLevel=20;p1.display();Product p2 = new Product();p2.barcode=4;p2.stockLevel=60;p2.display();}

}


Constructors

A constructor is something which 'makes' an object

A class will nearly always have a constructor defined

A constructor can have parameters, usually used to give initial values to fields.


Product constructorpublic class Product

{public Product(int initBarcode, int initStockLevel){

barcode=initBarcode;stockLevel=initStockLevel;

}public void display(){

System.out.println("Barcode = "+barcode);System.out.println("Stocklevel = "+stockLevel);System.out.println("=========================");

}public int barcode;public int stockLevel;}

check how a constructor is named


Using the constructor

new invokes constructor Check how initial values

are passed in the constructor

public class First{public static void main(String[] args){Product p1 = new Product(3,20);p1.display();Product p2 = new Product(4,60);p2.display();}

}


Task

Define an Employee class 2 data members – name (String) and

payrollNumber (int) Define a constructor Define a display method In First, instantiate and display 2 Employees Keep this for later use


Methods which take parameters

public class Product{..previous code omitted

public void deliver(int howMany){

stockLevel+=howMany;}

public int barcode;public int stockLevel;}

parameter


Using the deliver method

Product p1 = new Product(3,20);p1.deliver(10);p1.display();Product p2 = new Product(4,60);p2.deliver(20);p2.display();


Exercise

Copy the Product class (constructor, display and deliver methods)

Add a sell method. This takes a parameter of how many to sell, and it reduces the stock level.

If it is told to sell more than it has, the stock level should become 0.

Check it works


methods which return values

This is a method of the Product class

See how the return type (boolean) is included, like a C function return type

public boolean needMore(){ if (stockLevel==0) return true; else return false;}


Using the new method

Product p1 = new Product(3,20);p1.deliver(10);p1.sell(40);if (p1.needMore())

p1.deliver(50);p1.display();


Encapsulation

Central to the philosophy of OOP Means data in objects are 'closed up' Other parts of the application cannot

accidentally alter data within an object Increases modularity When you use a class, no need to worry about

messing it up. How to do it..


Encapsulation

This is part of the definition of the class Product

Data members should be declared private not public

Rest of code runs unaltered, but..

..private int barcode;private int stockLevel;..


Attempt to access private member from another class:

C:\Walter\java\javaprogs\First.java:11: stockLevel has private access in Productp1.stockLevel=22; ^1 error

Tool completed with exit code 1

Product p1 = new Product(3,20);p1.deliver(10);p1.sell(40);if (p1.needMore())

p1.deliver(50);p1.display();p1.stockLevel=22;


Validating access

Methods which alter data members should validate the change

Here the deliver method of Product checks for –ve quantity delivered

public void deliver(int howMany){ if (howMany<0) { System.out.println("Invalid delivery"); return; } else stockLevel+=howMany;}

Product p1 = new Product(3,20);p1.deliver(-10);p1.display();


Accessor methods

Data members should usually be private But often we want to find out the values of

those members from outside the class Or to change them Solution – public accessor methods Method to 'read' a data member XXX usually

called 'getXXX' Methods to 'write' to a data member XXX

called 'setXXX' Set methods must validate the change they

are making


Typical get method:

..public int getStockLevel(){

return stockLevel;}

..private int stockLevel;}

Product p1 = new Product(3,20);int x = p1.getStockLevel();System.out.println(x);

using it


Task

Go back to the Employee class Make the data members private Add public get and set methods Check it works


Overloading

You can have different versions of the same method with the same name

This is called overloading Different versions must have different

numbers or types of arguments For example..


Overloading example

One version delivers a default 100 units

Second version allows specifying the delivery quantity

public void deliver(){ stockLevel+=100;}

public void deliver(int howMany){ if (howMany<0) { System.out.println("Invalid delivery"); return; } else stockLevel+=howMany;}


Constructor overloading

Constructors are usually overloaded Such as..


Constructor overloadingpublic Product(int initBarcode, int initStockLevel){

barcode=initBarcode;stockLevel=initStockLevel;

}public Product(int initBarcode){

barcode=initBarcode;stockLevel=100;

}

Product p1 = new Product(3,20);p1.display();Product p2=new Product(4);p2.display();

One defaults to an initial stock level of 100

Other allows to specify it In use..


Default no-arg constructors

The constructor with no arguments is called 'the no-args constructor'

Like Product p = new Product(); If you do not define any constructors, then..

– the system calls a default version for you

If you do define some (with args), and you call Product p = new Product(); then

– You must explicitly define the no-arg version


Static variables

A static or class variable is a piece of data for the whole class, not for individual objects

For example, we need to ensure product barcodes are unique.

One way is to 'autonumber' them This means the Product class must remember

the last one used..


Using a static field

Revised constructor barcode and

stocklevel have different values for each Product object

There is a single lastBarcodeUsed value for the class

..public Product(){ lastBarcodeUsed++; barcode=lastBarcodeUsed; stockLevel=100;}..private static int lastBarcodeUsed=0;private int barcode;private int stockLevel;..


Using the revised constructor

Product p1 = new Product();Product p2=new Product();Product p3=new Product();p1.display();p2.display();p3.display();


Static methods

// a static method to the Product classpublic static int count(){ return lastBarcodeUsed;}

// use it..Product p1 = new Product();Product p2=new Product();Product p3=new Product();System.out.println("There are now "+Product.count()+" products");


public static void main()

This is a method It does not return a value (void) It is static (just one of it- don't need to construct

an object) It is public (so we can call it) It is special in that execution starts there – as for C


Review

Review the product class definition we have:


Product class definition

public class Product{

private static int lastBarcodeUsed=0;private int barcode;private int stockLevel;

public Product(){ lastBarcodeUsed++; barcode=lastBarcodeUsed; stockLevel=100;}

public Product(int initStock){ lastBarcodeUsed++; barcode=lastBarcodeUsed; stockLevel=initStock;}

What are these called?

What is this?

Two things same name – called what?

Why are these private?


More Product

public static int count(){ return lastBarcodeUsed;}

public void display(){ System.out.println("Barcode = "+barcode); System.out.println("Stocklevel = "+stockLevel); System.out.println("=========================");}

Why is this static?

Why is this int?

Why is this void?


Rest of Product

public void deliver(int howMany){ if (howMany<0) { System.out.println("Invalid delivery"); return; } else stockLevel+=howMany;} public int getStockLevel()

{ return stockLevel;}

}

Why do this?

When do we use getXXX?

What are these methods called?


Arrays of objects

To have an array of objects there are 2 steps: 1. make the array 2. make the objects to put in the array


Arrays of objects// declare the type of stock..Product stock[];// call the array constructor to make the array..stock = new Product[10];// make 10 objects and put them in the array:for (int i=0; i<10; i++)

stock[i]=new Product();// show they exist:for (int i=0; i<10; i++)

stock[i].display();


Object array exercise

Make an array of 10 Employee objects Give them data and display them

OOP in Java : © W. Milner 2005 : Slide 1 Java and OOP Part 2 – Classes and objects.

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Transcript of OOP in Java : © W. Milner 2005 : Slide 1 Java and OOP Part 2 – Classes and objects.