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2Inheritance.pdf-CPS209: Computer Science II Inher...
2Inheritance.pdf-CPS209: Computer Science II Inheritance Inheritance
2Inheritance.pdf-CPS209: Computer S...
2Inheritance.pdf-CPS209: Computer Science II Inheritance Inheritance
Page 11
Bank Account - example
public class
BankAccount
{
private long
accountNumber;
private double
balance;
public
BankAccount(
long
accountNumber)
{
this
.accountNumber = accountNumber;
balance = 0;
}
public double
getBalance()
{
return
balance;
}
// continued in the next slide...
Page 12
Bank Account - example
public void
deposit(
double
amount)
{
balance += amount;
}
public void
withdraw(
double
amount)
{
balance
-= amount;
}
public void
transfer(
double
amount, BankAccount targetAccount)
{
this.withdraw(amount);
targetAccount.deposit(amount);
}
}
Page 13
Bank Account – usage example
class BankAccountTester
{
public static void main( String[] args)
{
BankAccount aliceAcc =
new
BankAccount(1398723);
BankAccount bobAcc
=
new
BankAccount(1978394);
aliceAcc.deposit(900);
aliceAcc.transfer(700,bobAcc);
// Alice’s balance = 200 ; Bob’s balance = 700
}
}
Page 14
Inheritance: Simple Example –
Adding a new variable and method
public class
SavingsAccount
extends
BankAccount
{
private double
interestRate;
public
SavingsAccount(long account,
double
rate)
{
super(account);
interestRate = rate;
}
public void
addInterest()
{
double interest = getBalance() * interestRate / 100;
deposit(interest);
}
}
Page 15
Inheritance: Simple Example
Encapsulation
:
addInterest
method calls
getBalance
rather than directly updating the
balance
field of the superclass (balance field is
private
in superclass)
Note also that
addInterest
can call
getBalance
without specifying a “this” implicit
parameter (the calls apply to the same object)
Page 16
Inheritance: Simple Example
class SavingsAccountTester
{
public static void main( String[] args)
{
BankAccount aliceAcc
=
new
BankAccount(1398723);
SavingsAccount sallyAcc =
new
SavingsAccount(1978394,5.5);
sallyAcc.deposit(300);
aliceAcc.deposit(900);
sallyAcc.addInterest();
aliceAcc.transfer(700,sallyAcc);
System.out.println(“Sally Balance = “ + sallyAcc.getBalance());
}
}
Page 17
Inheritance: Simple Example
SavingsAccount
object
automatically
inherits the
balance
and
accountNumber
instance variables from
BankAccount
,
and adds one additional instance variable:
interestRate
:
SavingsAccount
accountNumber
balance
interestRate
Page 18
Self-Check
1.
What instance variables does an object of class
SavingsAccount
have?
2.
Name four methods that you can apply to
SavingsAccount
objects
Page 19
Answers
1.
Three instance fields:
balance, accountNumber
and
interestRat
e.
2.
deposit()
,
withdraw()
,
getBalance()
, and
addInterest()
.
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