COSC This week. Will Learn

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1 This week COSC Read chapters 9, 11 S tart thinking about assignment 2 Week 4. J anuary 26, 2004 Will Learn how to inherit and override superclass methods how to invoke superclass constructors about protected and package access control Will Learn About abstract classes about interfaces to convert between supertype and subtype references the concept of polymorphism how interfaces can be used to decouple classes common superclass Object and to override its tos tring, equals, and clone methods Inheritanc e: c onomy and Humility (Meyer) Don t redo what has already been done. We could reach so high only because we stand on the shoulders of giants (Newton) Without inheritance every new class has to define all services it offers. S upports Open-Closed Principle: Open: still available for extension Closed: available for use by clients public class A //superclas private int x; public A() x = 1; public int getx() return x; public void setx(int x) this.x = x; xtending classes public class B extends A // subclass private int y; public B() y = 1; public int gety() return y; public void sety(int y) this.y = y; 1

2 xtending classes Get all of the public/protected fields/methods S o getx, setx, gety, sety for B What happens when an instance of B is created? Note that B has two attributes x and y. xtending classes: constructors Construction of subclass involves construction of superclass Java will call a Constructor - by default A() public class B extends A private int y; public B() y = 1; public int gety() return y; public void sety(int y) this.y = y; xtending classes: constructors If the first statement of the constructor is not super( ) or this( ), J ava inserts a super() If a class has no constructor, java creates a default one public ThisClass() super(); xtending classes: casting You can cast from one class to another Always possible going up the hierarchy Can cast anything to Object Can only cast down if the object is an instance of the class or one of its subclasses A a = new A(); B b = new B(); A temp = (A)b; Object temp = (Object) a; B test = (B) a; // will fail xtending classes: casting instanceof operator objectinstance instance of Class is a boolean Returns true iff the cast would succeed. xtending classes: fields J ava supports early binding for fields Which field is to be used is defined at compile time Inherit field: All fields from the superclass are automatically inherited Add field: S upply a new field that doesn't exist in the superclass Can't override fields 2

3 xtending classes: class methods J ava supports early binding for class methods (static methods) Which class method to be used is defined at compile time. Inheritanc e and Methods Override method: S upply a different implementation of a method that exists in the superclass Inherit method: Don't supply a new implementation of a method that exists in the superclass Add method: S upply a new method that doesn't exist in the superclass xtending classes: methods J ava supports late binding for instance methods Which method to be used is defined at run time // from java in a nutshell class A // Define a class named A int i = 1; // An instance field int f() return i; // An instance method static char g() return 'A'; // A class method class B extends A // Define a subclass of A int i = 2; // Shadows field i in class A int f() return -i; // Overrides instance method f in class A static char g() return 'B'; // Shadows class method g() in class A public class OverrideTest public static void main(string args[]) B b = new B(); // Creates a new object of type B System.out.println(b.i); // Refers to B.i; prints 2 System.out.println(b.f()); // Refers to B.f(); prints -2 System.out.println(b.g()); // Refers to B.g(); prints B System.out.println(B.g()); // This is a better way to invoke B.g() A a = (A) b; // Casts b to an instance of class A System.out.println(a.i); // Now refers to A.i; prints 1 System.out.println(a.f()); // Still refers to B.f(); prints -2 System.out.println(a.g()); // Refers to A.g(); prints A System.out.println(A.g()); // This is a better way to invoke A.g() BankAc c ount xample SavingsAccount New method addinterest transfer deposit addinterest deposit balance withdraw transfer Balance interestrate getbalance getbalance withdraw 3

4 Bank Account xample SavingAccount objects behave like BankAccount objects in many ways Savings account = bank account with interest All methods of are automatically inherited May call on object An Inheritanc e Diagram BankAc c ount xample public class BankAccount public BankAccount() //constructor public BankAccount(double initialbalance) //constructor public void deposit(double amount) //update balance public void withdraw(double amount) //update balance public double getbalance() //accessor public void transfer(bankaccount other, double amount) private double balance; //data Class BankAccount public class BankAccount public BankAccount() balance = 0; public BankAccount(double initialbalance) balance = initialbalance; public void deposit(double amount) balance = balance + amount; public void withdraw(double amount) balance = balance - amount; public double getbalance() return balance; public void transfer(bankaccount other, double amount) withdraw(amount); other.deposit(amount); private double balance; Adding a Subclass Method Layout of a Subclass Object!" $ " %!&'((! 4

5 CheckingAccount deductfees is a new method; deposit and withdraw are redefined (overriden) deductfees deposit transfer balance transactioncount withdraw getbalance CheckingAccount Class First three transactions are free Charge $2 for every additional transaction Must override to increment transaction count ) method deducts accumulated fees, resets transaction count Bank Account Hierarchy /** CheckingAccount A checking account that charges transaction fees. */ public class CheckingAccount extends BankAccount public CheckingAccount(int initialbalance) //body delted public void deposit(double amount) //body deleted public void withdraw(double amount) //body deleted public void deductfees() //body deleted private int transactioncount; private static final int FR_TRANSACTIONS = 3; private static final double TRANSACTION_F = 2.0; Inherited Fields are Private Consider method of * * ++ &&,,, Inherited Fields are Private Consider method of * * ++, ", + Can't just add to is a private field of the superclass Will not WORK 5

6 Inherited Fields are Private Consider method of * * ++ Can't just call in method of * That is the same as, Calls the same method (infinite recursion) Invoking a Superclass Method Instead, invoke superclass me thod, Now calls method of class Complete method: * ++, CheckingAccount /** A checking account that charges transaction fees. */ public class CheckingAccount extends BankAccount public CheckingAccount(int initialbalance) //construct superclass super(initialbalance); //initialize transaction count transactioncount = 0; public void deposit(double amount) transactioncount++; //now add amount to balance super.deposit(amount); public void withdraw(double amount) transactioncount++; //now subtract amount from balance super.withdraw(amount); /** Deducts the accumulated fees and resets the transaction count. */ public void deductfees() if (transactioncount > FR_TRANSACTIONS ) double fees = TRANS ACTION_F * (transactioncount - FR_TRANS ACTIONS ); super.withdraw(fees); transactioncount = 0; private int transactioncount; private static final int FR_TRANSACTIONS = 3; private static final double TRANSACTION_F = 2.0; Converting from Subclasses to Superclasses Ok to convert subclass reference to superclass reference ) " '( ") -. -.") S uperclass references don't know:,$ &&/!!-! Why would anyone want to know le ss about an object? 6

7 Polymorphis m Generic method: Works with any kind of bank account (plain, checking, savings) 0, S ubclass object reference converted to superclass reference, '(((1* 2 Access Control Level (accessible by subclasses and package) package access (the default, no modifier) Recommended Access Levels Fields: Always constants Methods: or Classes: or package Beware of accidental package access (forgetting or ) Inheritanc e Hierarc hies Hierarchies of classes, subclasses, and sub-subclasses are common xample: S wing hierarchy S uperclass 4* has methods 56 class has methods to set/get button text and icon A Part of the Hierarchy of Swing UI Components Object: The Cosmic Superclass All classes extend -. Most useful methods:

8 The Object Class is the Superclass of very J ava Class Overriding the Method Returns a string representation of the object Useful for debugging xample:!, returns something like.,,! 8"91"'(":(";(< tostring used by concatenation operator means , prints class name and object address =:>?( Override 8 "@+ +@<@,,, Overriding the equals Method 3 7 tests for equal contents == tests for equal location Must cast the -.parameter to subclass 3 * * "* -.,7, AA "", Two References to qual Objec ts Two References to Same Objec t Overriding the Method Copying object reference gives two references to same object :" '; S ometimes, need to make a copy of the object Use clone: :" ', (); Must cast return value because return type is -. Define method to make new object: -. ", " 8

9 Abstract Class A class not fully specified contains abstract methods: empty body may contain concrete definitions of methods may contain instance variables Cannot be instantiated Can be extended A subclass may be abstract or concrete A concrete subclass must provide implementation for all abstract methods xample: java.lang.number Number class in java.lang package is an abstract class Integer and Double in java.lang extend Number Methods intvalue and doublevalue are abstract methods in Number, but implemented in concrete subclasses Abstract classes public abstract class Box public final int CAPACITY = 100; private int count; public BOX() count = 0; public int getcount() return count; public boolean ismpty() return count == 0; public boolean isfull () return count == CAPACITY; public abstract boolean put(object o); public abstract boolean remove(object o); public abstract boolean has(object o); Abstract classes Forces implementor of an extension to Box to define abstract methods public abstract boolean put(object o); public abstract boolean remove(object o); public abstract boolean has(object o); Implementor may choose Arrays for implementation or other data types public class ArrayBox extends Box public class ArrayBox extends Box public ArrayBox() items = new Objects[CAPACITY]; public boolean put(object o) if (isfull()) return false; else items[count++] = o; return true; private Object[] items; Abstract Classes Great for specifying what needs to be done rather than how it should be done They capture design decisions They facilitate code re use by capturing commonality They defer implementation to later phases 9

10 Interfac e: Motivation S uppose we would like to develop a class called DataS et for a collection of bank accounts or coins keeping track of: the bank account/coin with maximim balance/value in the collection the total sum of account-balances/coin values the count of bank accounts/coins seen Firs t Solution Develop a DataS et for bank accounts Develop a DataS et for coins B for Bank Accounts B B,,, "+, ""(CC, D, " ++ B for Coins B B,,, * "+,F F ""( CC,F F D,F F " ++ * * This solution is bad the code for bank account and coin are the same except each uses a different object repetition leads to code bloat and hence code complexity if they share an inte rface, then DataS et need not know which object it is using If it was not for a common interface, then you would need a new license each time you bought a different car :-) Interfac e S uppose various classes could agree on the same method name, Then B could call that method: "+, ""( CC, D, " Define an interface: 10

11 Generic B B for Meas urable Objec ts B,,, "+, ""( CC, D, " ++ Realizing an Interfac e Class names interface(s) in clause Class supplies definitions of interface methods class ClassName implements Measurable public double getmeasure() imple me ntation a dditional me thods and fie lds The class must define the methods as Making and * Classes Measurable * File DataSetTes t.java 1 /** 2 This program tests the DataSet class. 3 */ 4 public class DataSetTest 5 6 public static void main(string[] args) DataSet bankdata = new DataSet(); bankdata.add(new BankAccount(0)); 12 bankdata.add(new BankAccount(10000)); 13 bankdata.add(new BankAccount(2000)); S ystem.out.println("average balance = " 16 + bankdata.getaverage()); 17 Measurable max = bankdata.getmaximum(); 18 System.out.println("Highest balance = " 19 + max.getmeasure()); DataSet coindata = new DataSet(); coindata.add(new Coin(0.25, "quarter")); 24 coindata.add(new Coin(0.1, "dime")); 25 coindata.add(new Coin(0.05, "nickel")); S ystem.out.println("average coin value = " 28 + coindata.getaverage()); 29 max = coindata.getmaximum(); 30 S ystem.out.println("highest coin value = " 31 + max.getmeasure()); UML Diagram of DataSet and Related Classes Note that B is de couple d from * 11

12 Interfac e Objects need to know how to interact with one another Interfaces specify what messages are acceptable to be sent to an object A message is sent to an object by a method call Interfaces enforce API Interfac e Can think of it as an abstract class that defines no methods and has no instance varaibles public interface Account public void deposit(float amount); public float balance(); public void withdraw(float amount); Interfac e Can define constants (static final) Classes do not extend interfaces, but implement them public class SavingsAccount implements Account public void deposit(float amount).. Interfac e Can extend a class and implement an interface public class SeniorSavingsAccount extends SavingsAccount implements Account public void deposit(float amount).. Interfac e Interfaces can extend other interfaces public interface VisaAccount extends Account public float fee(); Classes can implement multiple interfaces public class SeniorSavingsAccount extends SavingsAccount implements Account, SomeOtherInterface public void deposit(float amount).. Interfac e A class that implements an interface, must implement all its methods This ensures the class have methods with the given signatures Interfaces like abstract classes are useful for specifying what is expected rather than how it should be carried out. 12

13 Converting Between Types Can convert from class type to realized interface type: " '(((( " &&-G S ame interface type variable can hold reference to Coin " * (,'@@ &&-G Cannot convert between unrelated types "! 9'(:(;( &&/!!-! Casts Add coin objects to B B B" B B, * (,:9@7@ B, * (,'@@,,, Get largest coin with method: " B, What can you do with it? It's not of type * ",H &&/!!-! You know it's a coin, but the compiler doesn't. Apply a cast: * * "* "*,H If you are wrong and is n't a coin, the compiler throws an exception Interfaces vs. Classes All methods in an interface are abstract--no implementation All methods in an interface are automatically public An interface doesn't have instance fields This week Read chapters 9 & 11 S tart thinking about assignment 2 13