Download Chapter 7 (Strings)

Strings
Chapter 7
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
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An object of the String class represents a
string of characters.
The String class belongs to the java.lang
package, which does not require an import
statement.
Like other classes, String has constructors
and methods.
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string: An object storing a sequence of text
characters.
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Two ways to create a string

As a new object through the String class
String name = new String("text");
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Convenient shorthand method
String name = "text";
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immutable.
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String name = "text";
Once created in memory, a string cannot be changed:
none of its methods changes the string
If a string variable is reassigned, the memory address is
deleted and a new memory address is created
Immutable objects are convenient because several
references can point to the same object safely: there is
no danger of changing an object through one reference
without the others being aware of the change.
Uses less memory.
String word1 = "Java";
String word2 = word1;
word1
"Java"
word2
OK
String word1 = “Java";
String word2 = new String(word1);
word1
"Java"
word2
"Java"
Less efficient:
wastes memory
Less efficient — you need to create a new string and
throw away the old one even for small changes.
String word = "java";
word = "HTML";
word
"java"
"HTML"
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Character class
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Contains standard methods for testing values of
characters
Methods that begin with “is”
Such as isUpperCase()
 Return Boolean value
 Can be used in comparison statements
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Methods that begin with “to”
Such as toUpperCase()
 Return character that has been converted to stated
format
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Characters of a string are numbered with 0-based
indexes:
String name = "Java fun";
index
0
1
2
3
character
J
a
v
a
4
5
6
7
f
u
n
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First character's index : 0
Last character's index : 1 less than the string's length
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The individual characters are values of type char
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Method name
Description
indexOf(str)
index where the start of the given string appears in
this string (-1 if not found)
length()
number of characters in this string
substring(index1, index2)
or
substring(index1)
the characters in this string from index1
(inclusive) to index2 (exclusive);
if index2 is omitted, grabs till end of string
toLowerCase()
a new string with all lowercase letters
toUpperCase()
a new string with all uppercase letters
charAt(index)
Returns the character at the specified index
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These methods are called using the dot notation:
String name = "Hello World";
System.out.println(name.length());
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It is sometimes useful to determine the length of a
string
Use the length( ) methods
String s = "Java is Fun";
int sLength = s.length();
System.out.println(sLength);
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// 11
The method counts the number of characters in the
string variable. It produces an integer result.
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Java is case sensitive
String s1 = "Java";
String s2 = "java";
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s1 and s2 represent two different strings
To get around this possible obstacle, Java has
methods that display a string in all upper case
letters or all lower case letters
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toUpperCase( )
toLowerCase( )
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These methods build and return a new string,
rather than modifying the current string.
To modify a variable's value, you must reassign
it:
String s = "Hello World";
s1 = s.toUpperCase();
s2 = s.toLowerCase();
System.out.println(s1 + " " + s2);
// HELLO WORLD hello world
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Another common task when handling strings is to see
whether one string can be found inside another. This is
useful when you are looking for a specific text inside a
large string (such as a document)
To look inside a string, use
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indexOf( string )
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If string is found, an integer that represents the starting position of
the first occurrence is returned
If string is not found, -1 is returned
lastIndexOf( string )
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Returns the position of the last occurrence
String s = "Java is fun";
int sfind = s.indexOf("fun");
System.out.println(sfind); // 8
// index
012345678901234567890123456
String name = "President George Washington";
name.indexOf
name.indexOf
name.indexOf
name.indexOf
('P');
('e');
("George");
('e', 3);
name.indexOf ("Bob");
name.lastIndexOf ('e');
Returns:
0
2
10
6 (starts searching
at position 3)
-1
15
(not found)
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We may also want to find and replace parts of a
large string
There are a few replace methods available
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replace(oldString, newString)
replaceFirst(oldString, newString)
replaceAll(oldString, newString)
String s = "Java is fan";
s1 = s.replace("fan","fun");
System.out.println(s1);
// Java is fun
// index
01234567890
String s1 = "Hello World";
System.out.println(s1.length());
System.out.println(s1.indexOf("r"));
System.out.println(s1.charAt(1));
System.out.println(s1.substring(3, 7));
String s2 = s1.substring(0, 5);
System.out.println(s2.toLowerCase());
11
8
e
lo W
hello
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One thing we will be testing often in our programs
is whether one string is the same as the other
Two basic commands:
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equals( )
compareTo( )
Do not compare two Strings using the ==
operator
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Not comparing values
Comparing computer memory locations
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equals() method
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Evaluates contents of two String objects to determine
if they are equivalent
Returns true if objects have identical contents
equalsIgnoreCase() method
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Ignores case when determining if two Strings
equivalent
Useful when users type responses to prompts in
programs
s1 = "Java";
s2 = "java";
boolean eq = s1.equals(s2);
System.out.println(eq);
// false
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compareTo() method
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Compares two Strings and returns:
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Zero
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Negative number
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Only if two Strings refer to same value
If calling object “less than” argument
Positive number
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If calling object “more than” argument
Method
Description
equals(str)
whether two strings contain the same characters
equalsIgnoreCase(str)
whether two strings contain the same characters,
ignoring upper vs. lower case
startsWith(str)
whether one contains other's characters at start
endsWith(str)
whether one contains other's characters at end
contains(str)
whether the given string is found within this one
These methods return a boolean value (true or false)
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Concatenation
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Join multiple variables together
Two ways to concatenate
 concat method
 String s3 = s1.concat(s2);
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Convenient shorthand
 String s3 = s1 + s2;
s1 = "Hello";
s2 = "World";
s3 = s1 + " " + s2;
Three ways to convert a number into a string:
1. String s = "" + num;
s = "" + 123;
//"123"
2. String s = Integer.toString (i);
String s = Double.toString (d);
s = Integer.toString(123); //"123"
s = Double.toString(3.14); //"3.14"
3. String s = String.valueOf (num);
s = String.valueOf(123);
//"123"
Integer and Double
are “wrapper” classes
from java.lang that
represent numbers as
objects. They also
provide useful static
methods.
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Integer and Double class
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Automatically imported into programs (java.lang)
Convert String to integer
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valueOf() method
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intValue() method
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Extract simple integer from wrapper class
parseInt() method
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Convert String to Integer class object
Returns its integer value
Convert String to double
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parseDouble() method
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Returns its double value
The StringBuilder/StringBuffer class is
an alternative to the String class.
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Can be used wherever a string is used.
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More flexible than String.
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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.
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Using StringBuilder objects
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Provides improved computer performance over String
objects
Can insert or append new contents into StringBuilder
StringBuilder constructors
public StringBuilder ()
public StringBuilder (int capacity)
public StringBuilder (String s)
StringBuilder name = new StringBuilder("Hello");
Method
Description
append(str)
Adds a string to the end of the StringBuilder object
delete(start, end)
Deletes characters at the specified index
insert(index, str)
Inserts string into the builder at the position offset
replace(start, end, str)
Replaces the characters in this builder at the specified
index with the specified string
setCharAt(index, char)
Changes a character at the specified index
CharAt(index)
Returns character at the specified index

Write a program to check whether a string is a
palindrome: a string that reads the same
forward and backward.
civic
radar
level
rotor
kayak
reviver
racecar
redder
import java.util.Scanner;
public class CheckPalindrome {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
System.out.print("Enter a string: ");
String s = input.nextLine();
if (isPalindrome(s))
System.out.println(s + " is a palindrome");
else
System.out.println(s + " is not a palindrome");
}
// continued on next page
public static boolean isPalindrome(String s) {
// Set index of the first and last characters in the string
int low = 0;
int high = s.length() - 1;
while (low < high) {
if (s.charAt(low) != s.charAt(high))
return false;
// Not a palindrome
low++;
high--;
}
return true;
}
}
// The string is a palindrome