Transcript Document

CS110 Lecture 15
Thursday, March 25, 2004
• Announcements
– hw6 due tonight
– pass/fail, withdraw deadline April 8
• Agenda
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Questions
Better BankAccount (Inheritance)
Overriding, class Object
toString
Trees
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Banking System (version 5)
• Several kinds of BankAccounts
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regular
checking
fee
savings
• Simulated time: every month
– fee accounts are charged a fee
– savings accounts accrue interest
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Bank Application (version 5)
Every class extends Object,
directly or indirectly
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class BankAccount
• What all BankAccounts share
– fields balance, transactionCount, issuingBank
– methods withdraw, deposit, incrementBalance,
countTransaction, getters and setters for fields
• Changes from BankAccount.java (4):
– abstract class (16): never new BankAccount()
– constructor (31): invoked by children only
– abstract method newMonth: (147) what each kind of
BankAccount does on first day of month,
implemented in each child class, called from Bank’s
report and newMonth methods
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class RegularAccount
• Inherit from abstract BankAccount
• Write as little as possible – take all the defaults
• Constructor:
public RegularAccount( int initialBalance,
Bank issuingBank )
{
super(initialBalance, issuingBank);
}
• implement abstract method newMonth
public void newMonth()
{
}
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class CheckingAccount
• Behavior: like RegularAccount, but can write
checks too. Checks cost money.
• Design
– inherit from BankAccount (not from RegularAccount!)
– write empty newMonth method
– write honorCheck method
public int honorCheck( int amount )
{
incrementBalance( - checkFee );
return withdraw( amount );
}
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class FeeAccount
• Behavior: like RegularAccount, but each
transaction costs money, and there’s a monthly fee
• Design
– inherit from BankAccount
– write newMonth method to charge monthly fee
– override countTransaction method!
subtract fee, then do whatever else is customary
public void countTransaction()
{
incrementBalance( -transactionFee );
super.countTransaction();
}
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class SavingsAccount
• Behavior:
– 5% annual interest credited monthly
– only three free transactions per month,
then each costs $1
• Design
– inherit from BankAccount
– write newMonth method to credit interest
– figure out how to keep track of number of
transactions in a month, override countTransaction
method to charge fee when appropriate
• Write it for hw7
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Where is the method?
• Send aMessage to an object of class SomeClass
• Java looks in SomeClass for method matching
aMessage with the right signature
• If not found, looks in parent class of SomeClass
(there always is a parent, except for class Object)
• If child and parent both have the method
– child’s method overrides parent’s
– if you really want the parent’s method, use super
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Inheritance – two uses
• Put features common to different kinds of
objects in an abstract superclass, often with
abstract methods
• Make a (small) change in the behavior of
some object by extending its class and
overriding a method
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toString
• Suppose
SomeClass foo = new SomeClass(
)
• Then these two expressions do the same thing:
System.out.println( foo.toString() );
System.out.println( foo );
• Every object knows how to respond to a
toString message since there’s a toString in
class Object
• For “foo” etymology, see the full online
dictionary of computer science at
http://foldoc.doc.ic.ac.uk
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class OverridingDemo
• It’s often nice to override toString, to provide an
informative String describing your particular kind
of object
• NamedObject overrides toString (71-74)
• Create NamedObject instances named by
command line arguments (33, 40)
• println …
34 nobj.toString()
35 nobj itself
36 toString from class Object
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implicit toString message
weird
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toString in class Object
• NamedObject@206fdf64
• Not very informative
• (class name)@(weird number)
• weird number is actually base 16 (hexadecimal)
(digits 0123456789abcde)
• weird number may change when program runs
again
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toString in class Boolean
• Wrapper class for primitive type boolean
• From file Boolean.java in library:
private boolean value; // field
public String toString() {
return value ? "true" : "false";
}
• Sun’s brace convention differs from ours
• test ? x : y expression on next slide
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test ? x : y
• Has value x if test is true, else has value y
if (a > b) {
max = a;
}
else {
max = b;
}
same as
max = ( a > b ) ? a : b;
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toString for collections
• TreeMapDemo.java
108 terminal.println(map.toString());
produces output
{one=1, three=3, two=1}
“{ (key.toString()=value.toString(), … }”
• ArrayList toString produces
“[ 0th item toString, 1st item toString … ]”
• Very useful for debugging
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Trees
• Common in computer science:
– Java class hierarchy (shows inheritance)
– Windows tree for files and directories (folders)
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Vocabulary: Tree, hierarchy
Root (often drawn at the top!)
Child, parent, branch, leaf, node
Draw with arrows, or in outline form
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Class hierarchy
Note descriptive words
root
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File system organization
• folder: place where Windows keeps information
• For historical reasons, we use “directory” as a synonym
for “folder”
• A directory can contain
– other directories (called subdirectories, subfolders)
– files
• Every directory is a subdirectory of its parent
• The world starts at the only directory (on the disk drive)
with no parent: the root directory, called “\”
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root
files are leaves
Tree for cs110
web page
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