1. Java Programming
Strings
Chapter 7

2. String Class
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.

3. Strings string: An object storing a sequence of text characters.
Two ways to create a string
As a new object through the String class
String name = new String("text");
Convenient shorthand method
String name = "text";

4. Immutability immutable.
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.

5. Advantages Of Immutability
Uses less memory.
String word1 = "Java";
String word2 = word1;
String word1 = “Java";
String word2 = new String(word1);
word1
"Java"
word1
"Java"
word2
"Java"
word2
Less efficient: wastes memory OK

6. Disadvantages of Immutability
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"

7. Manipulating Characters
Character class
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
Methods that begin with “to”
Such as toUpperCase()
Return character that has been converted to stated format

8. Indexes Characters of a string are numbered with 0-based indexes:
String name = "Java fun";
First character's index : 0
Last character's index : 1 less than the string's length
The individual characters are values of type char
index 1 2 3 4 5 6 7
character J a v f u n

9. String Methods These methods are called using the dot notation:
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
These methods are called using the dot notation:
String name = "Hello World";
System.out.println(name.length());

10. Length of a String
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); // 11
The method counts the number of characters in the string variable. It produces an integer result.

11. Modifying String Java is case sensitive
String s1 = "Java";
String s2 = "java";
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
toUpperCase( )
toLowerCase( )

12. Modifying String
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

13. Looking for a String
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
indexOf( string )
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 )
Returns the position of the last occurrence
String s = "Java is fun";
int sfind = s.indexOf("fun");
System.out.println(sfind); // 8

14. Looking for a String // index 012345678901234567890123456
String name = "President George Washington";
name.indexOf ('P');
name.indexOf ('e');
name.indexOf ("George");
name.indexOf ('e', 3);
name.indexOf ("Bob");
name.lastIndexOf ('e');
Returns:
(starts searching at position 3)
String has four overloaded versions of indexOf and four versions of lastIndexOf.
lastIndexOf(ch, fromPos) starts looking at fromPos and goes backward towards the beginning of the string.
(not found)

15. Replacing Part of a String
We may also want to find and replace parts of a large string
There are a few replace methods available
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

16. String methods // 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

17. Comparing Strings
One thing we will be testing often in our programs is whether one string is the same as the other
Two basic commands:
equals( )
compareTo( )
Do not compare two Strings using the == operator
Not comparing values
Comparing computer memory locations

18. Comparing Strings equals() method equalsIgnoreCase() method
Evaluates contents of two String objects to determine if they are equivalent
Returns true if objects have identical contents
equalsIgnoreCase() method
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

19. Comparing Strings compareTo() method Compares two Strings and returns:
Zero
Only if two Strings refer to same value
Negative number
If calling object “less than” argument
Positive number
If calling object “more than” argument

20. Comparing Strings These methods return a boolean value (true or false)
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)

21. String Concatenation Concatenation Join multiple variables together
Two ways to concatenate
concat method
String s3 = s1.concat(s2);
Convenient shorthand
String s3 = s1 + s2;
s1 = "Hello";
s2 = "World";
s3 = s1 + " " + s2;

22. Numbers to Strings Three ways to convert a number into a string:
1. String s = "" + num;
2. String s = Integer.toString (i);
String s = Double.toString (d);
3. String s = String.valueOf (num);
Integer and Double are “wrapper” classes from java.lang that represent numbers as objects. They also provide useful static methods.
s = "" + 123; //"123"
You can also convert a char to a string by using
String s = "" + ch; or
String s = ch + "";
By convention, a static method valueOf in a class converts something (its arguments) into an object of this class. For example:
public class Fraction {
public static Fraction valueOf(double x) {...}
should take a double and return a corresponding Fraction.
Here String.valueOf(x) returns a string.
s = Integer.toString(123); //"123"
s = Double.toString(3.14); //"3.14"
s = String.valueOf(123); //"123"

23. Strings to Numbers Integer and Double class
Automatically imported into programs (java.lang)
Convert String to integer
valueOf() method
Convert String to Integer class object
intValue() method
Extract simple integer from wrapper class
parseInt() method
Returns its integer value
Convert String to double
parseDouble() method
Returns its double value

24. StringBuilder
The StringBuilder/StringBuffer class is an alternative to the String class.
Can be used wherever a string is used.
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.

25. StringBuilder Using StringBuilder objects StringBuilder constructors
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");

26. StringBuilder 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

27. Finding Palindromes
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

28. Finding Palindromes 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

29. Finding Palindromes 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