1. 1 Chapter 9 Strings and Text I/O

2. 2 Objectives F To use the String class to process fixed strings. F To use the Character class to process a single character. F To discover file properties, delete and rename files using the File class. F To write data to a file using the PrintWriter class. F To read data from a file using the Scanner class.

3. 3 The String Class F Constructing a String: –String message = "Welcome to Java“; –String message = new String("Welcome to Java“); –String s = new String(); F Obtaining String length and Retrieving Individual Characters in a string F String Concatenation (concat) F Substrings (substring(index), substring(start, end)) F Comparisons (equals, compareTo) F String Conversions F Finding a Character or a Substring in a String F Conversions between Strings and Arrays F Converting Characters and Numeric Values to Strings

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5. 5 Constructing Strings String newString = new String(stringLiteral); String message = new String("Welcome to Java"); A String variable holds a reference to a String object that stores a string value. Since strings are used frequently, Java provides a shorthand initializer for creating a string: String message = "Welcome to Java";

6. 6 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";

7. 7 Trace Code String s = "Java"; s = "HTML"; animation

8. 8 Trace Code String s = "Java"; s = "HTML"; animation

9. 9 Interned Strings Since strings are immutable and are frequently used, to improve efficiency and save memory, the JVM uses a unique instance for string literals with the same character sequence. Such an instance is called interned. You can also use a String object’s intern method to return an interned string. For example, the following statements:

10. 10 Examples display s1 == s is false s2 == s is true s == s3 is true A new object is created if you use the new operator. If you use the string initializer, no new object is created if the interned object is already created.

11. 11 Trace Code animation

12. 12 Trace Code

13. 13 Trace Code

14. 14 Trace Code

15. 15 Finding String Length Finding string length using the length() method: message = "Welcome"; message.length() (returns 7 )

16. 16 Retrieving Individual Characters in a String  Do not use message[0]  Use message.charAt(index)  Index starts from 0

17. 17 Retrieving Individual Characters in a String public class StringProcess { public static void main (String[] args) { String s = "Java"; System.out.println(s.charAt(2)); }

18. 18 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);

19. 19 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";

20. 20 Extracting Substrings public class StringProcess { public static void main (String[] args) { String s1 = "Welcome to Java"; String s2 = s1.substring(0, 11) + "HTML"; System.out.println(s2); }

21. 21 String Comparisons F 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 }

22. 22 String Comparisons public class StringProcess { public static void main (String[] args) { String s1 = new String("Welcome"); String s2 = "Welcome"; System.out.println(s1.equals(s2)); // s1 and s2 have the same contents System.out.println(s1==s2); // s1 and s2 are the same object }

23. 23 String Comparisons, cont. F 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

24. 24 String Comparisons, cont. public class StringProcess { public static void main (String[] args) { String s1 = new String("Welcome"); String s2 = "Welcome"; System.out.println(s1.compareTo(s2) > 0); System.out.println(s1.compareTo(s2) == 0); System.out.println(s1.compareTo(s2) <0); } public class StringProcess { public static void main (String[] args) { String s1 = "abc"; String s2 = "abg"; System.out.println(s1.compareTo(s2)); }

25. 25 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: F toLowerCase –E.g. “Welcome”.toLowerCase() returns a new string, welcome. F toUpperCase –E.g. “Welcome”.toUpperCase() returns a new string, WELCOME. F trim –Returns a new string by eliminating blank characters from both ends of the string. F replace(oldChar, newChar) –Use to replace all occurrences of a character in the string with a new character.

26. 26 String Conversions public class StringProcess { public static void main (String[] args) { String s = "Welcome"; System.out.println(s.toLowerCase()); System.out.println(s.toUpperCase()); System.out.println(s.trim()); System.out.println(s.replace('e', 'A')); System.out.println(s.replaceFirst("e", "A")); System.out.println(s.replaceAll("e", "A")); }

27. 27 Finding a Character or a Substring in a String "Welcome to Java".indexOf('W') returns 0. "Welcome to Java".indexOf('x') returns -1. - Return the index of the first character in the string that matches the specified character. "Welcome to Java".indexOf('o', 5) returns 9. - Returns the index of the first character in the starting from the specified index that matches the specified character. "Welcome to Java".indexOf("come") returns 3. -Returns the index of the first character of the substring in the string that matches the specified string.

28. 28 "Welcome to Java".indexOf("Java", 5) returns 11. "Welcome to Java".indexOf("java", 5) returns - Returns the index of the first character of the substring in the string starting from the specified index that matches the specified string. "Welcome to Java".lastIndexOf('a') returns 14.

29. 29 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’.

30. 30 Convert String to Array char[] chars = “Java”.toCharArray(); F chars[0] is ‘J’, chars[1] is ‘a’, chars[2] is ‘v’ and chars[3] is ‘a’. F getChars(int srcBegin, int srcEnd, char[] dst, int dstBegin). The following copies a substring “3720” in “CS3720” from index 2 to index 6-1 into the character array dst starting from index 4: char[] dst = {‘J’,’a’,’v’,’a’,’1’,’3’,’0’,’1’}; “CS3720”.getChars(2, 6, dst, 4); dst becomes {‘J’,’a’,’v’,’a’,’3’,’7’,’2’,’0’}

31. 31 String str = new String(new char[]{‘J’,’a’, ‘v’, ‘a’}); Or String str = String.valueOf(new char[] {‘J’,’a’, ‘v’, ‘a’}); F String.valueOf(5.44) converts 5.44 to String, ‘5’, ‘.’,’4’ and’4’. F Double.parseDouble(str) or Integer.parseInt(str) to convert string double value or an int value.

32. 32 Example: Finding Palindromes F Objective: Checking whether a string is a palindrome: a string that reads the same forward and backward. CheckPalindrome Run

33. 33 1. import javax.swing.JOptionPane; 2. public class CheckPalindrome { 3. public static void main(String[] args) { 4. String s = JOptionPane.showInputDialog(“Enter a string:”); 5. String output = “”; 6. if (isPalindrome(s)) 7. output = s + “ is a palindrome”; 8. else 9. output = s + “ is not a palindrome”; 10. JOptionPane.showMessageDialog(null, output); 11. } 12. public static boolean isPalindrome(String s) { 13. int low = 0; 14. int high = s.length()-1; 15. while (low < high) { 16. if (s.charAt(low) != s.charAt(high)) 17. return false; 18. low++; 19. high--; 20. } 21. return true; 22. }}

34. 34 The Character Class

35. 35 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

36. 36 Example: Counting Each Letter in a String This example gives a program that counts the number of occurrence of each letter in a string. Assume the letters are not case-sensitive. Refer to page 298, Liang textbook, sixth edition). CountEachLetter Run

37. 37 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.

38. 38 Obtaining file properties and manipulating file

39. 39 Example: Using the File Class Objective: Write a program that demonstrates how to create files in a platform-independent way and use the methods in the File class to obtain their properties. Left figure shows a sample run of the program on Windows, and right figure a sample run on Unix.

40. 40 Example: Using the File Class public class TestFileClass { public static void main(String[] args) { java.io.File file = new java.io.File("image/us.gif"); System.out.println("Does it exist? " + file.exists()); System.out.println("Can it be read? " + file.canRead()); System.out.println("Can it be written? " + file.canWrite()); System.out.println("Is it a directory? " + file.isDirectory()); System.out.println("Is it a file? " + file.isFile()); System.out.println("Is it absolute? " + file.isAbsolute()); System.out.println("Is it hidden? " + file.isHidden()); System.out.println("Absolute path is " + file.getAbsolutePath()); System.out.println("Last modified on " + new java.util.Date(file.lastModified())); }

41. 41 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.

42. 42 Writing Data Using PrintWriter

43. 43 Writing Data Using PrintWriter public class WriteData { public static void main(String[] args) throws Exception { java.io.File file = new java.io.File("scores.txt"); if (file.exists()) { System.out.println("File already exists"); System.exit(0); } // Create a file java.io.PrintWriter output = new java.io.PrintWriter(file); // Write formatted output to the file output.print("John T Smith "); output.println(90); output.print("Eric K Jones "); output.println(85); // Close the file output.close(); } In file scores.txt, the following text will be printed: John T Smith 90 Eric K Jones 85

44. 44 Reading Data Using Scanner

45. 45 Reading Data Using Scanner public class ReadData { public static void main(String[] args) throws Exception { // Create a File instance java.io.File file = new java.io.File("scores.txt"); // Create a Scanner for the file java.util.Scanner input = new java.util.Scanner(file); // Read data from a file while (input.hasNext()) { String firstName = input.next(); String mi = input.next(); String lastName = input.next(); int score = input.nextInt(); System.out.println( firstName + " " + mi + " " + lastName + " " + score); } // Close the file input.close(); }

46. 46 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.

47. 47 Example: Replacing Text import java.io.*; import java.util.*; public class ReplaceText { public static void main(String[] args) throws Exception { // Check command line parameter usage if (args.length != 4) { System.out.println( "Usage: java ReplaceText sourceFile targetFile oldStr newStr"); System.exit(0); } // Check if source file exists File sourceFile = new File(args[0]); if (!sourceFile.exists()) { System.out.println("Source file " + args[0] + " does not exist"); System.exit(0); } // Check if target file exists File targetFile = new File(args[1]); if (targetFile.exists()) { System.out.println("Target file " + args[1] + " already exists"); System.exit(0); } // Create input and output files Scanner input = new Scanner(sourceFile); PrintWriter output = new PrintWriter(targetFile); while (input.hasNext()) { String s1 = input.nextLine(); String s2 = s1.replaceAll(args[2], args[3]); output.println(s2); } input.close(); output.close(); }