1	CS110 Lecture 16 Tuesday, March 30, 2004 Announcements hw7 due Thursday pass/fail, withdraw deadline April 8 Agenda Questions toString Bank (5) switch, flow control Trees JFile system
2	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
3	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
4	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
5	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
6	test ? x : y Has value x if test is true, else has value y same as
7	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
8	Bank(version 5) How does program decide which kind of account to open? How simulated time works Polymorphism Code in hw7/bank/Bank.java also answers hw6
9	switch (Bank.java 116) String accountName = atm.readWord ("Account name: " ); char accountType = atm.readChar ("Check/Fee/Reg/Sav? (c/f/r/s): " ); int start = atm.readInt("Initial deposit: " ); BankAccount newAccount; switch( accountType ) { case 'c': newAccount = new CA( bal, this ); break; case 'f': newAccount = new FA( bal, this ); break; default: atm.println ("invalid account type: " + accountType); return; }
10	switch Easier to read than if - else if - else if … . Variable whose value you switch on must be int or char (or long or short) Remember the break statement, lest you fall through to next case (a common error) Java design flaw – should use { … block …} new keywords: default, case, switch, break
11	Closing an account while (!(transaction = atm.readWord (" transaction: ")).equals("quit")) { . . . else if (transaction.equals("close")) { close(acct); // private in Bank break; } } Require whole word “close” to close account, use “cash check” or “check” to cash check
12	Leaving a loop body prematurely while or for ( … ) { … if ( … ) continue; if ( … ) break; if ( … ) return; … last in loop; } next statement; Sometimes makes for easy reading, avoids many else statements. Sometimes confusing - can’t trace flow without reading loop body.
13	newMonth Bank.java (203) private void newMonth() { month.next(); for (BankAccount acct: accountList) { acct.newMonth(); } } No cast: every subclass of BankAccount must implement the abstract newMonth method Real code is Java 1.4 (this is Java 1.5)
14	Polymorphism poly (many) + morph (shape) Bank maintains a list of BankAccount objects sends them messages without knowing what kinds of BankAccounts they are! Client refers to objects of type Parent that are really instances of a Child extending Parent Each child responds in its own particular way Powerful design tool - ignorance is bliss
15	Polymorphism atm.println(" withdrew " + acct.withdraw( amount )); Checking and Regular accounts just do it FeeAccount charges a fee SavingsAccount keeps track of transactions in this month No casting, since BankAccount has a withdraw method Polymorphism is in countTransaction, invoked by withdraw
16	Here we need a cast 146 process…ForAccount( BankAccount acct ) 165 else if ( trans.startsWith("ca" ) \|\| 166 trans.startsWith("ch" ) ) { 167 int amount = atm.readInt ( " amount of check: " ); 168 atm.println(" cashed check for " 169 ((CA)acct).honorCheck(amount )); 170 } Note use of \|\| (or). Use && for and. “c” is ambiguous since we can close account
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 In Windows, each drive (C:, A:) has a root directory, called “\”, with no parent Each directory is the root for the tree of things inside it
20	Tree for cs110 web page
21	Design problem Directory can store TextFiles and Directories Directory and TextFile both have owner, create/mod date (same meaning) size, contents (different meanings) Directory has methods to add to, get from and loop on its contents (the TreeMap of files in it) TextFile has methods to manipulate its text Can we write these classes without copying code?
22	Inheritance
23	JFile system uses two trees
24	JFile (easy part) private fields for String name Date createDate String owner Date modDate getters and setters as appropriate abstract getSize method since each child must provide its own implementation: number of JFiles in a Directory number of characters in a TextFile main for unit testing 1/4 of the source code tedious but straightforward – read it now
25	Testing JFile, Directory, TextFile JFile has static code for testing public static main private static methods out, list, type private static field for Terminal (visible in all static methods) main builds a tree of JFiles documented on lines 210-217 constructed on lines 218-229 explored on lines 231-251
26	Look at list list is passed a Directory to list – we might have put list in class Directory and asked a Directory to list itself. We didn’t, because Directory knows nothing about printing. Client sends a getContents message instead, and does its own printing The idiom test ? yes : no loop on contents (like unit test in hw4) line 238: jfile.getSuffix Send a message to a JFile object without knowing whether it’s a TextFile or a Directory
27	Polymorphism poly (many) + morph (shape) Directory.java maintains a list of JFile objects client retrieves them and sends them messages without knowing what kinds of JFiles they are! Client refers to objects of type Parent that are really instances of a Child extending Parent Powerful design tool - ignorance is bliss
28	JFile getSuffix ls -f on mars appends “/” for directory listing (e.g. hw5 in cs110) “@” for symbolic link (e.g. cs110 in your home) “*” for an executable file (e.g. mkdir in /bin) (on Unix many commands are really files) no suffix for ordinary text files, class files, ... We want JFiles to behave this way Ask a JFile to tell you its suffix by sending it a getSuffix message getSuffix is abstract in JFile.java
29	“\” vs “/” Windows uses one, Unix the other Java knows about both File.java (in the Java API) declares public static final String separator JFile.java declares public static String separator = File.separator
30	Managing the JFile tree A Directory keeps a TreeMap of JFiles (the Directory’s jfile field) keyed by name has methods to add and retrieve JFiles by name has a method that allows client to loop on contents A JFile has a parent field in which it keeps a reference to the Directory it belongs to (like BankAccount – Bank)
31	JFile constructor JFile.java, line 50 protected: visible to children, not public lines 52-53 are easy: they initialize fields if (parent != null) (line 54) parent.addJFile( name, this ); if this JFile has a parent (not top of JFile tree) send message to parent to add this JFile (Directory or TextFile) to its TreeMap, with name as key. (Directory.java line 69) Careful: parent directory != parent class
32	Constructors in a subclass Client creates a Directory with a name, an owner and in a particular directory (like mkdir): JFile.java line 236: Directory cs110 = new Directory(“cs110”, “eb”, home1); Directory.java constructor line 34: initialize TreeMap declared on 21 (familiar from Chapter 4, hw4) line 33: invoke parent class (JFile) constructor (java keyword super is “my parent”)