1	CS110 Lecture 22 Tuesday, April 20, 2004 Announcements hw10 due Thursday, April 22 exam next Tuesday, April 27 Agenda Questions Error handling (finally) What’s on the exam?
2	RumpelStiltskin Exception examples in a short standalone program The fairy tale: guess my name examples/RumpelStiltskin.java > Java RumpelStiltskin > Java RumpelStiltskin foo > Java RumpelStiltskin foo bar > Java RumpelStiltskin RumpelStiltskin
3	Java RumpelStiltskin Design (pseudocode) If there is no command line argument print usage message end the program Two possible implementation strategies test for args[0], proceed based on test result assume args[0] is there, catch Exception if not
4	Test first strategy if (args.length == 0 ) { System.out.println( "usage: java RumpelStiltskin guess"); System.exit(0); // leave program gracefully } // continue normal processing
5	Exception strategy try { System.out.println(”Are you " + args[0] +'?'); rumpelstiltskin.guessName(args[0]); System.out.println("Yes! How did you guess?"); System.exit(0); // leave program gracefully } // come here right away if there is no args[0] catch (IndexOutOfBoundsException e) { System.out.println( "usage: java RumpelStiltskin guess"); System.exit(0); // leave program gracefully }
6	java RumpelStiltskin foo sorry - foo is not my name Intentionally generate a NullPointerException, see what the Exception's toString method returns java.lang.NullPointerException Experiment with the printStackTrace() method: BadGuessException at java.lang.Throwable.<init>(Compiled Code) at java.lang.Exception.<init>(Compiled Code) at BadGuessException.<init>(Compiled Code) at Wizard.guessName(Compiled Code) at Wizard.makeMischief(Compiled Code) at RumpelStiltskin.main(Compiled Code) Look for a second command line argument, see what happens if it's not there: java.lang.ArrayIndexOutOfBoundsException: 1 at RumpelStiltskin.main(Compiled Code)
7	Equality In Java, “==” means “two variables have same value” Box and arrow pictures help: same value for primitive types is just what you expect same value for reference types: arrow points to the same Object In Object public boolean equals(Object o) { return this == o; } Override equals when you have a better idea about what equality should mean
8	Equals in AbstractList.java public boolean equals(Object o) { if (o == this) return true; if (!(o instanceof List)) return false; ListIterator e1 = listIterator(); ListIterator e2 = ((List) o).listIterator(); while(e1.hasNext() && e2.hasNext()) { Object o1 = e1.next(); Object o2 = e2.next(); if (!(o1==null ? o2==null : o1.equals(o2))) return false; } return !(e1.hasNext() \|\| e2.hasNext()); }
9	How ArrayLists work private Object elementData[]; private int size; public ArrayList() { this(10); } public boolean add(Object o) { ensureCapacity(size + 1); elementData[size++] = o; return true; }
10	How ArrayLists grow public void ensureCapacity(int minCapacity) { int oldCapacity = elementData.length; if (minCapacity > oldCapacity) { Object oldData[] = elementData; int newCapacity = (oldCapacity * 3)/2 + 1; if (newCapacity < minCapacity) newCapacity = minCapacity; elementData = new Object[newCapacity]; System.arraycopy(oldData, 0, elementData, 0, size); } }
11	hw10 Test scripts Error handling for all shell commands Error handling to register and login Count fraction of Juno that’s there for error handling (class Profile)
12	Characters Type char is primitive in Java A char is really an int In the old days characters were just small integers The ASCII character set contains 128 characters numbered 0-127 one byte, 8 bits: 00000000 to 11111111 in binary (0-255 decimal) ascii codes are the bytes with the high bit 0 Googling for ASCII code will find lots of information
13	Characters (continued) Printable characters are 32-126 (decimal) – other bytes are visible in emacs (look at a class file) used for emacs commands, like ^S To represent them in Java use escape character: \ ‘\ddd’ // ddd is base 8 number < 256 System.out.println(‘\007’); //ring bell ‘\n’, ‘\b’, ‘\t’, ‘\”’, ‘\\’ See Escape.java in joi/examples/
14	Unicode Unicode extends character set to 16 bits (0 to 2¹⁶-1) for kanji, Arabic, Hebrew, mathematics, … Type char in Java really is a 16 bit int We usually write these values as hexadecimal strings: 16 bits is four hex digits ‘\uXXXX’ (X = 0, 1, …, 9, A, … , F) Internationalization (I18N) locale collation sequence time, date, number format
15	class Character Wrapper class for primitive type char Static methods to process char values Use Character to save char in a Collection Character(char ch) // constructor public char charValue() static int getNumericValue(char ch) // unicode value static boolean isDigit(char ch) static char toUpperCase(char ch) … see API for more
16	Strings ... The String API - lots there to use when needed constructors equality comparisons substrings character contents changing String contents (not) Read (some of) String.java
17	String constructors String s; s = “hello”; //common and convenient s = new String(“hello”); char[ ] charArray = {‘O’, ‘K’} ; s = new String( charArray ); String t = new String(s);
18	String matches and searches boolean equals(String anotherString); boolean equalsIgnoreCase(String anotherString); int compareTo(String anotherString); // +,-,0 boolean startsWith(String prefix); boolean endsWith(String suffix); int indexOf(int ch); int indexOf(String str); int indexOf(..., int fromIndex); int lastIndexOf(...);