1	CS110 Lecture 23 Thursday, April 22, 2004 Announcements hw10 due tonight exam next Tuesday, April 27 final exam Wednesday, May 20, 8:00 AM McCormack, Floor 01, Room 0608 Agenda Questions Characters Strings Files
2	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
3	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/
4	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
5	class Character Wrapper class for primitive type char Static methods to process char values Use Character to save char in a Collection (happens automatically in Java 1.5) 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
6	Strings ... The String API - lots there to use when needed constructors equality comparisons substrings character contents changing String contents (not) finding meaning in Strings Read (some of) String.java See StringDemo.java in JOI/examples
7	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);
8	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); // -1 if not found int indexOf(String str); int indexOf(String str, int fromIndex); int lastIndexOf(...);
9	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 in class String extends Object to compare Strings character by character (that’s what equality for String should mean).
10	Equality String s = “hello”; String t = “hello”; s == t false (sometimes) s.equals(t) true s.equals(“hello”) true “hello”.equals(s) true (weird) See EqualsDemo.java in JOI/examples
11	String.java About 1/3 of it is comment! public final class String { … } Implementation uses a character array: private char[] value; private int offset; private int count; The characters that make up this String are stored in value at positions offset … offset+count-1 (usually offset = 0)
12	Comparing Strings public int compareTo(String s) // not boolean “hello”.compareTo(“help!”) -4 // = ‘l’ - ‘p’ “hello”.compareTo(“hell”) 1 // 5-4 (lengths) x.compareTo(y) == 0 just when x.equals(y) compareTo is wonderful for sorting (alphabetizing)
13	compareTo pseudocode march through the character arrays looking for first char difference (be sure not to run off the end, since lengths may differ) if you find a char difference, return it (numerically) if no difference when you reach the end of the shorter string, return the difference in lengths (0 if the same)
14	"public int compareTo(String anotherString)" public int compareTo(String anotherString) { int len1 = this.count; // a field int len2 = anotherString.count; int n = Math.min(len1, len2); char v1[] = this.value; // another field char v2[] = anotherString.value; int i = offset; // pretend offset=0 int j = anotherString.offset; while (n-- != 0) { // code like C char c1 = v1[i++]; // i++ in loop char c2 = v2[j++]; if (c1 != c2) { // (first) mismatch return c1 - c2; // subtract ints } } return len1 - len2; // same length? }
15	Substrings String s = “hello, world”; s.startsWith(“hello”) returns true s.substring(3,8) returns “lo, w” // from index 3 to index 8-1 s.substring(7) returns “world” // index 7-end s.indexOf(“world”) returns 7 s.indexOf(“hello”) returns 0 s.indexOf(“foo”) returns -1
16	Seeing characters in a String String s = “hello”; for (int i = 0; i < s.length(); i++){ char c = s.charAt(i); // do whatever with c }
17	Changing String contents (NOT) These methods return a new String: they do not change the String getting the message! s.toUpperCase() returns “HELLO, WORLD” s.replace(‘o’,‘x’) returns “hellx, wxrld” s.concat(“!”) returns “hello, world!” s += “!” returns “hello, world!” s.concat(‘!’) returns “hello, world!” “ x y z \t\b”.trim() returns “x y z”
18	Methods Returning/Constructing new Strings concat(String str); // Can also use + replace(char old, char new); // Not in place! substring(int beginIndex); // new String! substring(int beginIndex, int endIndex); toLowerCase(); // new String! toLowerCase(Locale locale); toUpperCase(); toUpperCase(Locale locale); trim();
19	Class StringBuffer Like a String, but with direct access to char contents – therefore mutable Much more efficient StringBuffer buf = new StringBuffer("World"); buf.insert(0, "Jello, "); buf.append("!"); buf.setCharAt(0,'H'); // now "Hello, World!" buf.reverse(); // now "!dlroW ,olleH" String s = buf.toString();
20	Conversions Strings have no meaning: “1001” is not 1001 To convert a String to an integer: int n; String s = “1001”; try { n = Integer.parseInt(s); // String ® int } catch (NumberFormatException e) { // take corrective action }
21	Conversions To convert an integer to a String int n = 1001; String s; s = String.valueOf(n); // “1001” s = “” + n; // rely on smart concatenation s = Integer.toHexString(n) // “3e9” s = Integer.toBinaryString(n) // “1111101001” More such static methods in String, Integer, Double To convert any Object to a String: Object obj … ; s = obj.toString(); s = “” + obj; // rely on smart concatenation
22	I/O programming I/O = input/output I/O is hard deals with real world beyond programmers control Output easier than input (programmer knows more) System.out.println() is straightforward Terminal readLine() wraps hard to use System.in java.io package provides lots of useful classes I/O programming may throw many Exceptions Even good tools are hard to use when topic is hard Count on borrowing from code that works
23	public class File Information about files, not their contents (Juno should be redesigned this way) Constructors File(String path) or (String path, String name) or (File dir, String name) Methods boolean exists(), isFile(), isDirectory(), canRead(), canWrite(); long length(), lastModified(); boolean delete(), mkdir(), renameTo(File dest); String getName(), getParent(), getPath(), getAbsolutePath()
24	Useful final Fields In class File Windows Unix File.pathSeparator; ";" ":" File.separator; "\" "/" In class System System.getProperty ("line.separator") "\n\r" "\n public static final FileDescriptor in; public static final FileDescriptor out; public static final FileDescriptor err;
25	Profile main in Profile.java (pseudocode): declare and initialize counters open Java source for reading while (get a line from source file) classify line, increment counters close source file print results
26	Copy Classic example dealing with file contents Write Windows command line copy in Java: java Copy sourcefile targetfile main in Copy.java (pseudocode): open sourcefile for reading open targetfile for writing while (get stuff from sourcefile) write stuff to targetfile close both files
27	Copy1.java FileReader inStream = null; // 26, outside try FileWriter outStream = null; // 27, outside try try { inStream = new FileReader(args[0]); // 32 outStream = new FileWriter(args[1]); // 33 while (…) { // 36-38 copy loop } catch // various errors 40: faulty command line input - give usage message 44: source file not found (or not readable) target file not writeable 47: something went wrong in actual read/write
28	Keyword finally try { } catch() { { finally { code here runs whether or not try works } Copy1.java 53, 61: close files whether or not there was an error in processing (underlying OS may limit number of files you may have open) try (lines 51, 63) since even closing a file may throw an Exception
29	FileReader/FileWriter i/o int ch; // character read as an int (line 28) while ((ch = inStream.read()) != -1) { // 36 outStream.write(ch); } Java (and C) idiom: assignment statement x = y gets value of x , so (ch = inStream.read()) != EOF sends instream a read() message assigns returned int to variable ch compares that int to EOF, declared final static, used by read() to signal end of file result is true or false, so useful inside while( )
30	Copy2 using BufferedReader/Writer BufferedReader inStream = null;// lines 24, 25 BufferedReader inStream = null String line; try inStream = new BufferedReader ( new FileReader(argv[0])); outStream = ... while ((line = inStream.readLine()) != null) outStream.write( line ); outStream.newLine(); // no ‘\n’ in line BufferedReader/Writer handle whole lines (Strings) readLine returns null at EOF
31	Streams/filters data can be characters, Strings, bytes, objects,… Streams connect to file, terminal, String, net, … Always use same methods: read, write (polymorphism) Examples: copy: stream of characters, or of lines (Strings) Profile: stream of lines, program counts kinds TV: input stream from cable, output stream to screen
32	*Stream classes