1. Lecture 231 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. Lecture 232 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. Lecture 233 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. Lecture 234 Unicode Unicode extends character set to 16 bits (0 to 2 16 -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. Lecture 235 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. Lecture 236 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. Lecture 237 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. Lecture 238 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. Lecture 239 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. Lecture 2310 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. Lecture 2311 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. Lecture 2312 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. Lecture 2313 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. Lecture 2314 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? } from String.java

15. Lecture 2315 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. Lecture 2316 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. Lecture 2317 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. Lecture 2318 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. Lecture 2319 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. Lecture 2320 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. Lecture 2321 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. Lecture 2322 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 S ystem.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. Lecture 2323 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. Lecture 2324 Useful final Fields In class File WindowsUnix 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. Lecture 2325 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. Lecture 2326 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. Lecture 2327 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. Lecture 2328 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. Lecture 2329 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. Lecture 2330 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. Lecture 2331 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 data coming indata going out program

32. Lecture 2332 *Stream classes