Lecture 23 1

CS110 Lecture 23Thursday, 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

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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

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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/

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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

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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

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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

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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);

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String matches and searchesboolean 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 foundint indexOf(String str);int indexOf(String str, int fromIndex);int lastIndexOf(...);

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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 Objectpublic 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).

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EqualityString 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

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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)

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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)

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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)

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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

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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

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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

}

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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”

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Methods Returning/Constructing new Stringsconcat(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();

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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();

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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}

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Conversions• To convert an integer to a String

int n = 1001;String s;s = String.valueOf(n); // “1001”s = “” + n; // rely on smart concatenations = 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

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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

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public class File• Information about files, not their contents (Juno

should be redesigned this way)• ConstructorsFile(String path) or (String path, String name) or

(File dir, String name)

• Methodsboolean exists(), isFile(), isDirectory(),

canRead(), canWrite();

long length(), lastModified();

boolean delete(), mkdir(), renameTo(File dest);

String getName(), getParent(), getPath(), getAbsolutePath()

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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;

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Profile• main in Profile.java (pseudocode):

declare and initialize countersopen Java source for reading while (get a line from source file)

classify line, increment countersclose source fileprint results

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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 writingwhile (get stuff from sourcefile)

write stuff to targetfileclose both files

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Copy1.javaFileReader inStream = null; // 26, outside tryFileWriter outStream = null; // 27, outside trytry { 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

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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

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FileReader/FileWriter i/oint 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( )

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Copy2 using BufferedReader/WriterBufferedReader inStream = null;// lines 24, 25BufferedReader inStream = nullString 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

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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 in data going outprogram

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*Stream classes