1	CS110 Lecture 15 Thursday, March 25, 2004 Announcements hw6 due tonight pass/fail, withdraw deadline April 8 Agenda Questions Better BankAccount (Inheritance) Overriding, class Object toString Trees
2	Banking System (version 5) Several kinds of BankAccounts regular checking fee savings Simulated time: every month fee accounts are charged a fee savings accounts accrue interest
3	Bank Application (version 5)
4	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() protected constructor (33): invoked by children only abstract method newMonth: (185) what each kind of BankAccount does on first day of month, implemented in each child class, called from Bank’s report and newMonth methods
5	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() { }
6	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 ); }
7	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(); }
8	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
9	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
10	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
11	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
12	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 implicit toString message 36 toString from class Object weird
13	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
14	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
15	test ? x : y Has value x if test is true, else has value y same as
16	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 “[ 0^th item toString, 1^st item toString … ]” Very useful for debugging
17	Trees Common in computer science: Java class hierarchy (shows inheritance) Windows tree for files and directories (folders) Vocabulary: Tree, hierarchy Root (often drawn at the top!) Child, parent, branch, leaf, node Draw with arrows, or in outline form
18	Class hierarchy
19	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 “\”
20	Tree for cs110 web page