Binghamton University. CS-140 Fall Chapter 9. Inheritance

Transcription

1 Chapter 9 Inheritance 1

2 Shapes Created class Point for (x,y) points Created classes: Rectangle, Circle, RightTriangle All have llc field All have dimensions, but different for each shape All have identical move method All have different grow methods Created Shapes interface Identified methods with the same signature Same Parameters, same return type, same name 2

3 Re-Use Package everything that is common to all shapes in Shapes class! What is common to all shapes? The llc field The part of the constructor that initializes the llc field The move method What is different from shape to shape? The dimension fields The grow method Various special shape-specific methods (Most of) the tostring methods 3

4 Shape class common to all shapes public class Shape { private Point llc; public Shape(Point llc) { this.llc = llc; } public void move(double xd, double yd) { llc=llc.move(xd,yd); } public Point getllc() { return llc; } public String tostring() { return " + llc ; } } 4

5 Extending the concept of Shape Classes: Rectangle, Circle, and RightTriangle extend a Shape Extend means do everything a Shape does and do more All inherit the fields defined by Shape (llc) All inherit the methods defined by Shape (move) May have their own fields (extend the field list) May have their own methods (extend the method list) May re-define (override) Shape methods (tostring) 5

6 Example Child Class public class RightTriangle extends Shape { double base; double height; public RightTriangle(Point llc, double base, double height) { super(llc); // Contains this.llc=llc this.base = base; this.height = height; } 6

7 Inheritance The subclass inherits all the fields from its parent If a field is private in the parent, the child cannot access it The subclass inherits all the methods from its parent class If a method is private in the parent, the child cannot use it Multiple terminology: class B extends A B is the subclass, A is the superclass (hence the super keyword) B is the child class, A is the parent class B is the derived class, A is the base class 7

8 Diagramming Inheritance (UML) Shape Rectangle Circle RightTriangle Slides 6 8

9 Unified Modeling Language(UML) Shape Shape llc move(double xd, double yd) getllc() tostring() Rectangle Recgangle width height grow(double factor) getul() getlr() getur() tostring() 9

10 Comparing Shape w/ Rectangle llc Shape Rectangle llc width height Rectangle extends Shape 10

11 Classifying Objects Labrador Dogs Greyhound Animals Tabby Cats Manx 11

12 Classification Paradox Larger sets are described by smaller amounts of information If I don t know the species of an animal, I still know it s an animal If I know the species, and the species is cat, I know the animal is in the subset CAT If I know both the species and the breed, then I know this animal is a cat, and that it is a Manx Cat Animal large amount of information, small population 12

13 Duality of Parent/Child Objects Can be thought of as a Shape object With field llc With methods move, getllc, and tostring Can be though of as a Rectangle object With fields llc, width and height Shape With methods move, grow, getllc, getul, getlr, getur, and tostring llc width height Rectangle WARNING: Don t make a child class field with the same name as a parent class field! 13

14 The super keyword In a child class, the keyword super refers to the parent class For instance, if Rectangle is a subclass of Shape, in Rectangle: super() refers to the Shape constructor, tostring() refers to the Rectangle tostring method super.tostring() refers to the Shapes tostring method 14

15 Parent/Child Object Construction 1. Space is allocated for the child object 2. All fields are initialized with 0 or null 3. Java invokes the no-argument parent constructor if 1. the parent has a constructor with no parameters, AND 2. the child constructor does not invoke a parent constructor. 4. The child constructor is invoked The child constructor may explicitly invoke the parent constructor with or without parameters! If so, super( ) must come first! Note: A parent constructor must be invoked. 15

16 How do we treat methods? Rectangle.move inherits Shape.move Rectangle.getllc inherits Shape.getllc Rectangle.grow uses Rectangle.grow Same for all other unique Rectangle methods Rectangle.toString uses Rectangle.toString Overrides Shape.toString Rectangle must use super.tostring to access Shapes.toString 16

17 Inheritance and Public/Private The child class has access to all PUBLIC fields and methods in the superclass The child class DOES NOT have access to PRIVATE fields and methods in the superclass. 17

18 Example of parent field privacy public llc Point getul() { Point ul=new Point(llc); ul=ul.move(0,height); return ul; } private llc Point getul() { Point ul=new Point(getllc()); ul=ul.move(0,height); return ul; } 18

19 Inherited implicit this When an inherited method gets an implicit this argument, this is a reference to the child class not the parent class! For instance, in Shape.toString(): return this.getclass().getsimplename() + " + llc ; When Shape.toString is invoked from Rectangle.toString as super.tostring, this refers to a Rectangle object in the Rectangle class this refers to a Rectangle object in the Shapes class as well! r1 is (-7.0,2.0) -> (-2.0,7.0) 19

20 Static Methods and Inheritance Static (class) methods are NOT inherited/overridden You can always access a static method by specifying ClassName.methodName( ) No need for inheritance you can always get at class methods without inheritance 20

21 Memory Image of a Rectangle r1 llc Rectangle width height 5 5 x y Point

22 The cosmic Object Class If a class does not extend a super-class, it implicitly extends the Object class Every class descends from Object All classes inherit some important methods from Object clone() equals() hashcode() tostring() getclass() 22

23 The default tostring in Object public String tostring() { return getclass().getname() + } "@" + Integer.toHexString(hashCode( )); Typical override of tostring: public class BankAccount { // has a field called balance } } public String tostring { return BankAccount[balance= +balance+ ] ; 23

24 When are objects equal By default, two objects are equal ONLY if they are the same object. Object A = new Object(); Object B = A; if (A.equals(B)) { System.out.println( A and B are the same ); B = new Object(); if (!A.equals(B)) { System.out.println( A and B are different ); 24

25 Should equals work differently? Rectangle A = new Rectangle(p1,10,20); Rectangle B = new Rectangle(p1,10,20); if (A.equals(B)) { System.out.println( Same Rectangle ); } else { System.out.println( Different Rectangles! ); } 25

26 hashcode Special Topic 15.1 More later, but for now, think of the hashcode function as something which returns a semi-unique integer for each object Hash codes are used for quick look-up of an object For now, we only need worry about the rule: If two objects are equal according to the equals(object) method, then calling the hashcode method on each of the two objects must produce the same integer result. 26

27 hash vs. equals This is OK Java likes this This is NOT OK java has a problem a.hash()==b.hash() a.hash()==b.hash() a.equals(b) a.equals(b) 27

28 <Class>.Class object When Java loads a class, it creates a <Class>.Class object The.Class object contains a pointer to the constructor code The.Class object contains static (class) variables The.Class object contains pointers to static (class) methods second Rectangle llc width height ChoiceQuestion.class Constructor 28

29 Class Object You can find out all kinds of things with a class object For example: <object>.getclass().getname() returns the name of the class <object>.getclass().getpackage() returns the package for this class See Class documentation in Javadoc for details 29

30 The instanceof operator Java has an instanceof operator Left argument is a reference to an object Right argument is a type typically a class name, but also Interface names Return value is true if object is an instance of the type specified Rectangle r1 = new Rectangle(p2,4,4); if (r1 instanceof Rectangle) { if (r1 instanceof Shape) { if (r1 instanceof Shapes) { 30

31 Polymorphism Fancy name for a single method name used in different contexts One invocation may invoke different methods sref may be Rectangle or Circle invokes either Rectangle.toString Rectangle r1 = new Rectangle(p2,4,4); or Circle.toString System.out.println(shapeInfo("r1 is",r1)); Circle c1=new Circle(p1,4.5); System.out.println(shapeInfo("c1 is",c1)); public static String shapeinfo(string string,shape sref) { return string+" "+sref.tostring(); } Example 31

32 Virtual Method Table (VMT) Internal table computed by Java for each class Columns: 1) List of method names, 2) Pointer to method code Shape VMT Rectangle VMT Method Code Method Code move Shape.java:9 move Shape.java:9 getllc Shape.java:10 getllc Shape.java:10 tostring Shape.java:11 grow Rectangle.java:12 tostring Rectangle.java:16 32

33 Forcing sub-class to override methods Banking Example : Super class Account Sub-Classes: Savings, Checking, CreditCard, DebitCard, etc Each sub-class has a different fee structure What the bank charges the customer to maintain the account How does the Account class force the sub-classes to charge fees? All accounts should have a deductfees method The generic Account cannot define how to deduct fees 33

34 Method 1: Throw an Exception In Account: public void deductfees() { throw new UnsupportedOperationException( "This class does not implement deductfees"); } Sub classes must implement deductfees override method 34

35 Method 2: Abstract Methods/Classes public abstract class Account { } public abstract void deductfees(); abstract means I know what to do, but not how to do it concrete means I know how to do it as well. If child does not override deductfees, child is abstract as well Cannot make an object for abstract classes! 35

36 Need a concrete sub-class public class CheckingAccount extends Account { public void deductfees() { } } 36

37 Abstract classes and Interfaces Abstract class not required to implement all interface methods public abstract class Account implements Comparable<Account> { } // compareto not implemented! but abstract! public class CheckingAccount extends Account { public void deductfees() {. } public int compareto(account other) { } } 37

38 The final keyword If a method is declared as final, sub-class CANNOT override! public final boolean checkpassword(string pwd) { } // no-one can change this in a subclass If a class is declared as final, no sub-classes! public final class String { } // cannot make a subclass 38

39 Field Access Private only this class can use this field Protected only sub-classes or the same package can use this field There are problems with protected field see the text for details Public All java code can use this field Special Topic

40 Why Inherit? The concept of a standard library goes back to the 1970 s (C) We showed how you can sort arrays or collections as long as the elements are comparable With inheritance, we can provide framework classes in a standard library The framework classes do MOST of what we want them to do We can extend the framework to finish the job All we have to do is implement interfaces and/or extend library classes Browse through Javadoc packages to get a feel for the breadth 40

41 Example of using Inheritance After Steve Jobs left Apple (for a while), he created the NeXT computer The NeXT operating system and developer libraries use a lot of Objective C (this code was brought back to the Apple Macs and the iphone) The developer toolkits include a full scale text editor. Tim Berners- Lee was able to use that framework, combined with a few of his own subclasses, to create the first web browser 41

42 Quote: I wrote the program using a NeXT computer. I could do in a couple of months what would take more like a year on other platforms, because on the NeXT, a lot of it was done for me already. There was an application builder to make all the menus as quickly as you could dream them up. there were all the software parts to make a wysiwyg (what you see is what you get - in other words direct manipulation of text on screen as on the printed - or browsed page) word processor. I just had to add hypertext, (by subclassing the Text object) 42

43 The power of Object Orientation What he could do alone in a few months by leveraging the Next development kits took the Mozilla developers and Microsoft teams over a year to reproduce from scratch in C and C++ The difference was not the language used but the very well designed OO toolkits that had been built for the Next computers (in Objective C) We can still use some of these libraries for iphone and ipad programming 43