Java and OOP Part 3 Extending classes OOP in Java : W. Milner 2005 : Slide 1
Inheritance Suppose we want a version of an existing class, which is slightly different from it. We want to avoid starting again from scratch We can define the new class to be a sub-class of the first class. OOP in Java : W. Milner 2005 : Slide 2
Terms used The original class is called the base class, the ancestor class or the super class The process of designing the sub-class from the base class is called 'sub-classing' the base class OOP in Java : W. Milner 2005 : Slide 3
Sub-classing The subclass inherits all members and methods of the first where needed we can write new versions of inherited methods which replace the old method we can add extra members and methods You can t loose a member or method No limit to levels of inheritance OOP in Java : W. Milner 2005 : Slide 4
Example Common type of sub-classing is specialization Suppose we want a type of product which is perishable When we deliver new stock, we throw away old stock not add to it First review the Product class: OOP in Java : W. Milner 2005 : Slide 5
public class Product public Product() lastbarcodeused++; barcode=lastbarcodeused; stocklevel=100; public Product(int initstock) lastbarcodeused++; barcode=lastbarcodeused; stocklevel=initstock; Product class definition public static int count() return lastbarcodeused; public void display() System.out.println("Barcode = "+barcode); System.out.println("Stocklevel = "+stocklevel); System.out.println("========================="); OOP in Java : W. Milner 2005 : Slide 6
Rest of Product public int getstocklevel() return stocklevel; public void deliver(int howmany) if (howmany<0) System.out.println("Invalid delivery"); return; else stocklevel+=howmany; private static int lastbarcodeused=0; private int barcode; protected int stocklevel; OOP in Java : W. Milner 2005 : Slide 7
Implementation of Perishable public class Perishable extends Product public void deliver(int howmany) stocklevel=howmany; OOP in Java : W. Milner 2005 : Slide 8
Protected Problem the deliver method in Perishable references the private field stocklevel not allowed Solution use access control modifier protected Excerpt from modified Product definition.. private static int lastbarcodeused=0; private int barcode; protected int stocklevel; OOP in Java : W. Milner 2005 : Slide 9
public class First public static void main(string[] args) Product prod = new Product(); prod.deliver(100); Perishable perish1 = new Perishable(); Perishable perish2 = new Perishable(); perish1.deliver(50); perish2.deliver(60); prod.display(); perish1.display(); perish2.display(); Using the subclass All 3 use default constructor =stocklevel 100 Barcode = 1 Stocklevel = 200 ==================== Barcode = 2 Stocklevel = 50 ==================== Barcode = 3 Stocklevel = 60 ==================== OOP in Java : W. Milner 2005 : Slide 10
Constructors of sub-classes Constructors are not methods They are not inherited If you don't define one the no-arg constructor of the base class is called for you see last example OOP in Java : W. Milner 2005 : Slide 11
super() super(); can be used as the first statement in a constructor It means the corresponding superclass constructor is called Further statements can take further action For example..suppose Perishable products have an extra store location code.. OOP in Java : W. Milner 2005 : Slide 12
Using super() public Perishable(int initstock, int initlocationcode) super(initstock); locationcode = initlocationcode; public Product(int initstocklevel) barcode=lastbarcodeused++; stocklevel=initstocklevel; OOP in Java : W. Milner 2005 : Slide 13
More on super() super() cannot be anywhere except the first line of a constructor If you don t use super(), the system executes it anyway IOW a subclass constructor first executes the no-arg constructor of the super class OOP in Java : W. Milner 2005 : Slide 14
Exercise Define an Employee class, with fields payrollnumber and rateofpay Define a Manager class as a sub-class of Employee. They are paid monthly define their pay() method to display their pay Define a Clerical class as a sub-class of Employee. They are hourly paid. Add an hoursworked field, and a pay() method. OOP in Java : W. Milner 2005 : Slide 15
Object All classes descend from the class Object public class MyClass.. Is in effect: public class MyClass extends Object.. While if you say public class MyClass extends MySuperClass Then MySuperClass, or its ancestor, descends from Object Object objects have few useful methods Except tostring(), which converts the object to a descriptive string Which is what System.out.println calls For example.. OOP in Java : W. Milner 2005 : Slide 16
Object example Object someobject= new Object(); System.out.println(someObject); Output: java.lang.object@187c6c7 OOP in Java : W. Milner 2005 : Slide 17
Changing and using tostring In Perishable definition.... public String tostring() return "Perishable ID="+barcode+" Loc="+locationCode+" stock="+stocklevel;.. in use.. Perishable p1 = new Perishable(20,30); Perishable p2 = new Perishable(20,45); System.out.println(p1); System.out.println(p2); Output: Perishable ID=0 Loc=30 stock=20 Perishable ID=1 Loc=45 stock=20 calls tostring() of Perishable objects OOP in Java : W. Milner 2005 : Slide 18
final methods A method declared as final in a superclass cannot be altered in a subclass For example.. OOP in Java : W. Milner 2005 : Slide 19
Defining a method as final In Product.. public final void display() System.out.println("Barcode = "+barcode); System.out.println("Stocklevel = "+stocklevel); System.out.println("========================="); In Perishable.. public void display().... In use: Perishable p1 = new Perishable(20,30); Output on next slide OOP in Java : W. Milner 2005 : Slide 20
Trying to override final - compile time error C:\Walter\JavaProgs\Perishable.java:19: display() in Perishable cannot override display() in Product; overridden method is final public void display() ^ 1 error OOP in Java : W. Milner 2005 : Slide 21
Final classes and why A class declared as final cannot be subclassed Methods and classes are usually declared as final for security Otherwise a subclass of a standard superclass might be defined, with.. Unpleasant overridden methods But at run-time a subclass object would look like the superclass Eg the String class is final for this reason OOP in Java : W. Milner 2005 : Slide 22
Abstract classes Superclasses which are 'general' can be declared abstract Used when subclasses will all implement the same method in different ways, but The superclass is too general to actually implement the method itself For example.. OOP in Java : W. Milner 2005 : Slide 23
Example abstract class Suppose we had a superclass called Shape And subclasses called Triangle, Rectangle, Square and so on. Each would need a draw method But we could not program the draw method of a Shape instance So the draw method of Shape is declared abstract As is the class as a whole This means Shape cannot be instantiated OOP in Java : W. Milner 2005 : Slide 24
Example abstract class public abstract class Shape public Shape(int initheight, int initwidth) width=initwidth; height=initheight; public abstract void draw(); protected int width; protected int height; OOP in Java : W. Milner 2005 : Slide 25
public class Rectangle extends Shape public Rectangle(int h, int w) super(h,w); Subclass of abstract class public void draw() for (int i=0; i<height; i++) for (int j=0; j<width; j++) System.out.print("*"); System.out.println(); OOP in Java : W. Milner 2005 : Slide 26
Using the subclass Rectangle r = new Rectangle(4,5); r.draw(); OOP in Java : W. Milner 2005 : Slide 27
Copy the Shape class Exercise Define a Triangle class by subclassing Shape define the draw method How would you deal with a Square class? OOP in Java : W. Milner 2005 : Slide 28