1. 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

2. 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

3. Cay Horstmann's Bank Account Example
// File Name BankAccount.java
public class BankAccount {
private double balance;
public BankAccount()
{ balance = 0; }
public BankAccount(double initialBalance)
{ balance = initialBalance;
Cay Horstmann's Bank Account Example

4. Cay Horstmann's Bank Account Example
public void deposit(double amount)
{ balance = balance + amount; }
public void withdraw(double amount)
{ balance = balance - amount;
public double getBalance()
{ return balance;
Cay Horstmann's Bank Account Example

5. Cay Horstmann's Bank Account Example
// again but let’s add more members
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;
Cay Horstmann's Bank Account Example

6. Cay Horstmann's Bank Account Example
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;
Cay Horstmann's Bank Account Example

7. A Savings Account “IS A” Bank Account
public class SavingsAccount extends BankAccount {
// new methods
// new instance variables
// we already have deposit, withdraw, etc. }
Cay Horstmann's Bank Account Example

8. Cay Horstmann's Bank Account Example
Inheritance Diagram
Object
Unlike C++,
Java is singly rooted
Bank Account
Savings Account
Cay Horstmann's Bank Account Example

9. 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);
Cay Horstmann's Bank Account Example

10. Cay Horstmann's Bank Account Example
public static void main(String args[]) {
SavingsAccount rainyDay = new SavingsAccount(100,.10);
rainyDay.addInterest();
System.out.println(rainyDay.getAccountNumber());
System.out.println(rainyDay.getBalance());
SavingsAccount collegeFund = new SavingsAccount(1000,.10);
collegeFund.addInterest();
System.out.println(collegeFund.getAccountNumber());
System.out.println(collegeFund.getBalance()); }
Cay Horstmann's Bank Account Example

11. Cay Horstmann's Bank Account Example
Output
C:\heinz\90-876\examples\Inherit\BankAccount>java SavingsAccount
121.0 1
1210.0
Cay Horstmann's Bank Account Example

12. Converting Between Class Types
A SavingsAccount “is a” BankAccount.
A BankAccount “is a” Object.
Is the following legal?
Object o = new SavingsAccount(100,.10);
Cay Horstmann's Bank Account Example

13. Converting Between Class Types
Object o = new SavingsAccount(100,.10);
Assignment is fine!
An Object
Cay Horstmann's Bank Account Example

14. Converting Between Class Types
Is the following legal?
SavingsAccount s = new SavingsAccount(100,.10);
Object o = s;
Sure!
Both references point to the same object.
Cay Horstmann's Bank Account Example

15. Converting Between Class Types
Is the following legal?
SavingsAccount s = new SavingsAccount(100,.10);
BankAccount b = s;
Sure!
Cay Horstmann's Bank Account Example

16. Converting Between Class Types
Is the following legal?
SavingsAccount s = new SavingsAccount(100,.10);
BankAccount b = s;
b.addInterest()
Cay Horstmann's Bank Account Example

17. Converting Between Class Types
Is the following legal?
SavingsAccount s = new SavingsAccount(100,.10);
BankAccount b = s;
b.addInterest()
NO!
A BankAccount object has no addInterest method!
Cay Horstmann's Bank Account Example

18. Converting Between Class Types
Is the following legal?
SavingsAccount s = new SavingsAccount(100,.10);
BankAccount b = s;
((SavingsAccount)b).addInterest();
Yes!
We tell the compiler we will take the risk!
Cay Horstmann's Bank Account Example

19. Converting Between Class Types
How about the following?
SavingsAccount s = new SavingsAccount(100,.10);
Object o = s;
((SavingsAccount)o).addInterest();
Cay Horstmann's Bank Account Example

20. Converting Between Class Types
How about the following?
SavingsAccount s = new SavingsAccount(100,.10);
Object o = s;
((SavingsAccount)o).addInterest();
Sure!
Why?
Are we taking a risk?
Cay Horstmann's Bank Account Example

21. Converting Between Class Types
How about the following?
SavingsAccount s = new SavingsAccount(100,.10);
Rectangle r = s;
((SavingsAccount)r).addInterest();
Cay Horstmann's Bank Account Example

22. Converting Between Class Types
How about the following?
SavingsAccount s = new SavingsAccount(100,.10);
Rectangle r = s;
((SavingsAccount)r).addInterest();
No!
Why?
Cay Horstmann's Bank Account Example

23. Cay Horstmann's Bank Account Example
Inheritance Diagram
Object
Rectangle
Bank Account
Savings Account
Cay Horstmann's Bank Account Example

24. Cay Horstmann's Bank Account Example
Polymorphism
Consider the following static method:
public void static display(BankAccount b) {
System.out.println(“Acct. Number:” + b.getAccountNumber());
System.out.println(“Balance $” + b.getBalance()); }
Cay Horstmann's Bank Account Example

25. Cay Horstmann's Bank Account Example
Polymorphism
Consider the following static method:
public void static display(BankAccount b) {
System.out.println(“Acct. Number:” + b.getAccountNumber());
System.out.println(“Balance $” + b.getBalance()); }
// Suppose we call it with the following code. What happens?
public static void main(String args[]) {
BankAccount collegeFund = new BankAccount(100);
display(collegeFund);
Cay Horstmann's Bank Account Example

26. Cay Horstmann's Bank Account Example
Polymorphism
How about with this code?
public void static display(BankAccount b) {
System.out.println(“Acct. Number:” + b.getAccountNumber());
System.out.println(“Balance $” + b.getBalance()); }
public static void main(String args[]) {
SavingsAccount rainyDay = new SavingsAccount(100,.10);
display(rainyDay);
Cay Horstmann's Bank Account Example

27. Inheritance Diagram (UML)
Object
Rectangle
BankAccount
SavingsAccount
CDAccount
Cay Horstmann's Bank Account Example

28. Cay Horstmann's Bank Account Example
Polymorphism
Is this OK?
public void static display(BankAccount b) {
System.out.println(“Acct. Number:” + b.getAccountNumber());
System.out.println(“Balance $” + b.getBalance()); }
public static void main(String args[]) {
CDAccount retirement= new CDAccount(100,.10,5);
display(retirement);
Cay Horstmann's Bank Account Example

29. Only accessible within the class
Java Access Control
public
Interface Access
Only accessible within the class
private
protected
“Sort of private”
Cay Horstmann's Bank Account Example

30. Class member accessibility
Java Access Control
Class member accessibility
Yes No
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
Classes are either public or package access
Cay Horstmann's Bank Account Example

31. Interfaces and Abstract classes
Cay Horstmann's Bank Account Example

32. Cay Horstmann's Bank Account Example
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?
Cay Horstmann's Bank Account Example

33. Cay Horstmann's Bank Account Example
Inheritance
static void foo(BankAccount x) {
x.withdraw(1000);
x.deposit(50.0); : }
foo() can call those
methods on x that are
provided by the BankAccount
class
Cay Horstmann's Bank Account Example

34. Cay Horstmann's Bank Account Example
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?
Cay Horstmann's Bank Account Example

35. Cay Horstmann's Bank Account Example
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!!
Cay Horstmann's Bank Account Example

36. Cay Horstmann's Bank Account Example
Inheritance
Often we want to write a method that is able to handle any object
that meets certain requirements.
In the example above, foo() requires that it receive BankAccount
objects. 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 certain
operations will be available. Interfaces take this a step further…
Cay Horstmann's Bank Account Example

37. Cay Horstmann's Bank Account Example
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)
Cay Horstmann's Bank Account Example

38. Cay Horstmann's Bank Account Example
The BankAccount Again
interface Account {
public double getBalance();
public void setBalance(double x); }
Any class that implements this
interface MUST have these
two methods defined exactly
as specified.
Cay Horstmann's Bank Account Example

39. Cay Horstmann's Bank Account Example
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;
Cay Horstmann's Bank Account Example

40. Cay Horstmann's Bank Account Example
public void deposit(double amount)
{ balance = balance + amount; }
public void withdraw(double amount)
{ balance = balance - amount;
public double getBalance()
{ return balance;
Cay Horstmann's Bank Account Example

41. Cay Horstmann's Bank Account Example
public void setBalance(double a) {
balance = a; }
public void update() {
balance = balance + balance * rate;
We have provided
implementations for
the two methods.
Cay Horstmann's Bank Account Example

42. Cay Horstmann's Bank Account Example
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 implements
interface Account
Cay Horstmann's Bank Account Example

43. Cay Horstmann's Bank Account Example
public static void foo(Account a) {
double m = a.getBalance();
System.out.println(m); }
The name of
an interface
Any other class that implements Account can
be passed to foo().
Cay Horstmann's Bank Account Example

44. 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?
Cay Horstmann's Bank Account Example

45. 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.
Cay Horstmann's Bank Account Example

46. Cay Horstmann's Bank Account Example
INTERFACES
public interface Comparable {
int compareTo(Object other); }
public class Student implements Comparable {
// this class MUST define compareTo()
Automatically public
Found in java.lang
Cay Horstmann's Bank Account Example

47. Cay Horstmann's Bank Account Example
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?
Cay Horstmann's Bank Account Example

48. Cay Horstmann's Bank Account Example
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.
Cay Horstmann's Bank Account Example

49. Cay Horstmann's Bank Account Example
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
Cay Horstmann's Bank Account Example

50. Cay Horstmann's Bank Account Example
Abstract Classes
Consider
public class BankAccount {
public void deductFees() { …body goes here ? … } : }
public class SavingsAccount extends BankAccount{
What should this method do?
We silently get the
deductFees() method
Cay Horstmann's Bank Account Example

51. Cay Horstmann's Bank Account Example
Abstract Classes
No body..It’s up to the
derived class to complete.
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 }
Cay Horstmann's Bank Account Example

52. Cay Horstmann's Bank Account Example
Abstract Classes
Abstract
void foo(BankAccount s) {
make calls on s.deductFees(); }
A real object
at run time.
Many classes can extend the abstract class,
get some code reuse, but are forced to define
the abstract method deductFees() .
Cay Horstmann's Bank Account Example

53. Cay Horstmann's Bank Account Example
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
Cay Horstmann's Bank Account Example

54. 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"); }

55. 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 ClassCastException
is thrown when we try to convert the BigInteger
pointed to by o to a String.

56. 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.

57. instanceof and isInstanceOf
// Given an object, check its type:
public class ShortTest {
public static void main(String args[]) {
Object o = new Fruit();
if(o instanceof Fruit)
System.out.println("it's a Fruit"); }

58. instanceof and isInstanceOf
// Given a type, check an object
public class ShortTest {
public static void main(String args[]) {
Object o = new Fruit();
Class c = o.getClass();
if(c.isInstance(o))
System.out.println("it's a Fruit"); }

59. The Class Object interface BabySitter { void respondFast(); }
class Dad implements BabySitter {
public Dad() {}
public void respondFast() {}

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

61. 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); }

62. C:\McCarthy\www\JustJava\Examples>java RTTITest
Dad
false
BabySitter
true
java.lang.Object

63. 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; } }

64. 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 TestFruit
Grams:0 Calories per gram:0

65. 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();
Cay Horstmann's Bank Account Example

66. Cay Horstmann's Bank Account Example
// 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);
Cay Horstmann's Bank Account Example

67. Cay Horstmann's Bank Account Example
C:\McCarthy\www\JustJava\Examples>java TestFruit
Fruit extends
java.lang.Object extends
Grams:0 Calories per gram:0
Cay Horstmann's Bank Account Example

68. Cay Horstmann's Bank Account Example
TestException.java
class BankBalanceException extends Exception {
BankBalanceException(String errorMessage) {
super(errorMessage); }
Cay Horstmann's Bank Account Example

69. Cay Horstmann's Bank Account Example
class BankAccount {
private double balance;
public BankAccount()
{ balance = 0; }
public BankAccount(double initialBalance)
throws BankBalanceException
if (initialBalance < 0) throw
new BankBalanceException("balance less than 0");
balance = initialBalance;
Cay Horstmann's Bank Account Example

70. Cay Horstmann's Bank Account Example
public void deposit(double amount)
{ balance = balance + amount; }
public void withdraw(double amount)
{ balance = balance - amount;
public double getBalance()
{ return balance;
Cay Horstmann's Bank Account Example

71. Cay Horstmann's Bank Account Example
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 must
be caught, or it must
be declared in the throws clause of this method.
BankAccount billG = new BankAccount(100.0); ^
1 error
Cay Horstmann's Bank Account Example

72. Cay Horstmann's Bank Account Example
Handle the exception
public class TestException {
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");
Cay Horstmann's Bank Account Example

73. Cay Horstmann's Bank Account Example
C:\McCarthy\www\95-713\examples>java TestException
Balance initialized too low
BankBalanceException: balance less than 0
Exiting
End of program
Cay Horstmann's Bank Account Example

74. Exceptions in Java Object Throwable Error Exception RunTimeException
Less severe conditions
Unrecoverable problems
- e.g. Corrupted class file, out
of memory
Can be handled but rare
- These are all unchecked
RunTimeException
Checked
e.g.
ClassNotFoundException
CloneNotSupportedException
Can occur anywhere
Unchecked
e.g. Null Pointer Exception,
ArrayIndexOutOfBounds

75. Exceptions
Checked Exceptions must be explicitly thrown or handled. The compiler
will enforce this rule.
Unchecked exceptions, for example ArrayIndexOutOfBounds, need
not 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. Exceptions try { // 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()
Java in a Nutshell

77. Error is an Unchecked Exception
class BankBalanceException extends Error {
BankBalanceException(String errorMessage) {
super(errorMessage); }
Cay Horstmann's Bank Account Example

78. Cay Horstmann's Bank Account Example
class BankAccount {
private double balance;
public BankAccount()
{ balance = 0; }
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 need
a throws
on our method
for an unchecked exception.
Cay Horstmann's Bank Account Example

79. Cay Horstmann's Bank Account Example
public void withdraw(double amount)
{ balance = balance - amount; }
public double getBalance()
{ return balance;
public class TestException {
public static void main(String a[]) {
BankAccount billG = new BankAccount(100.0);
BankAccount mikeM = new BankAccount(-230.0);
We don’t
have to throw
or catch an
unchecked
exception.
Cay Horstmann's Bank Account Example

80. Cay Horstmann's Bank Account Example
C:\McCarthy\www\95-713\examples>javac TestException.java
C:\McCarthy\www\95-713\examples>java TestException
Exception in thread "main" BankBalanceException:
balance less than 0
at BankAccount.<init>(TestException.java:18)
at TestException.main(TestException.java:39)
Cay Horstmann's Bank Account Example