1 / 32

CS110 Lecture 16 Tuesday, March 30, 2004

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. toString. Suppose SomeClass foo = new SomeClass( )

agatha
Télécharger la présentation

CS110 Lecture 16 Tuesday, March 30, 2004

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. CS110 Lecture 16Tuesday, March 30, 2004 • Announcements • hw7 due Thursday • pass/fail, withdraw deadline April 8 • Agenda • Questions • toString • Bank (5) • switch, flow control • Trees • JFile system Lecture 16

  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 Lecture 16

  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 Lecture 16

  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 Lecture 16

  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’sbrace convention differs from ours • test ? x : y expression on next slide Lecture 16

  6. test ? x : y • Has value x if test is true, else has value y same as if (a > b) { max = a; } else { max = b; } max = ( a > b ) ? a : b; Lecture 16

  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 “[ 0th item toString, 1st item toString … ]” • Very useful for debugging Lecture 16

  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 Lecture 16

  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; } Lecture 16

  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 see JOI/examples/SwitchDemo.java Lecture 16

  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 Lecture 16

  12. Leaving a loop body prematurely redo loop from top (do test) 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. leave loop return from method Lecture 16 see JOI/examples/BreakandContinueDemo.java

  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) Lecture 16

  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 Lecture 16

  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 Lecture 16

  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 Lecture 16

  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 Lecture 16

  18. Class hierarchy Note descriptive words root Lecture 16

  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 aroot directory, called “\”, with no parent • Each directory is the root for the tree of things inside it Lecture 16

  20. Tree for cs110 web page root files are leaves Lecture 16

  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? Lecture 16

  22. Inheritance class Object class JFile - fields and methods needed by all child classes (deal with owner and Dates), abstract getSize method class TextFile - fields and methods just for TextFile (String contents, append …) class Directory - fields and methods just for Directory (TreeMap jfiles, add and retrieve JFiles, ...) Lecture 16

  23. JFile system uses two trees Java class hierarchy Directory and TextFile hierarchy class Object root eb hello class JFile insult cs110 diary bill class Directory class TextFile Lecture 16

  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 Lecture 16

  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 Lecture 16

  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 Lecture 16

  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 Lecture 16

  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 Lecture 16

  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 Lecture 16

  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) Lecture 16

  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 Lecture 16

  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”) Lecture 16

More Related