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Chapter 8 – Strings and Characters 1

Introduction

• String and character processing– Class java.lang.String– Class java.lang.StringBuffer– Class java.lang.Character– Class java.util.StringTokenizer

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Fundamentals of Strings

• String– Series of characters treated as single unit– May include letters, digits, etc.– Object of class String– You should refer to http://java.sun.com for the API details– Once created the content stored in a String object cannot

be changed (immutable).

• Java provides a rich library for manupilating Strings.

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String Constructors

• String has the following constructors:

public String();

public String(byte ascii[]);

public String(byte ascii[], int offset, int count);

public String(char value[]);

public String(char value[], int offset, int count);

public String(String value);

public String(StringBuffer buffer);

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Example// StringConstructors.java// This program demonstrates the String class constructors.

public class StringConstructors { public static void main (String args[]) {

char charArray[] = { 'b', 'i', 'r', 't', 'h', ' ', 'd', 'a', 'y' }; byte byteArray[] = { 'n', 'e', 'w', ' ', 'y', 'e', 'a', 'r' }; StringBuffer buffer; String s, s1, s2, s3, s4, s5, s6, s7;

s = new String( "hello" ); buffer = new StringBuffer(); buffer.append( "Welcome to Java Programming!" );

// use the String constructors s1 = new String(); s2 = new String( s ); s3 = new String( charArray ); s4 = new String( charArray, 6, 3 ); s5 = new String( byteArray, 4, 4 ); s6 = new String( byteArray ); s7 = new String( buffer );

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System.out.println( "s1 = " + s1 ); System.out.println( "s2 = " + s2 ); System.out.println( "s3 = " + s3 ); System.out.println( "s4 = " + s4 ); System.out.println( "s5 = " + s5 ); System.out.println( "s6 = " + s6 ); System.out.println( "s7 = " + s7 ); }}

Output

s1 =s2 = hellos3 = birth days4 = days5 = years6 = new years7 = Welcome to Java Programming!

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String Methods length, charAt and getChars

• Method length– Determine String length

• Like arrays, Strings always “know” their size• Unlike array, Strings do not have length instance variable

• Method charAt– Get character at specific location in String – Like array, the first character begins with index 0

• Method getChars– Get entire set of characters in String – void getChars(int srcBegin, int srcEnd, char dst[], int dstBegin);

– Copies characters from this string into the destination char array. The first character to be copied is at index srcBegin; the last character to be copied is at index srcEnd-1.

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Examplepublic class StringMiscellaneous { public static void main( String args[] ){ String s1, output = ""; char charArray[] = new char[ 5 ];; s1 = new String( "hello there" );

// output the string System.out.println("s1: " + s1);

// test length method System.out.println("Length of s1: " + s1.length());

// loop through characters in s1 and display reversed System.out.println("The string reversed is: ");

for ( int count = s1.length() - 1; count >= 0; count-- ) System.out.print( s1.charAt(count) + " " );

// copy characters from string into char array s1.getChars( 6, 11, charArray, 0 ); System.out.println("\nThe character array is: ");

for ( int count = 0; count < charArray.length; count++ ) output += charArray[ count ];

System.out.println(output); }} // end class StringMiscellaneous

>java StringMiscellaneouss1: hello thereLength of s1: 11The string reversed is:e r e h t o l l e hThe character array is:there

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Comparing Strings

• You CANNOT use relational operators like ==, <= to compare two Strings.

• String class provides the following methods for comparing two stringspublic int compareTo(String anotherString);

Returns the value 0 if the argument string is equal to this string; a value less than 0 if this string is lexicographically less than the string argument; and a value greater than 0 if this string is lexicographically greater than the string argument.

public boolean equals(Object anObject);Returns true if the Strings are equal; false otherwise.

public boolean equalsIgnoreCase(String anotherString); Returns true if the argument is not null and the Strings are equal, ignoring case; false otherwise.

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Examplepublic class StringCompare {

public static void main(String args[]){ String s1, s2, s3, s4, output;

s1 = new String( "hello" ); s2 = new String( "good bye" ); s3 = new String( "Happy Birthday" ); s4 = new String( "happy birthday" );

output = "s1 = " + s1 + "\ns2 = " + s2 + "\ns3 = " + s3 + "\ns4 = " + s4 + "\n\n";

// test for equality if ( s1.equals( "hello" ) ) output += "s1 equals \"hello\"\n"; else output += "s1 does not equal \"hello\"\n";

// test for equality with == if ( s1 == "hello" ) output += "s1 equals \"hello\"\n"; else output += "s1 does not equal \"hello\"\n";

>java StringCompares1 = hellos2 = good byes3 = Happy Birthdays4 = happy birthday

s1 equals "hello"s1 does not equal "hello"

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Example // test for equality (ignore case) if ( s3.equalsIgnoreCase( s4 ) ) output += "s3 equals s4\n"; else output += "s3 does not equal s4\n";

// test compareTo output += "\ns1.compareTo( s2 ) is " + s1.compareTo( s2 ) + "\ns2.compareTo( s1 ) is " + s2.compareTo( s1 ) + "\ns1.compareTo( s1 ) is " + s1.compareTo( s1 ) + "\ns3.compareTo( s4 ) is " + s3.compareTo( s4 ) + "\ns4.compareTo( s3 ) is " + s4.compareTo( s3 ) + "\n\n";

System.out.print( output ); }}

>java StringCompares1 = hellos2 = good byes3 = Happy Birthdays4 = happy birthday.......s3 equals s4

s1.compareTo( s2 ) is 1s2.compareTo( s1 ) is -1s1.compareTo( s1 ) is 0s3.compareTo( s4 ) is -32s4.compareTo( s3 ) is 32

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StartsWith and EndsWith

• boolean startsWith(String prefix)

– Tests if this string starts with the specified prefix.

• boolean endsWith(String suffix)

– Tests if this string ends with the specified suffix.

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Examplepublic class StringStartEnd { public static void main( String args[] ) { String strings[] = { "started", "starting", "ended", "ending" }; String output = "";

// test method startsWith for ( int count = 0; count < strings.length; count++ )

if ( strings[ count ].startsWith( "st" ) ) output += "\"" + strings[ count ] + "\" starts with \"st\"\n";

output += "\n";

// test method endsWith for ( int count = 0; count < strings.length; count++ )

if ( strings[ count ].endsWith( "ed" ) ) output += "\"" + strings[ count ] + "\" ends with \"ed\"\n";

System.out.print( output ); }

} // end class StringStartEnd

>java StringStartEnd"started" starts with "st""starting" starts with "st"

"started" ends with "ed""ended" ends with "ed"

13Locating Characters and Substrings in Strings

• To locate a character or a substring in a String Object, you can use the overloaded indexOf()and lastIndexOf()

public int indexOf(int ch);

public int indexOf(int ch, int fromIndex);

the index of the first occurrence of the character in the character sequence represented by this object that is greater than or equal to fromIndex, or -1 if the character does not occur.

public int indexOf(String str);

public int indexOf(String str, int fromIndex);

public int lastIndexOf(int ch);

public int lastIndexOf(int ch, int fromIndex);

public int lastIndexOf(String str);

public int lastIndexOf(String str, int fromIndex);

If the string argument occurs as a substring within this object at a starting index no greater than fromIndex then the index of the first character of the last such substring is returned. If it does not occur as a substring starting at fromIndex or earlier, -1 is returned.

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Exampleclass StringIndexMethods { public static void main (String args[]) {

String letters = "abcdefghijklmabcdefghijklm";

// test indexOf to locate a character in a string System.out.println("'c' is located at index " + letters.indexOf('c') );

System.out.println("'a' is located at index " + letters.indexOf('a', 1 ) );

System.out.println("'$' is located at index " + letters.indexOf('$' ) );

System.out.println();

// test lastIndexOf to find a substring in a string System.out.println("Last \"def\" is located at index " + letters.lastIndexOf( "def" ) );

System.out.println("Last \"def\" is located at index " + letters.lastIndexOf( "def", 10 ) );

System.out.println("Last \"hello\" is located at index " + letters.lastIndexOf( "hello" ) ); }}

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Output

>java StringIndexMethods'c' is located at index 2'a' is located at index 13'$' is located at index -1

Last "def" is located at index 16Last "def" is located at index 3Last "hello" is located at index -1

letters = "abcdefghijklmabcdefghijklm";

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Extracting Substrings from Strings• String class provides the following methods for extracting

substrings:public String substring(int beginIndex);

public String substring(int beginIndex, int endIndex);

Returns a new string that is a substring of this string. The substring begins at the specified beginIndex and extends to the character at index endIndex - 1. Thus the length of the substring is endIndex-beginIndex.

• Example: if s = "hamburger",

– s.substring(3) returns "burger"

– s.substring(6) returns "ger"

– s.substring(9) returns "" (an empty string)

– s.substring(4, 8) returns "urge"

– s.substring(1, 3) returns "am"

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Exampleclass StringSubstring { public static void main (String args[]) {

String letters = "abcdefghijklmabcdefghijklm";

// test substring System.out.println( "\nSubstring from index 20 to end is " + "\"" + letters.substring( 20 ) + "\"");

System.out.println( "Substring from index 0 upto 6 is " + "\"" + letters.substring( 0, 6 ) + "\""); }}

>java StringSubstring

Substring from index 20 to end is "hijklm"Substring from index 0 upto 6 is "abcdef"

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Miscellaneous String Methods

• String class has several methods that return modified copies of a string object to a character array:– public String concat(String str);

– public String replace(char oldChar, char newChar);• returns a string derived from this string by replacing every

occurrence of oldChar with newChar.

– public char[] toCharArray();

– public String toLowerCase();

– public String toString();

– public String toUpperCase();

– public String trim(); • returns this string, with whitespace removed from the front and

end

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Example

class StringMisc { public static void main (String args[]) {

String s1 = new String( "hello" ), s2 = new String( "GOOD BYE" ), s3 = new String( " spaces " );

System.out.println( );

// test method replace System.out.println( "Replace 'l' with 'L' in s1: " + s1.replace( 'l', 'L' ) ); System.out.println( );

// test toLowerCase and toUpperCase System.out.println( "s1 after toUpperCase = " + s1.toUpperCase() ); System.out.println( "s2 after toLowerCase = " + s2.toLowerCase() ); System.out.println( );

>java StringMisc

Replace 'l' with 'L' in s1: heLLo

s1 after toUpperCase = HELLOs2 after toLowerCase = good bye

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Example

// test trim method System.out.println( "s3 after trim = \"" + s3.trim() + "\""); System.out.println( );

// test toString method System.out.println( "s1 = " + s1.toString() ); System.out.println( );

// test toCharArray method char charArray[] = s1.toCharArray(); System.out.println( "s1 as a character array = " ); for ( int i = 0; i < charArray.length; i++ ) { System.out.print( charArray[i]); // use method print(char) System.out.print( " " ); } System.out.println(); }}

>java StringMisc.......

s3 after trim = "spaces"

s1 = hello

s1 as a character array =h e l l o

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Using String Method valueOf• Java provides a set of static valueOf methods that takes

arguments of various types, convert those arguments to strings and return them as String objects.– public static String valueOf(boolean b);

– public static String valueOf(char c);

– public static String valueOf(char data[]);

– public static String valueOf(char data[], int offset, int count);

– public static String valueOf(double d);

– public static String valueOf(float f);

– public static String valueOf(int i);

Code Output System.out.print(String.valueOf(true)); true System.out.print(String.valueOf(12000)); 12000 System.out.print(String.valueOf('A')); A System.out.print(String.valueOf(24.72)); 24.72

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Important!

• All the above String methods do NOT modify the String itself. It just return a NEW String objects.

class S2 { public static void main (String args[]) {

String s = " Hello World! ";

// test substring System.out.println( "\nSubstring from index 5 to end is " + "\"" + s.substring( 5 ) + "\""); System.out.println( "s after trim = \"" + s.trim() + "\"");

System.out.println( "s = \"" + s + "\""); }}

>java S2Substring from index 5 to end is "llo World! "s after trim = "Hello World!"s = " Hello World! "

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Example• Write a program that asks for user's name (always consists of

two names) and then writes it back with the first name as entered, and the second name in capital letters.

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import javax.swing.*;

class CapName { public static void main (String args[]) { String inName; String outName; int spaceLoc;

inName = JOptionPane.showInputDialog("Enter your name:");

// find space spaceLoc = inName.indexOf(' ');

// get first name and last name outName = inName.substring(0, spaceLoc) + " " + inName.substring(spaceLoc+1).toUpperCase();

JOptionPane.showMessageDialog(null, "Welcome, " + outName); System.exit(0);

}}

Michael Jackson

spaceLoc

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Command Line Arguments

• When runs an application, you can supply a list of arguments - command line argument

• The arguments are passed as an array of String (args in the above example) to the main method.

• public static void main(String args[])

What does this parameter mean?

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Example 1

import java.util.*;

public class CommandLineArgument { public static void main( String args[] ) { for (int i=0; i<args.length; i++) {

System.out.println(args[i]); }

}}

>java CommandLineArgument

>java CommandLineArgument Hello Peter ChanHelloPeterChan

>java CommandLineArgument "Hello Peter Chan" "Hello Mary Wong"Hello Peter ChanHello Mary Wong

One string object

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Example 2

import java.util.*;

public class AddTwoIntegers { public static void main( String args[] ) { int num1, num2; if (args.length != 2) { System.out.println( "Usage : java AddTwoIntegers <num1> <num2>" ); System.exit(0); }

num1 = Integer.parseInt(args[0]); num2 = Integer.parseInt(args[1]);

System.out.println("The sum is " + (num1+num2)); }}

>java AddTwoIntegersUsage : java AddTwoIntegers <num1> <num2>

>java AddTwoIntegers 23 55The sum is 78

• Write a program to sum up two command line arguments.

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Exercise

>java AddIntegersThe sum is 0

>java AddIntegers 22 35The sum is 57

>java AddIntegers 12 3 5 26The sum is 44

• Write a program to sum up all command line arguments.