Chapter 9 Strings and Text I O 1

Chapter 9 Strings and Text I/O 1

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

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

4

Constructing Strings String new. String = new String(string. Literal); 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"; 5

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"; 6

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

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

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: 9

Examples display s 1 == s is false s 2 == s is true s == s 3 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. 10

animation Trace Code 11

Trace Code 12

Trace Code 13

Trace Code 14

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

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

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

String Concatenation String s 3 = s 1. concat(s 2); String s 3 = s 1 + s 2; s 1 + s 2 + s 3 + s 4 + s 5 same as (((s 1. concat(s 2)). concat(s 3)). concat(s 4)). concat(s 5); 18

Extracting Substrings You can extract a single character from a string using the char. At method. You can also extract a substring from a string using the substring method in the String class. String s 1 = "Welcome to Java"; String s 2 = s 1. substring(0, 11) + "HTML"; 19

Extracting Substrings public class String. Process { public static void main (String[] args) { String s 1 = "Welcome to Java"; String s 2 = s 1. substring(0, 11) + "HTML"; System. out. println(s 2); } } 20

String Comparisons public class String. Process { public static void main (String[] args) { String s 1 = new String("Welcome"); String s 2 = "Welcome"; System. out. println(s 1. equals(s 2)); // s 1 and s 2 have the same contents System. out. println(s 1==s 2); // s 1 and s 2 are the same object } } 22

String Comparisons, cont. F compare. To(Object object) String s 1 = new String("Welcome"); String s 2 = "Welcome"; if (s 1. compare. To(s 2) > 0) { // s 1 is greater than s 2 } else if (s 1. compare. To(s 2) == 0) { // s 1 and s 2 have the same contents } else // s 1 is less than s 2 23

String Comparisons, cont. public class String. Process { public static void main (String[] args) { String s 1 = new String("Welcome"); String s 2 = "Welcome"; public class String. Process { public static void main (String[] args) { String s 1 = "abc"; String s 2 = "abg"; System. out. println(s 1. compare. To(s 2) > 0); System. out. println(s 1. compare. To(s 2) == 0); System. out. println(s 1. compare. To(s 2) <0); } } System. out. println(s 1. compare. To(s 2)); } } 24

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 to. Lower. Case – E. g. “Welcome”. to. Lower. Case() returns a new string, welcome. F to. Upper. Case – E. g. “Welcome”. to. Upper. Case() returns a new string, WELCOME. F trim – Returns a new string by eliminating blank characters from both ends of the string. F replace(old. Char, new. Char) – Use to replace all occurrences of a character in the string with a new character. 25

String Conversions public class String. Process { public static void main (String[] args) { String s = "Welcome"; System. out. println(s. to. Lower. Case()); System. out. println(s. to. Upper. Case()); System. out. println(s. trim()); System. out. println(s. replace('e', 'A')); System. out. println(s. replace. First("e", "A")); System. out. println(s. replace. All("e", "A")); } } 26

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

"Welcome to Java". index. Of("Java", 5) returns 11. "Welcome to Java". index. Of("java", 5) returns -1. - 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". last. Index. Of('a') returns 14. 28

Convert Character and Numbers to Strings The String class provides several static value. Of methods for converting a character, an array of characters, and numeric values to strings. These methods have the same name value. Of with different argument types char, char[], double, long, int, and float. For example, to convert a double value to a string, use String. value. Of(5. 44). The return value is string consists of characters ‘ 5’, ‘. ’, ‘ 4’, and ‘ 4’. 29

Convert String to Array char[] chars = “Java”. to. Char. Array(); F F chars[0] is ‘J’, chars[1] is ‘a’, chars[2] is ‘v’ and chars[3] is ‘a’. get. Chars(int src. Begin, int src. End, char[] dst, int dst. Begin). The following copies a substring “ 3720” in “CS 3720” 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’}; “CS 3720”. get. Chars(2, 6, dst, 4); dst becomes {‘J’, ’a’, ’v’, ’a’, ’ 3’, ’ 7’, ’ 2’, ’ 0’} 30

String str = new String(new char[]{‘J’, ’a’, ‘v’, ‘a’}); Or String str = String. value. Of(new char[] {‘J’, ’a’, ‘v’, ‘a’}); F String. value. Of(5. 44) converts 5. 44 to String, ‘ 5’, ‘. ’, ’ 4’ and’ 4’. F Double. parse. Double(str) or Integer. parse. Int(str) to convert string double value or an int value. 31

Example: Finding Palindromes FObjective: Checking whether a string is a palindrome: a string that reads the same forward and backward. Check. Palindrome Run 32

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

The Character Class 34

Examples Character char. Object = new Character('b'); char. Object. compare. To(new Character('a')) returns 1 char. Object. compare. To(new Character('b')) returns 0 char. Object. compare. To(new Character('c')) returns -1 char. Object. compare. To(new Character('d') returns – 2 char. Object. equals(new Character('b')) returns true char. Object. equals(new Character('d')) returns false 35

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). Count. Each. Letter Run 36

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. 37

Obtaining file properties and manipulating file 38

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. 39

Example: Using the File Class public class Test. File. Class { 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. can. Read()); System. out. println("Can it be written? " + file. can. Write()); System. out. println("Is it a directory? " + file. is. Directory()); System. out. println("Is it a file? " + file. is. File()); System. out. println("Is it absolute? " + file. is. Absolute()); System. out. println("Is it hidden? " + file. is. Hidden()); System. out. println("Absolute path is " + file. get. Absolute. Path()); System. out. println("Last modified on " + new java. util. Date(file. last. Modified())); } } 40

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 Print. Writer classes. 41

Writing Data Using Print. Writer 42

Writing Data Using Print. Writer public class Write. Data { 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. Print. Writer output = new java. io. Print. Writer(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 43

Reading Data Using Scanner 44

Reading Data Using Scanner public class Read. Data { 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. has. Next()) { String first. Name = input. next(); String mi = input. next(); String last. Name = input. next(); int score = input. next. Int(); System. out. println( first. Name + " " + mi + " " + last. Name + " " + score); } // Close the file input. close(); } } 45

Example: Replacing Text Write a class named Replace. Text 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 Replace. Text source. File target. File old. String new. String For example, invoking java Replace. Text Format. String. java t. txt String. Builder String. Buffer replaces all the occurrences of String. Builder by String. Buffer in Format. String. java and saves the new file in t. txt. 46

Example: Replacing Text import java. io. *; import java. util. *; public class Replace. Text { public static void main(String[] args) throws Exception { // Check command line parameter usage if (args. length != 4) { System. out. println( "Usage: java Replace. Text source. File target. File old. Str new. Str"); System. exit(0); } // Check if source file exists File source. File = new File(args[0]); if (!source. File. exists()) { System. out. println("Source file " + args[0] + " does not exist"); System. exit(0); } // Check if target file exists File target. File = new File(args[1]); if (target. File. exists()) { System. out. println("Target file " + args[1] + " already exists"); System. exit(0); } // Create input and output files Scanner input = new Scanner(source. File); Print. Writer output = new Print. Writer(target. File); while (input. has. Next()) { String s 1 = input. next. Line(); String s 2 = s 1. replace. All(args[2], args[3]); output. println(s 2); } input. close(); output. close(); } } 47