1 lecture 3 “just java” chapter 6 - inheritance, polymorphism and the class whose name is class...

Cay Horstmann's Bank Account Example

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Lecture 3

“Just Java” Chapter 6 - Inheritance,Polymorphism and the class whose name is Class“Just Java” Chapter 7 Exceptions

BankAccount example is from Cay Horstmann’s “Java 2 Essentials” Chapter 9


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Important Programming Concepts

• Inheritance and code reuse

• Supertype• Base Class• Subtype• Derived Class• The Extends keyword• The “is a” relationship

• Inheritance Diagram• The Object class• Converting Between

Class Types• Polymorphism• RTTI• Exceptions


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// File Name BankAccount.java

public class BankAccount{ private double balance;

public BankAccount() { balance = 0; }

public BankAccount(double initialBalance) { balance = initialBalance; }


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public void deposit(double amount) { balance = balance + amount; }

public void withdraw(double amount) { balance = balance - amount; }

public double getBalance() { return balance; }

}


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class BankAccount {

static private int accountNumber = 0; private double balance; private int acctNum; public BankAccount() { balance = 0; acctNum = accountNumber++; } public BankAccount(double initialBalance) { balance = initialBalance; acctNum = accountNumber++; }

// again but let’s add more members


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public void deposit(double amount) { balance = balance + amount;}public void withdraw(double amount) { balance = balance - amount;}public int getAccountNumber() { return acctNum;}public double getBalance() { return balance;}

}


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A Savings Account “IS A” Bank Account

public class SavingsAccount extends BankAccount { // new methods // new instance variables // we already have deposit, withdraw, etc.

}


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Inheritance Diagram

Object

Bank Account

Savings Account

Unlike C++,Java is singly rooted


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Specializing the base class

class SavingsAccount extends BankAccount {

private double rate;

public SavingsAccount(double initialDeposit, double interestRate) { super(initialDeposit); rate = interestRate; }

void addInterest() { double interest = getBalance() * rate; deposit(interest); }


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public static void main(String args[]) { SavingsAccount rainyDay = new SavingsAccount(100,.10); rainyDay.addInterest(); rainyDay.addInterest(); System.out.println(rainyDay.getAccountNumber()); System.out.println(rainyDay.getBalance());

SavingsAccount collegeFund = new SavingsAccount(1000,.10); collegeFund.addInterest(); collegeFund.addInterest(); System.out.println(collegeFund.getAccountNumber()); System.out.println(collegeFund.getBalance()); }}


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Output

C:\heinz\90-876\examples\Inherit\BankAccount>java SavingsAccount0121.011210.0


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Converting Between Class Types

A SavingsAccount “is a” BankAccount.

A BankAccount “is a” Object.

Is the following legal?

Object o = new SavingsAccount(100,.10);


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Object o = new SavingsAccount(100,.10);

An ObjectAssignment is fine!


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SavingsAccount s = new SavingsAccount(100,.10); Object o = s;

Sure!

Both references point to the same object.


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SavingsAccount s = new SavingsAccount(100,.10); BankAccount b = s;

Sure!


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SavingsAccount s = new SavingsAccount(100,.10); BankAccount b = s; b.addInterest()


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SavingsAccount s = new SavingsAccount(100,.10); BankAccount b = s; b.addInterest()

NO!

A BankAccount object has no addInterest method!


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SavingsAccount s = new SavingsAccount(100,.10); BankAccount b = s; ((SavingsAccount)b).addInterest();

Yes!

We tell the compiler we will take the risk!


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How about the following?

SavingsAccount s = new SavingsAccount(100,.10); Object o = s; ((SavingsAccount)o).addInterest();


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SavingsAccount s = new SavingsAccount(100,.10); Object o = s; ((SavingsAccount)o).addInterest();

Sure!

Why?

Are we taking a risk?


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SavingsAccount s = new SavingsAccount(100,.10); Rectangle r = s; ((SavingsAccount)r).addInterest();


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SavingsAccount s = new SavingsAccount(100,.10); Rectangle r = s; ((SavingsAccount)r).addInterest();

No!

Why?


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Inheritance Diagram

Object

Bank Account

Savings Account

Rectangle


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PolymorphismConsider the following static method:

public void static display(BankAccount b) { System.out.println(“Acct. Number:” + b.getAccountNumber()); System.out.println(“Balance $” + b.getBalance());}


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PolymorphismConsider the following static method:


// Suppose we call it with the following code. What happens?public static void main(String args[]) {

BankAccount collegeFund = new BankAccount(100); display(collegeFund);}


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PolymorphismHow about with this code?


public static void main(String args[]) {

SavingsAccount rainyDay = new SavingsAccount(100,.10); display(rainyDay);}


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Inheritance Diagram (UML)

Object

BankAccount

SavingsAccount

Rectangle

CDAccount


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PolymorphismIs this OK?



CDAccount retirement= new CDAccount(100,.10,5); display(retirement);}


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Java Access Control

public Interface Access

private

protected

Only accessible within the class

“Sort of private”


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Class member accessibility

Yes

No

No

No

Yes Yes Yes

Yes Yes Yes

Yes Yes No

Yes No No

Same class

Class in same package

Subclass indifferent package

Non-subclass different package

Private

Member Visibility

Public Protected Package

Accessible to

Java Access Control

Classes are either public or package access


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Interfaces and Abstract classes


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Review Inheritance

• The inheritance relationship is often called the “is-a” relationship.• For example, a CheckingAccount “is-a” BankAccount.• Now, suppose that we have a routine that manipulates BankAccount objects --

static void foo(BankAccount x) { // do things to x }

• What kind of things can foo() do to x?


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Inheritance

static void foo(BankAccount x) { x.withdraw(1000); x.deposit(50.0); : : } foo() can call those

methods on x that areprovided by the BankAccountclass


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Inheritance

// A CheckingAccount “is-a” BankAccount.

class CheckingAccount extends BankAccount {

}

static void foo(BankAccount x) { // do things to x }

Should we be able to pass a CheckingAccount object to this routine?


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Inheritance

// A CheckingAccount “is-a” BankAccount.

class CheckingAccount extends BankAccount {

}

static void foo(BankAccount x) { // do things to x } Should we be able to pass a CheckingAccount object to this routine?

SURE!!


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Inheritance

Often we want to write a method that is able to handle any objectthat meets certain requirements.

In the example above, foo() requires that it receive BankAccountobjects. The objects may be CD Account objects or Checking Account objects etc.. As long as the object that is passed to foo() extends the BankAccount class, the writer of foo() knows that the object has methods called “deposit” and “withdraw”.

Since the object “is a” BankAccount, we are promised that certainoperations will be available. Interfaces take this a step further…


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INTERFACES

• Interfaces

• Replace multiple inheritance• Have no instance variables• Have only abstract methods (all parameters but no bodies)• Have only public methods• Are implemented not extended as in inheritance• Are not classes…you can’t create interface objects• May be referenced• May contain constants (all are public static final by default)


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The BankAccount Again

interface Account { public double getBalance(); public void setBalance(double x);}

Any class that implements thisinterface MUST have thesetwo methods defined exactly as specified.


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class BankAccount implements Account { private double balance; private double rate;

public BankAccount() { balance = 0; } public BankAccount(double initialBalance, double arate) { rate = arate / 100;; balance = initialBalance; }


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public double getBalance() { return balance; }


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public void setBalance(double a) { balance = a; } public void update() { balance = balance + balance * rate; }}

We have providedimplementations forthe two methods.


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public class BankAccountTest{ public static void main(String[] args) { BankAccount myAccount = new BankAccount(1000,10); int month; for (month = 1; month <= 2; month++) { myAccount.update(); } myAccount.deposit(100); foo(myAccount); }

Call foo() with an object that implementsinterface Account


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public static void foo(Account a) {

double m = a.getBalance(); System.out.println(m); } }

The name ofan interface

Any other class that implements Account canbe passed to foo().


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Consider A Student Class

public class Student {

}

Student x = new Student(“Joe”,2.3);

Student y = new Student(“Zack”,1.7);

Student z = new Student(“Amy”,3.0);

How would you put these three in order?


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Consider A Student Class

public class Student {

}

Student x = new Student(“Joe”,2.3);

Student y = new Student(“Zack”,3.7);

Student z = new Student(“Amy”,3.0);

It depends on how they are compared.


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INTERFACES

public interface Comparable { int compareTo(Object other); }

public class Student implements Comparable { // this class MUST define compareTo() }

Automatically public

Found in java.lang


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INTERFACES

Suppose we have a function foo

void foo(Comparable x[]) {

}

Can we pass an array of Student objects to this function? Why would we want to?


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INTERFACES

Suppose we have a function foo

void foo(Comparable x[]) {

}

Can we pass an array of Student objects to this function? SURE Why would we want to? Perhaps foo() sorts.


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Abstract Classes

• When you extend an existing class, you have a choice whether or not to redefine the methods of the superclass. If you don’t redefine the method, it will appear in the derived class as it appears in the superclass.

• Sometimes it is desirable to force derived class programmers to redefine a method

• There may be no good (superclass) default for the method

• Only the subclass programmer can know how to implement the method


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Abstract Classes

• Consider

public class BankAccount { public void deductFees() { …body goes here ? … } : : }

public class SavingsAccount extends BankAccount{

}

We silently get the deductFees() method

What should this method do?


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Abstract Classes

• Writing an Abstract class

public abstract class BankAccount { public abstract void deductFees(); public double getBalance() { return balance; } : }

public class SavingsAccount extends BankAccount{ public void deductFees() { // deduct fees according to // the account type } }

No body..It’s up to thederived class to complete.


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Abstract Classes

void foo(BankAccount s) {

make calls on s.deductFees();}

Abstract

A real objectat run time.

Many classes can extend the abstract class,get some code reuse, but are forced to definethe abstract method deductFees() .


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Abstract Classes

• An abstract method has no implementation•You can’t construct objects of classes with abstract methods• You can only create objects of concrete classes• The class must be declared with the keyword abstract• It’s fine to have handles that reference abstract classes• Abstract classes, unlike interfaces, may have concrete methods and instance variables


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RTTI Run-Time Type Identification

If we have a reference to an object’s base type we can find out the exact type of the object.

public static void foo(Object o) {

if(o instanceof Fruit) System.out.println("Found a Fruit"); if(o instanceof BigInteger) System.out.println("Found a Big Int"); }


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Some type checking is automatic

Object o = new BigInteger("1234");BigInteger x = (BigInteger)o;String st = (String)o;

All casts are checked at run time. A ClassCastExceptionis thrown when we try to convert the BigInteger pointed to by o to a String.


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But we have access to The Class Object

• There is a Class object for each class in your program.

• The Class object for a particular class holds information about that class.

• It will be loaded at run time if not already in the JVM.

• A Java program is not completely loaded before it starts.

• The Class object is of class Class.


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instanceof and isInstanceOf

// Given an object, check its type:

public class ShortTest {


Object o = new Fruit(); if(o instanceof Fruit) System.out.println("it's a Fruit"); } }


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instanceof and isInstanceOf // Given a type, check an object

public class ShortTest {


Object o = new Fruit(); Class c = o.getClass();

if(c.isInstance(o)) System.out.println("it's a Fruit"); } }


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The Class Object

interface BabySitter {

void respondFast();}class Dad implements BabySitter {

public Dad() {} public void respondFast() {}}


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public class RTTITest {

public static void displayInfo(Class c) { System.out.println(c.getName()); System.out.println(c.isInterface()); System.out.println(c.isArray()); }


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public static void main(String args[]) throws Exception { Class a = Class.forName("Dad"); // name could be entered at run // time. Dad is on the classpath. displayInfo(a); Class b = Class.forName("BabySitter"); displayInfo(b);

Class[] faces = a.getInterfaces(); for(int i = 0; i < faces.length; i++) displayInfo(faces[i]); Class sup = a.getSuperclass(); displayInfo(sup); }}


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C:\McCarthy\www\JustJava\Examples>java RTTITestDadfalsefalseBabySittertruefalseBabySittertruefalsejava.lang.Objectfalsefalse


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Creating an Object from a Name

public class Fruit { private int grams; private int calsPerGram; public Fruit() { this(0,0); } public Fruit(int g, int c) { grams = g; calsPerGram = c; } public String toString() { return "Grams:" + grams + " Calories per gram:" + calsPerGram; }}


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public class TestFruit {

public static void main(String a[]) throws Exception {

String any = "Fruit"; // The compiler has no idea what class // we will be working with Object m = Class.forName(any).newInstance(); System.out.println(m); } }

C:\McCarthy\www\JustJava\Examples>java TestFruitGrams:0 Calories per gram:0


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It gets more interesting

// Sample code modified from "Java in a Nutshell"

import java.lang.reflect.*;public class TestFruit {

public static void main(String a[]) throws Exception {

String any = "Fruit"; // any class in the classpath // build an object of that class Object o = Class.forName(any).newInstance();

// get the Class object Class c = o.getClass();


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// if it's an array figure out its base type while(c.isArray()) c = c.getComponentType();

// if c is not primitive then print its class hierarchy if(!c.isPrimitive()) { for(Class s = c; s != null; s = s.getSuperclass()) System.out.println(s.getName() + " extends"); }

// get a toString() method from the class passing an empty // array of arg types Method m = c.getMethod("toString", new Class[] {} ); // Call the method pointed to by m // include the calling instance and, in this case, an empty array // of parameters String s = (String) m.invoke(o, new Object[] {}); System.out.println(s); } }


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C:\McCarthy\www\JustJava\Examples>java TestFruitFruit extendsjava.lang.Object extendsGrams:0 Calories per gram:0


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TestException.javaclass BankBalanceException extends Exception {

BankBalanceException(String errorMessage) { super(errorMessage); }}


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class BankAccount{ private double balance;


public BankAccount(double initialBalance) throws BankBalanceException { if (initialBalance < 0) throw new BankBalanceException("balance less than 0"); balance = initialBalance; }


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public double getBalance() { return balance; }}


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public class TestException {

public static void main(String a[]) {

BankAccount billG = new BankAccount(100.0); BankAccount mikeM = new BankAccount(-230.0); }}

C:\McCarthy\www\95-713\examples>javac TestException.java

TestException.java:38: Exception BankBalanceException mustbe caught, or it must be declared in the throws clause of this method. BankAccount billG = new BankAccount(100.0); ^1 error


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public static void main(String a[]) { try { BankAccount billG = new BankAccount(100.0); BankAccount mikeM = new BankAccount(-230.0); } catch(BankBalanceException e) { System.out.println("Balance initialized too low"); System.out.println(e); } finally { System.out.println("Exiting"); } System.out.println("End of program"); }}

Handle the exception


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C:\McCarthy\www\95-713\examples>java TestExceptionBalance initialized too lowBankBalanceException: balance less than 0ExitingEnd of program


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Exceptions in Java

Object

Throwable

Error ExceptionUnrecoverable problems- e.g. Corrupted class file, out of memory-Can be handled but rare-- These are all unchecked

Less severe conditions

RunTimeExceptionCan occur anywhereUncheckede.g. Null Pointer Exception,ArrayIndexOutOfBounds

Checkede.g. ClassNotFoundExceptionCloneNotSupportedException

Java in a Nutshell 75

ExceptionsChecked Exceptions must be explicitly thrown or handled. The compilerwill enforce this rule.

Unchecked exceptions, for example ArrayIndexOutOfBounds, neednot be caught or explicitly throw but may be.

When a throw is executed:

jump to the handler in current block if no handler exists then check next higher enclosing block and so on… if no handler exists in the method then jump to the calling statement and look for a handler in that code if this goes back to main and there is no handler then stop the interpreter and print the error with a stack trace

Java in a Nutshell 76

Exceptionstry { // exception x thrown here }catch(SomeThrowableClass e) { // The first catch clause that matches x runs // x caught here if x is a class or a subclass of e}catch(SomeOtherThrowableClass e) {

} finally { // this code runs unless the try performed System.exit()}


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Error is an Unchecked Exception

class BankBalanceException extends Error {

BankBalanceException(String errorMessage) { super(errorMessage); }}


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class BankAccount{ private double balance;


public BankAccount(double initialBalance) { if (initialBalance < 0) throw new BankBalanceException("balance less than 0"); balance = initialBalance; } public void deposit(double amount) { balance = balance + amount; }

We don’t needa throwson our methodfor an unchecked exception.


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public double getBalance() { return balance; }}


public static void main(String a[]) { BankAccount billG = new BankAccount(100.0); BankAccount mikeM = new BankAccount(-230.0); }}

We don’t have to throwor catch an unchecked exception.


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C:\McCarthy\www\95-713\examples>javac TestException.java

C:\McCarthy\www\95-713\examples>java TestExceptionException in thread "main" BankBalanceException: balance less than 0 at BankAccount.<init>(TestException.java:18) at TestException.main(TestException.java:39)

1 lecture 3 “just java” chapter 6 - inheritance, polymorphism and the class whose name is class...

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public class bankaccount

cay horstmanns java

return balance

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public double getbalance

exceptions bankaccount

public void depositdouble

public void withdrawdouble