Chapter 8 Strings and Text I/O

1Liang, Introduction to Java Programming, Sixth Edition, (c) 2005 Pearson Education, Inc. All rights reserved. 0-13-148952-6

Chapter 8 Strings and Text I/O

Chapter 7 Objects and Classes

Chapter 8 Strings and Text I/O

Chapter 9 Inheritance and Polymorphism

Chapter 6 Arrays

§10.2, “Abstract Classes”

§10.4, “Interfaces”

Chapter 13 Graphics

Chapter 14 Event-Driven Programming

Chapter 17 Exceptions and Assertions

Chapter 18 Binary I/O

Exception and binary I/O can be covered after Chapter 9

GUI can be covered after §10.2, “Abstract Classes” Chapter 12 GUI Basics

Chapter 11 Object-Oriented Design


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).


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


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


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 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";


Trace Code

String s = "Java";

s = "HTML";

: String

String object for "Java"

s

After executing String s = "Java";

After executing s = "HTML";

: String


: String

String object for "HTML"

Contents cannot be changed

This string object is now unreferenced

s

animation


Trace Code

String s = "Java";

s = "HTML";

: String


s

After executing String s = "Java";

After executing s = "HTML";

: String


: String

String object for "HTML"

Contents cannot be changed

This string object is now unreferenced

s

animation


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:


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 = s.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


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

: String


s

animation



: String


: String


s

s1



: String


: String


s

s1

s2



: String


: String


s

s1

s2

s3


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

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


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";

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)


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 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 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’.


Example:Finding Palindromes

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

CheckPalindromeCheckPalindrome RunRun


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


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


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.

CountEachLetterCountEachLetter RunRun


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 contentsinto a string buffer. However, the value ofa String object is fixed once the string is created.


java.lang.StringBuffer

+StringBuffer()

+StringBuffer(capacity: int)

+StringBuffer(str: String)

+append(data: char[]): StringBuffer

+append(data: char[], offset: int, len: int): StringBuffer

+append(v: aPrimitiveType): StringBuffer

+append(str: String): StringBuffer

+capacity(): int

+charAt(index: int): char

+delete(startIndex: int, endIndex: int): StringBuffer

+deleteCharAt(int index): StringBuffer

+insert(index: int, data: char[], offset: int, len: int): StringBuffer

+insert(offset: int, data: char[]): StringBuffer

+insert(offset: int, b: aPrimitiveType): StringBuffer

+insert(offset: int, str: String): StringBuffer

+length(): int

+replace(int startIndex, int endIndex, String str): StringBuffer

+reverse(): StringBuffer

+setCharAt(index: int, ch: char): void

+setLength(newLength: int): void

+substring(startIndex: int): String

+substring(startIndex: int, endIndex: int): String

Constructs an empty string buffer with capacity 16

Constructs a string buffer with the specified capacity

Constructs a string buffer with the specified string

Appends a char array into this string buffer

Appends a subarray in data into this string buffer

Appends a primitive type value as string to this buffer

Appends a string to this string buffer

Returns the capacity of this string buffer

Returns the character at the specified index

Deletes characters from startIndex to endIndex

Deletes a character at the specified index

Inserts a subarray of the data in the array to the buffer at the specified index

Inserts data to this buffer at the position offset

Inserts a value converted to string into this buffer

Inserts a string into this buffer at the position offset

Returns the number of characters in this buffer

Replaces the characters in this buffer from startIndex to endIndex with the specified string

Reveres the characters in the buffer

Sets a new character at the specified index in this buffer

Sets a new length in this buffer

Returns a substring starting at startIndex

Returns a substring from startIndex to endIndex


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 charactersas the string argument. Initial capacity 16plus the length of the string argument.


Appending New Contentsinto a String Buffer

StringBuffer strBuf = new StringBuffer();strBuf.append("Welcome");strBuf.append(' ');strBuf.append("to");strBuf.append(' ');strBuf.append("Java");


Example:Checking Palindromes Ignoring Non-alphanumeric Characters

PalindromeIgnoreNonAlphanumeric PalindromeIgnoreNonAlphanumeric RunRun


Main Method Is Just a Regular Method

public class A { public static void main(String[] args) { String[] strings = {"New York", "Boston", "Atlanta"}; B.main(strings); } }

class B { public static void main(String[] args) { for (int i = 0; i < args.length; i++) System.out.println(args[i]); } }

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


ProcessingCommand-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.


Example: Using Command-Line Parameters

Objective: Write a program that will perform binary operations on integers. The program receives three parameters: an operator and two integers.

CalculatorCalculator

java Calculator 2 + 3

java Calculator 2 - 3

RunRun java Calculator 2 / 3

java Calculator 2 “*” 3


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

matches is a method in the String class.

"Java".matches("Java"); //returns true

"Java".equals("Java"); // returns true

Yet, following calls also return true

"Java is fun".matches("Java.*")

"Java is cool".matches("Java.*")

"Java is powerful".matches("Java.*")


Regular Expression Syntax

Regular Expression Matches Example x a specified character x Java matches Java . any single character Java matches J..a (ab|cd) a, b, or c ten matches t(en|im] [abc] a, b, or c Java matches Ja[uvwx]a [âbc] any character except Java matches Ja[ârs]a

a, b, or c [a-z] a through z Java matches [A-M]av[a-d] [â-z] any character except Java matches Jav[^b-d] a through z [a-e[m-p]] a through e or Java matches m through p [A-G[I-M]]av[a-d] [a-e&&[c-p]] intersection of a-e Java matches with c-p [A-P&&[I-M]]av[a-d] \d a digit, same as [1-9] Java2 matches "Java[\\d]" \D a non-digit $Java matches "[\\D][\\D]ava" \w a word character Java matches "[\\w]ava" \W a non-word character $Java matches "[\\W][\\w]ava" \s a whitespace character "Java 2" matches "Java\\s2" \S a non-whitespace char Java matches "[\\S]ava" p* zero or more Java matches "[\\w]*" occurrences of pattern p p+ one or more Java matches "[\\w]+" occurrences of pattern p p? zero or one Java matches "[\\w]?Java" occurrence of pattern p Java matches "[\\w]?ava" p{n} exactly n Java matches "[\\w]{4}" occurrences of pattern p p{n,} at least n Java matches "[\\w]{3,}" occurrences of pattern p p{n,m} between n and m Java matches "[\\w]{1,9}" occurrences (inclusive)


Replacing and Splitting Strings

java.lang.String

+matches(regex: String): boolean

+replaceAll(regex: String, replacement: String): String

+replaceFirst(regex: String, replacement: String): String

+split(regex: String): String[]

Returns true if this string matches the pattern.

Returns a new string that replaces all matching substrings with the replacement.

Returns a new string that replaces the first matching substring with the replacement.

Returns an array of strings consisting of the substrings split by the matches.


ExamplesString s = "Java Java Java".replaceAll("v\\w", "wi") ;s become “Jawi Jawi Jawi”.

String s = "Java Java Java".replaceFirst("v\\w", "wi") ;

s become “Jawi Java Java”.

String[] s = "Java1HTML2Perl".split("\\d");s[0] holds: “Java”s[1] holds: “HTML”s[2] holds: “Perl”


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.


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


Example: Using the File Class

TestFileClassTestFileClass RunRun

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. Figure 16.1 shows a sample run of the program on Windows, and Figure 16.2 a sample run on Unix.


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

WriteDataWriteData RunRun

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.”


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.

ReadDataReadData RunRun


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.

ReplaceTextReplaceText RunRun


Scanning Primitive Type ValuesIf 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.

JDK 1.5Feature

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:

JDK 1.5Feature

System.out.print("Please enter an int value: ");Scanner scanner = new Scanner(System.in);int i = scanner.nextInt();

Documents

Chapter 8 Strings and Text I/O