Chapter08.ppt

* Chapter 8 Strings and Text I/O * Objectives To use the String class to process fixed strings (§8.2). To use the Character class to process a single character (§8.3). To use the StringBuilder/StringBuffer class to process flexible strings (§8.4). To know the differences between the String, StringBuilder, and StringBuffer classes (§8.2-8.4). To learn how to pass strings to the main method from the command line (§8.5). (Optional) To use the regular expressions to represent patterns for matching, replacing, and splitting strings (§8.6). To discover file properties, delete and rename files using the File class (§8.7). To write data to a file using the PrintWriter class (§8.8.1). To read data from a file using the Scanner class (§8.8.2). (Optional GUI) To add components to a frame (§8.9). * The String Class Constructing a String: String message = "Welcome to Java?; String message = new String("Welcome to Java?); String s = new String(); Obtaining String length and Retrieving Individual Characters in a string String Concatenation (concat) Substrings (substring(index), substring(start, end)) Comparisons (equals, compareTo) String Conversions Finding a Character or a Substring in a String Conversions between Strings and Arrays Converting Characters and Numeric Values to Strings * * Constructing Strings String newString = new String(stringLiteral);   String message = new String("Welcome to Java"); Since strings are used frequently, Java provides a shorthand initializer for creating a string: String message = "Welcome to Java"; * Strings Are Immutable A String object is immutable; its contents cannot be changed. Does the following code change the contents of the string? String s = "Java"; s = "HTML"; * Trace Code String s = "Java"; s = "HTML"; animation * Trace Code String s = "Java"; s = "HTML"; animation * Finding String Length Finding string length using the length() method: message = "Welcome"; message.length() (returns 7) * Retrieving Individual Characters in a String Do not use message[0] Use message.charAt(index) Index starts from 0 * String Concatenation String s3 = s1.concat(s2); String s3 = s1 + s2; s1 + s2 + s3 + s4 + s5 same as (((s1.concat(s2)).concat(s3)).concat(s4)).concat(s5); * Extracting Substrings You can extract a single character from a string using the charAt method. You can also extract a substring from a string using the substring method in the String class. String s1 = "Welcome to Java"; String s2 = s1.substring(0, 11) + "HTML"; * String Comparisons equals String s1 = new String("Welcome?); String s2 = "welcome"; if (s1.equals(s2)){ // s1 and s2 have the same contents } if (s1 == s2) { // s1 and s2 have the same reference } * String Comparisons, cont. compareTo(Object object) String s1 = new String("Welcome?); String s2 = "welcome"; if (s1.compareTo(s2) > 0) { // s1 is greater than s2 } else if (s1.compareTo(s2) == 0) { // s1 and s2 have the same contents } else // s1 is less than s2 * String Conversions The contents of a string cannot be changed once the string is created. But you can convert a string to a new string using the following methods: toLowerCase toUpperCase trim replace(oldChar, newChar) * Finding a Character or a Substring in a String "Welcome to Java".indexOf('W') returns 0. "Welcome to Java".indexOf('x') returns -1. "Welcome to Java".indexOf('o', 5) returns 9. "Welcome to Java".indexOf("come") returns 3. "Welcome to Java".indexOf("Java", 5) returns 11. "Welcome to Java".indexOf("java", 5) returns -1. "Welcome to Java".lastIndexOf('a') returns 14. * Convert Character and Numbers to Strings The String class provides several static valueOf methods for converting a character, an array of characters, and numeric values to strings. These methods have the same name valueOf with different argument types char, char[], double, long, int, and float. For example, to convert a double value to a string, use String.valueOf(5.44). The return value is string consists of characters ?5?, ?.?, ?4?, and ?4?. * The Character Class * Examples Character charObject = new Character('b'); charObject.compareTo(new Character('a')) returns 1 charObject.compareTo(new Character('b')) returns 0 charObject.compareTo(new Character('c')) returns -1 charObject.compareTo(new Character('d') returns ?2 charObject.equals(new Character('b')) returns true charObject.equals(new Character('d')) returns false * StringBuilder and StringBuffer The StringBuilder/StringBuffer class is an alternative to the String class. In general, a StringBuilder/StringBuffer can be used wherever a string is used. StringBuilder/StringBuffer is more flexible than String. You can add, insert, or append new contents into a string buffer, whereas the value of a String object is fixed once the string is created. * StringBuilder vs. StringBuffer The StringBuilder class, introduced in JDK 1.5, is similar to StringBuffer except that the update methods in StringBuffer are synchronized. Use StringBuffer if it may be accessed by multiple tasks concurrently. Using StringBuilder is more efficient if it is accessed by a single task. The constructors and methods in StringBuffer and StringBuilder are almost the same. This book covers StringBuffer. You may replace StringBuffer by StringBuilder. The program can compile and run without any other changes. * The StringBuffer Class The StringBuffer class is an alternative to the String class. In general, a string buffer can be used wherever a string is used. StringBuffer is more flexible than String. You can add, insert, or append new contents into a string buffer. However, the value of a String object is fixed once the string is created. * * StringBuffer Constructors public StringBuffer() No characters, initial capacity 16 characters. public StringBuffer(int length) No characters, initial capacity specified by the length argument. public StringBuffer(String str) Represents the same sequence of characters as the string argument. Initial capacity 16 plus the length of the string argument. * Appending New Contents into a String Buffer StringBuffer strBuf = new StringBuffer(); strBuf.append("Welcome"); strBuf.append(' '); strBuf.append("to"); strBuf.append(' '); strBuf.append("Java"); * Main Method Is Just a Regular Method You can call a regular method by passing actual parameters. Can you pass arguments to main? Of course, yes. For example, the main method in class B is invoked by a method in A, as shown below: * Command-Line Parameters class TestMain { public static void main(String[] args) { ... } } java TestMain arg0 arg1 arg2 ... argn * Processing Command-Line Parameters In the main method, get the arguments from args[0], args[1], ..., args[n], which corresponds to arg0, arg1, ..., argn in the command line. * Regular Expressions A regular expression (abbreviated regex) is a string that describes a pattern for matching a set of strings. Regular expression is a powerful tool for string manipulations. You can use regular expressions for matching, replacing, and splitting strings. * Matching Strings "Java".matches("Java"); "Java".equals("Java"); "Java is fun".matches("Java.*") "Java is cool".matches("Java.*") "Java is powerful".matches("Java.*") * Regular Expression Syntax * Replacing and Splitting Strings * Examples String s = "Java Java Java".replaceAll("v\\w", "wi") ; s contains ?Jawi Jawi Jawi? String s = "Java Java Java".replaceFirst("v\\w", "wi") ; s contains ?Jawi Java Java? * The File Class The File class is intended to provide an abstraction that deals with most of the machine-dependent complexities of files and path names in a machine-independent fashion. The filename is a string. The File class is a wrapper class for the file name and its directory path. * Obtaining file properties and manipulating file * Text I/O A File object encapsulates the properties of a file or a path, but does not contain the methods for reading/writing data from/to a file. In order to perform I/O, you need to create objects using appropriate Java I/O classes. The objects contain the methods for reading/writing data from/to a file. This section introduces how to read/write strings and numeric values from/to a text file using the Scanner and PrintWriter classes. * Writing Data Using PrintWriter * Reading Data Using Scanner * Example: Replacing Text Write a class named ReplaceText that replaces a string in a text file with a new string. The filename and strings are passed as command-line arguments as follows: java ReplaceText sourceFile targetFile oldString newString For example, invoking java ReplaceText FormatString.java t.txt StringBuilder StringBuffer replaces all the occurrences of StringBuilder by StringBuffer in FormatString.java and saves the new file in t.txt. * The Scanner Class The delimiters are single characters in StringTokenizer. You can use the new JDK 1.5 java.util.Scanner class to specify a word as a delimiter. String s = "Welcome to Java! Java is fun! Java is cool!"; Scanner scanner = new Scanner(s); scanner.useDelimiter("Java"); while (scanner.hasNext()) System.out.println(scanner.next()); Creates an instance of Scanner for the string. Sets ?Java? as a delimiter. hasNext() returns true if there are still more tokens left. The next() method returns a token as a string. Welcome to ! is fun! is cool! Output * Scanning Primitive Type Values If a token is a primitive data type value, you can use the methods nextByte(), nextShort(), nextInt(), nextLong(), nextFloat(), nextDouble(), or nextBoolean() to obtain it. For example, the following code adds all numbers in the string. Note that the delimiter is space by default. String s = "1 2 3 4"; Scanner scanner = new Scanner(s);   int sum = 0; while (scanner.hasNext()) sum += scanner.nextInt();   System.out.println("Sum is " + sum); * Console Input Using Scanner Another important application of the Scanner class is to read input from the console. For example, the following code reads an int value from the keyboard: System.out.print("Please enter an int value: "); Scanner scanner = new Scanner(System.in); int i = scanner.nextInt(); NOTE: StringTokenizer can specify several single characters as delimiters. Scanner can use a single character or a word as the delimiter. So, if you need to scan a string with multiple single characters as delimiters, use StringTokenizer. If you need to use a word as the delimiter, use Scanner.