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Chapter 9 Strings and Text I/O

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Objectives 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 PrintWriter class. To read data from a file using the Scanner class.

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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

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java.lang.String

+String()

+String(value: String)

+String(value: char[])

+charAt(index: int): char

+compareTo(anotherString: String): int

+compareToIgnoreCase(anotherString: String): int

+concat(anotherString: String): String

+endsWith(suffix: String): boolean

+equals(anotherString: String): boolean

+equalsIgnoreCase(anotherString: String): boolean

+getChars(int srcBegin, int srcEnd, char[] dst, int dstBegin): void

+indexOf(ch: int): int

+indexOf(ch: int, fromIndex: int): int

+indexOf(str: String): int

+indexOf(str: String, fromIndex: int): int

+lastIndexOf(ch: int): int

+lastIndexOf(ch: int, fromIndex: int): int

+lastIndexOf(str: String): int

+lastIndexOf(str: String, fromIndex: int): int

+regionMatches(toffset: int, other: String, offset: int, len: int): boolean

+length(): int

+replace(oldChar: char, newChar: char): String

+startsWith(prefix: String): boolean

+subString(beginIndex: int): String

+subString(beginIndex: int, endIndex: int): String

+toCharArray(): char[]

+toLowerCase(): String

+toString(): String

+toUpperCase(): String

+trim(): String

+copyValueOf(data: char[]): String

+valueOf(c: char): String

+valueOf(data: char[]): String

+valueOf(d: double): String

+valueOf(f: float): String

+valueOf(i: int): String

+valueOf(l: long): String

Constructs an empty string

Constructs a string with the specified string literal value

Constructs a string with the specified character array

Returns the character at the specified index from this string

Compares this string with another string

Compares this string with another string ignoring case

Concat this string with another string

Returns true if this string ends with the specified suffix

Returns true if this string is equal to anther string

Checks if this string equals anther string ignoring case

Copies characters from this string into the destination character array

Returns the index of the first occurrence of ch

Returns the index of the first occurrence of ch after fromIndex

Returns the index of the first occurrence of str

Returns the index of the first occurrence of str after fromIndex

Returns the index of the last occurrence of ch

Returns the index of the last occurrence of ch before fromIndex

Returns the index of the last occurrence of str

Returns the index of the last occurrence of str before fromIndex

Returns true if the specified subregion of this string exactly matches the specified subregion of the string argument

Returns the number of characters in this string

Returns a new string with oldChar replaced by newChar

Returns true if this string starts with the specified prefix

Returns the substring from beginIndex

Returns the substring from beginIndex to endIndex-1.

Returns a char array consisting characters from this string

Returns a new string with all characters converted to lowercase

Returns a new string with itself

Returns a new string with all characters converted to uppercase

Returns a string with blank characters trimmed on both sides

Returns a new string consisting of the char array data

Returns a string consisting of the character c

Same as copyValueOf(data: char[]): String

Returns a string representing the double value

Returns a string representing the float value

Returns a string representing the int value

Returns a string representing the long value

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Constructing StringsString 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";

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Strings Are ImmutableA String object is immutable; its contents cannot be changed.

Does the following code change the contents of the string?

String s = "Java";

s = "HTML";

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Trace Code

String s = "Java";

s = "HTML";

: String

String object for "Java"

s

After executing String s = "Java";

After executing s = "HTML";

: String

String object for "Java"

: String

String object for "HTML"

Contents cannot be changed

This string object is now unreferenced

s

animation

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Trace Code

String s = "Java";

s = "HTML";

: String

String object for "Java"

s

After executing String s = "Java";

After executing s = "HTML";

: String

String object for "Java"

: String

String object for "HTML"

Contents cannot be changed

This string object is now unreferenced

s

animation

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Interned StringsSince 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:

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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.

String s = "Welcome to Java"; String s1 = new String("Welcome to Java"); String s2 = s1.intern(); String s3 = "Welcome to Java"; System.out.println("s1 == s is " + (s1 == s)); System.out.println("s2 == s is " + (s2 == s)); System.out.println("s == s3 is " + (s == s3));

: String

Interned string object for "Welcome to Java"

: String

A string object for "Welcome to Java"

s

s1

s2

s3

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Trace Code String s = "Welcome to Java"; String s1 = new String("Welcome to Java"); String s2 = s1.intern(); String s3 = "Welcome to Java";

: String

Interned string object for "Welcome to Java"

s

animation

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Trace Code String s = "Welcome to Java"; String s1 = new String("Welcome to Java"); String s2 = s1.intern(); String s3 = "Welcome to Java";

: String

Interned string object for "Welcome to Java"

: String

A string object for "Welcome to Java"

s

s1

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Trace Code String s = "Welcome to Java"; String s1 = new String("Welcome to Java"); String s2 = s1.intern(); String s3 = "Welcome to Java";

: String

Interned string object for "Welcome to Java"

: String

A string object for "Welcome to Java"

s

s1

s2

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Trace Code String s = "Welcome to Java"; String s1 = new String("Welcome to Java"); String s2 = s1.intern(); String s3 = "Welcome to Java";

: String

Interned string object for "Welcome to Java"

: String

A string object for "Welcome to Java"

s

s1

s2

s3

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Finding String Length

Finding string length using the length() method:

message = "Welcome";

message.length() (returns 7)

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Retrieving Individual Characters in a String

Do not use message[0]

Use message.charAt(index)

Index starts from 0

W e l c o m e t o J a v a

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

message

Indices

message.charAt(0) message.charAt(14) message.length() is 15

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Retrieving Individual Characters in a String

public class StringProcess {public static void main (String[] args) { String s = "Java";

System.out.println(s.charAt(2));}}

public class StringProcess {public static void main (String[] args) { String s = "Java";

System.out.println(s.charAt(2));}}

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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);

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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";

W e l c o m e t o J a v a

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

message

Indices

message.substring(0, 11) message.substring(11)

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

System.out.println(s2);}}

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);}}

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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 }

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String Comparisonspublic 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

}}

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

}}

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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

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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 = 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));}}

public class StringProcess {public static void main (String[] args) { String s1 = "abc"; String s2 = "abg";

System.out.println(s1.compareTo(s2));}}

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String ConversionsThe 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– E.g. “Welcome”.toLowerCase() returns a new string, welcome.

toUpperCase– E.g. “Welcome”.toUpperCase() returns a new string, WELCOME.

trim– Returns a new string by eliminating blank characters from both

ends of the string. replace(oldChar, newChar)

– Use to replace all occurrences of a character in the string with a new character.

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String Conversionspublic 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"));}}

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"));}}

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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.

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"Welcome to Java".indexOf("Java", 5) returns 11.

"Welcome to Java".indexOf("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".lastIndexOf('a') returns 14.

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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’.

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Convert String to Arraychar[] chars = “Java”.toCharArray();

chars[0] is ‘J’, chars[1] is ‘a’, chars[2] is ‘v’ and chars[3] is ‘a’.

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’}

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String str = new String(new char[]{‘J’,’a’, ‘v’, ‘a’});

Or

String str = String.valueOf(new char[] {‘J’,’a’, ‘v’, ‘a’});

String.valueOf(5.44) converts 5.44 to String, ‘5’, ‘.’,’4’ and’4’.

Double.parseDouble(str) or Integer.parseInt(str) to convert string double value or an int value.

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Example:Finding Palindromes

Objective: Checking whether a string is a palindrome: a string that reads the same forward and backward.

CheckPalindromeCheckPalindrome RunRun

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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. else9. 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. }}

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The Character Class

java.lang.Character

+Character(value: char)

+charValue(): char

+compareTo(anotherCharacter: Character): int

+equals(anotherCharacter: Character): boolean

+isDigit(ch: char): boolean

+isLetter(ch: char): boolean

+isLetterOrDigit(ch: char): boolean

+isLowerCase(ch: char): boolean

+isUpperCase(ch: char): boolean

+toLowerCase(ch: char): char

+toUpperCase(ch: char): char

Constructs a character object with char value

Returns the char value from this object

Compares this character with another

Returns true if this character equals to another

Returns true if the specified character is a digit

Returns true if the specified character is a letter

Returns true if the character is a letter or a digit

Returns true if the character is a lowercase letter

Returns true if the character is an uppercase letter

Returns the lowercase of the specified character

Returns the uppercase of the specified character

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Examples

Character charObject = new Character('b');

charObject.compareTo(new Character('a')) returns 1charObject.compareTo(new Character('b')) returns 0charObject.compareTo(new Character('c')) returns -1charObject.compareTo(new Character('d') returns –2charObject.equals(new Character('b')) returns truecharObject.equals(new Character('d')) returns false

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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).

CountEachLetterCountEachLetter RunRun

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The File ClassThe 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.

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java.io.File

+File(pathname: String)

+File(parent: String, child: String)

+File(parent: File, child: String)

+exists(): boolean

+canRead(): boolean

+canWrite(): boolean

+isDirectory(): boolean

+isFile(): boolean

+isAbsolute(): boolean

+isHidden(): boolean

+getAbsolutePath(): String

+getCanonicalPath(): String

+getName(): String

+getPath(): String

+getParent(): String

+lastModified(): long

+delete(): boolean

+renameTo(dest: File): boolean

Creates a File object for the specified pathname. The pathname may be a directory or a file.

Creates a File object for the child under the directory parent. child may be a filename or a subdirectory.

Creates a File object for the child under the directory parent. parent is a File object. In the preceding constructor, the parent is a string.

Returns true if the file or the directory represented by the File object exists.

Returns true if the file represented by the File object exists and can be read.

Returns true if the file represented by the File object exists and can be written.

Returns true if the File object represents a directory.

Returns true if the File object represents a file.

Returns true if the File object is created using an absolute path name.

Returns true if the file represented in the File object is hidden. The exact definition of hidden is system-dependent. On Windows, you can mark a file hidden in the File Properties dialog box. On Unix systems, a file is hidden if its name begins with a period character '.'.

Returns the complete absolute file or directory name represented by the File object.

Returns the same as getAbsolutePath() except that it removes redundant names, such as "." and "..", from the pathname, resolves symbolic links (on Unix platforms), and converts drive letters to standard uppercase (on Win32 platforms).

Returns the last name of the complete directory and file name represented by the File object. For example, new File("c:\\book\\test.dat").getName() returns test.dat.

Returns the complete directory and file name represented by the File object. For example, new File("c:\\book\\test.dat").getPath() returns c:\book\test.dat.

Returns the complete parent directory of the current directory or the file represented by the File object. For example, new File("c:\\book\\test.dat").getParent() returns c:\book.

Returns the time that the file was last modified.

Deletes this file. The method returns true if the deletion succeeds.

Renames this file. The method returns true if the operation succeeds.

Obtaining file properties and manipulating file

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Example: Using the File ClassObjective: 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.

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Example: Using the File Classpublic 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())); }}

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())); }}

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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.

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Writing Data Using PrintWriter

java.io.PrintWriter

+PrintWriter(filename: String)

+print(s: String): void

+print(c: char): void

+print(cArray: char[]): void

+print(i: int): void

+print(l: long): void

+print(f: float): void

+print(d: double): void

+print(b: boolean): void

Also contains the overloaded println methods.

Also contains the overloaded printf methods.

.

Creates a PrintWriter for the specified file.

Writes a string.

Writes a character.

Writes an array of character.

Writes an int value.

Writes a long value.

Writes a float value.

Writes a double value.

Writes a boolean value.

A println method acts like a print method; additionally it prints a line separator. The line separator string is defined by the system. It is \r\n on Windows and \n on Unix.

The printf method was introduced in §3.6, “Formatting Console Output and Strings.”

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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 90Eric K Jones 85

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 90Eric K Jones 85

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Reading Data Using Scanner

java.util.Scanner

+Scanner(source: File)

+Scanner(source: String)

+close()

+hasNext(): boolean

+next(): String

+nextByte(): byte

+nextShort(): short

+nextInt(): int

+nextLong(): long

+nextFloat(): float

+nextDouble(): double

+useDelimiter(pattern: String): Scanner

Creates a Scanner that produces values scanned from the specified file.

Creates a Scanner that produces values scanned from the specified string.

Closes this scanner.

Returns true if this scanner has another token in its input.

Returns next token as a string.

Returns next token as a byte.

Returns next token as a short.

Returns next token as an int.

Returns next token as a long.

Returns next token as a float.

Returns next token as a double.

Sets this scanner’s delimiting pattern.

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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(); }}

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(); }}

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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, invokingjava 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.

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Example: Replacing Textimport 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(); }}

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(); }}