Handout 9 OO Inheritance.

Transcription

1 Handout 9 CS603 Object-Oriented Programming Fall 2016 Page 1 of 11 Handout 9 OO Inheritance. All classes in Java form a hierarchy. The top of the hierarchy is class Object Example: classicalarchives.com website maintains a list of its customers and members, who are identified by their address. Members maintain a password. CLASS Object.... all other classes CLASS Customer String set () get () Notes: Subclass "IS a more specific Kind of " Superclass. Every object of a subclass is also an object of the superclass. The reverse is not true. Superclass (also called parent class) contains variables and methods common to all subclasses. A subclass (a.k.a. child class) automatically inherits all the data fields (variables) and (public) behaviors (methods) of the superclass (a.k.a. parent) class. Subclass can define additional methods and variables that are unique to it. This will not affect objects of the parent class, or siblings. Subclass can define its own version of an inherited method (will see lated). This is called overriding. The Object class: CLASS Member String password setpassword() superclass of all Java classes, i.e. Root of the entire Java class hierarchy. Defines the default implementation of the tostring() method to return a Java internal object ID string. 1

2 Handout 9 CS603 Object-Oriented Programming Fall 2016 Page 2 of 11 The extend keyword is used to create a subclass of a class. class subclass_name extends superclass_name {... // Customer.java - implements a customer of online service public class Customer { private String ; // login name public String get () {return this. ; public void set (String addr) {this. = addr; // Member.java - subclass of Customer public class Member extends Customer { private String password; public void setpassword(string passwd) { this.password = passwd; Member can access (i.e. use) all public and protected methods/instance vars and methods of Customer. Member inherits instance variable from the Customer class, but must use accessor/mutator to access it, since is private! /* Demonstrates inheritance of methods and instance vars, * Assignment between subclass and superclass type variables * and type casting. */ public class DemoInheritance { public static void main(string[] args) { Customer c = new Customer(); c.set ("madeye@hg.mg"); System.out.println(c.get ()); Member m = new Member(); m.set ("cbrown@bentley.edu"); //inherited from Customer m.setpassword("foo1234"); // defined in class Member System.out.println(m.get ()); //inherited from Customer // Also note, every Member is a Customer, // so the following assignment is legal: Customer c1 = m; System.out.println(c1.get ()); 2

3 Handout 9 CS603 Object-Oriented Programming Fall 2016 Page 3 of 11 Notes: // System.out.println(c1.getPassword()); does not work, but ((Member) c1).setpassword("foo");// works since c1 is // typecast as Member // but not every Customer is a Member, // so the following would not work: // Member m1 = c; // Error: cannot convert from Customer to Member, i.e. //and Member m1 = (Member) c; generates a runtime error 1. A variable (or method parameter) of superclass type can store (be passed) a value of any of its subclass types. (e.g. a var of Customer type can be assigned a Member-type object). e.g. Customer c1 = m; where since declaration is Member m; 2. The reverse of 1. is not allowed. 3. When a method is called, the compiler checks that the method is defined for the declared class of the variable storing the calling object or its ancestor, e.g. m.set ("cbrown"); is allowed since declaration is Member m; m.setpassword("foo1234"); is allowed, since declaration is Member m; c1.getpassword(); is NOT allowed since declaration is Customer c1 because getpassword() is defined only in Member 4. A variable of the superclass type can be typecast as a subclass, e.g. ((Member) c1).setpassword("foo"); If the value stored in c1 is not an object of type Member, a run-time error stating: setpassword() is not applicable to c1 will ensue. 5. To check if an object is an instance of a class use instanceof operator, e.g. o m instanceof Member -> true o m instanceof Customer -> true o m instanceof String -> false To obtain the class name of an object s class (bottom of the inheritance hierarchy) use method getclass() o c.getclass() -> Customer o m.getclass() -> Member (Practice question: which class defines method getclass()?) 3

4 Handout 9 CS603 Object-Oriented Programming Fall 2016 Page 4 of public and protected methods and variables of the superclass can be accessed directly from the subclass. private variables of superclass must be accessed using superclass public methods from the subclass Example: // won t work public Member(String ) { this. = name; // will work public Member(String ) { this.set ( ); // or just: set ( ); *** Within a subclass, use the set and get methods of the superclass for accessing the private instance variables of that superclass. 2. Overriding: Defining another version of an inherited method. The overriding method has the same signature (different from overloading different signature). Example tostring() method. It s defined in the Object class. When classes define their own tostring() method they override tostring() of Object class. Dynamic Binding: When an overridden method is called, the run-time class of the calling object (i.e. type of the actual object) determines which method is called. Example: download() method (see code on next pages): Business rules Customers are allowed only 3 downloads. Members are allowed unlimited number of downloads. Implementation: Customer class defines download() method. Member class re-defines, i.e. overrides it. 4

5 Handout 9 CS603 Object-Oriented Programming Fall 2016 Page 5 of 11 /* Customer.java - now with download. Allows only up to MAX_DOWNLOADS downloads */ public class Customer { public static final int MAX_DOWNLOADS = 3; private String ; // login name private int numdownloads = 0; // counter of number of downloads // made by this customer public String get () {return this. ; public void set (string addr){this. = addr; /** Check if reached maximum allowed number of downloads, or just increment numdownloads by 1 */ public void download(){ if (this.numdownloads < Customer.MAX_DOWNLOADS){ this.incrementnumdownloads(); System.out.println((Customer.MAX_DOWNLOADS this.numdownloads) + " downloads left."); else System.out.println("Cannot download more than "+ Customer.MAX_DOWNLOADS+" times."); /* Increment numdownloads by 1 */ public void incrementnumdownloads(){ this.numdownloads++; /* Member.java - subclass of Customer Overrides download() method allowing unlimited downloads*/ public class Member extends Customer { private String password; public void setpassword(string passwd) { this.password = passwd; public void download(){ // note this.numdownloads++ would not work, because it's private // Would have worked if it were public or protected System.out.println("Number of downloads is not limited."); this.incrementnumdownloads(); // iherited from superclass 5

6 Handout 9 CS603 Object-Oriented Programming Fall 2016 Page 6 of 11 When an overridden method is called, the run-time class of the calling object (i.e. type of the actual object) determines which method is called. /* Demonstrates inheritance of methods and instance vars, * Assignment between subclass and superclass type variables, dynamic binding. */ public class DemoOverriding { public static void main(string[] args) { Customer c = new Customer(); c.set ("madeye@hg.mg"); Member m = new Member(); m.set ("cbrown@bentley.edu"); m.setpassword("foo1234"); System.out.println(c.get ()); for (int i = 1; i<=4; i++){ c.download(); /* Prints: madeye@hg.mg 2 downloads left. 1 downloads left. 0 downloads left. Cannot download more than 3 times. */ System.out.println("\n"+m.get ()); for (int i = 1; i<=4; i++){ m.download(); /* Prints: cbrown@bentley.edu Number of downloads is not limited. Number of downloads is not limited. Number of downloads is not limited. Number of downloads is not limited. */ Customer next; String whoisnext = JOptionPane.showInputDialog ("Who downloads next: " + c.get () + "[1] or " + m.get () + "[anything]?" ); if (whoisnext.equals("1")) next = c; else next = m; // What will be printed? System.out.println("\nPicked "+next.get ()); next.download(); /* Depending on value stored in next, prints Picked madeye@hg.mg Cannot download more than 3 times. or Picked cbrown@bentley.edu Number of downloads is not limited. */ 6

7 Handout 9 CS603 Object-Oriented Programming Fall 2016 Page 7 of 11 Note: Changing the number or type of parameters is called method overloading, not overriding. Warning: if a subclass method is identical in all ways to a superclass method except that it has a different return type, then a compiler error will result. Polymorphism: (a.k.a: dynamic binding, late binding, or runtime binding) Means can take many forms Methods with the same name perform different actions on different objects Implemented by overloading and overriding in Java Polymorphic method (e.g. download() in the Customer/Member example, tostring() in other examples) Has many structures (different signatures, when overloaded) Can operate on different types of objects. Each object knows how it is to be acted on. Overriding is also referred to as: dynamic typing, dynamic method dispatch, or pure polymorphism. Dynamic ~ Run Time (vs. compile time) 3. Inheritance and Constructors If constructor not included, default one (no-args) is supplied automatically Constructors are never inherited. The superclass constructor is invoked from a subclass constructor by super() (possibly, with parameters) as a first statement. If invocation of a superclass constructor is not included in subclass constructor as he first statement, default (no-argument) constructor of superclass is called automatically before executing the first line of the subclass constructor. If superclass default constructor (i.e., no-arg) does not exist, but a constructor with args does, then error will result. // Customer.java - with constructors public class Customer { public static final int MAX_DOWNLOADS = 3; private String ; // login name private int numdownloads = 0; // counter of number of downloads // made by this customer public Customer() { this. = "unknown"; public Customer(String ) { this. = ; // the rest of Customer class follows... 7

8 Handout 9 CS603 Object-Oriented Programming Fall 2016 Page 8 of 11 // Member.java no constructor defined public class Member extends Customer { private String password; public void setpassword(string passwd) { this.password = passwd; /* Increments counter of downloads, prints a message */ public void download(){... code omitted Given the above Member m = new Member(); System.out.println(m.get ); prints: unknown To explicitly reference a superclass constructor, must be the first statement inside of the constructor for the subclass, along with any required arguments. /* Member.java - subclass of Customer, now with constructors */ public class Member extends Customer { private String password; public Member() { // implicitely calls super(); public Member(String , String passwd) { super( ); //calls 1-arg constructor of Customer class this.password = passwd; public void setpassword(string passwd) { this.password = passwd; /* Increments counter of downloads, prints a message */ public void download(){... code omitted 8

9 Handout 9 CS603 Object-Oriented Programming Fall 2016 Page 9 of 11 Another Example: overriding, using super to access methods of superclass Class Object CLASS BankAccount... balance deposit() withdraw() CLASS CheckingAccount minbalance charge processcheck() withdraw() CLASS SavingsAccount interestrate postinterest() The withdraw method can be defined for the superclass BankAccount, and for the subclass CheckingAccount public class BankAccount { private double balance; // instance variable // constructors public BankAccount(){ // no arg constructor public BankAccount(double b) {balance = b; // starting balance // mutator methods - increment balance public void setbalance(double amount) {balance = amount; public void deposit(double amount) { balance += amount; public void withdraw(double amount) { if (balance >= amount) balance -= amount; else System.out.println("Insufficient funds"); // accessor method return balance public double getbalance() {return balance; 9

10 Handout 9 CS603 Object-Oriented Programming Fall 2016 Page 10 of 11 public class CheckingAccount extends BankAccount { private double minbalance; private double charge; public CheckingAccount(double minbalance, double charge) { // will look for no-arg super constructor first // since not explicitly invoking a super constructor this.minbalance = minbalance; this.charge = charge; public void processcheck(double amount) { this.withdraw(amount); public void withdraw(double amount) { if (getbalance() >= minbalance) super.withdraw(amount); else super.withdraw(amount + charge); // this not required Instances of a subclass can always be used where an object of the superclass is expected. An instance of CheckingAccount always inherits from BankAccount a CheckingAccount is always a BankAccount Another example: (are BankAccount or CheckingAcount methods used?) BankAccount myaccount = new CheckingAccount(100, 20); myaccount.deposit(500); myaccount.withdraw(10.0); The following code is not legal: myaccount.processcheck(25.00); because type of myaccount variable is BankAccound, and processcheck is not defined in BankAccount. Compiler can't tell that myaccount is also a CheckingAccount processcheck is not defined for class BankAccount Solution: either of the three below would work Declare myaccount as a CheckingAccount from the start Define a dummy, or shell processcheck method for BankAccount to be overridden by certain subclasses. Cast myaccount to CheckingAccount 10

11 Handout 9 CS603 Object-Oriented Programming Fall 2016 Page 11 of 11 Problems: public class SavingsAccount extends BankAccount { private double interestrate; // subclass instance variable public SavingsAccount(double amount, double rate) { super(amount); // must be first! interestrate = rate; public void postinterest() { double balance = getbalance(); double interest = interestrate/100*balance; setbalance(balance + interest); public String tostring() { return "\tsavingsaccount balance = " + this.getbalance(); 1. Given the earlier definitions for BankAccount and the above definition for SavingsAccount, is this legal? BankAccount myaccount = new BankAccount(); SavingsAccount saccount = new SavingsAccount(200.0,5.0); saccount = myaccount; 2. Which of the following will result in a compiler error if account anaccount is declared as follows? Identify the class of the invoked method if no error results. (a) SavingsAccount anaccount= new SavingsAccount(100, 20); (b) BankAccount anaccount= new SavingsAccount(100, 20); anaccount.postinterest(); anaccount.withdraw(5.0); anaccount.processcheck(10.0); anaccount.setbalance(50.0); anaccount.getbalance(); 11