The String class has a number of methods for examining the contents of strings, finding characters or substrings within a string, changing case, and other tasks.

Getting Characters and Substrings by Index You can get the character at a particular index within a string by invoking the charAt() accessor method.

The index of the first character is 0, while the index of the last character is length()-1.

For example, the following code gets the character at index 9 in a string:

 
String anotherPalindrome = "Niagara. O roar again!"; 
char aChar = anotherPalindrome.charAt(9);

Indices begin at 0, so the character at index 9 is 'O', as illustrated in the following figure:

If you want to get more than one consecutive character from a string, you can use the substring method.

The substring method has two versions, as shown in the following table:

The `substring` Methods in the `String` Class
Method	Description
`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`.
`String substring(int beginIndex)`	Returns a new string that is a substring of this string. The integer argument specifies the index of the first character. Here, the returned substring extends to the end of the original string.

The following code gets from the Niagara palindrome the substring that extends from index 11 up to, but not including, index 15, which is the word "roar":

 
String anotherPalindrome = "Niagara. O roar again!"; 
String roar = anotherPalindrome.substring(11, 15);

Other Methods for Manipulating Strings Here are several other String methods for manipulating strings:

Other Methods in the `String` Class for Manipulating Strings
Method	Description
`String[] split(String regex)` `String[] split(String regex, int limit)`	Searches for a match as specified by the string argument (which contains a regular expression) and splits this string into an array of strings accordingly. The optional integer argument specifies the maximum size of the returned array. Regular expressions are covered in the lesson titled "Regular Expressions."
`CharSequence subSequence(int beginIndex, int endIndex)`	Returns a new character sequence constructed from `beginIndex` index up until `endIndex` - 1.
`String trim()`	Returns a copy of this string with leading and trailing white space removed.
`String toLowerCase() String toUpperCase()`	Returns a copy of this string converted to lowercase or uppercase. If no conversions are necessary, these methods return the original string.

Searching for Characters and Substrings in a String Here are some other String methods for finding characters or substrings within a string.

The String class provides accessor methods that return the position within the string of a specific character or substring: indexOf() and lastIndexOf().

The indexOf() methods search forward from the beginning of the string, and the lastIndexOf() methods search backward from the end of the string.

If a character or substring is not found, indexOf() and lastIndexOf() return -1.

The String class also provides a search method, contains, that returns true if the string contains a particular character sequence.

Use this method when you only need to know that the string contains a character sequence, but the precise location isn't important.

The following table describes the various string search methods.

The Search Methods in the `String` Class
Method	Description
`int indexOf(int ch) int lastIndexOf(int ch)`	Returns the index of the first (last) occurrence of the specified character.
`int indexOf(int ch, int fromIndex) int lastIndexOf(int ch, int fromIndex)`	Returns the index of the first (last) occurrence of the specified character, searching forward (backward) from the specified index.
`int indexOf(String str) int lastIndexOf(String str)`	Returns the index of the first (last) occurrence of the specified substring.
`int indexOf(String str, int fromIndex) int lastIndexOf(String str, int fromIndex)`	Returns the index of the first (last) occurrence of the specified substring, searching forward (backward) from the specified index.
`boolean contains(CharSequence s)`	Returns true if the string contains the specified character sequence.

Note: CharSequence is an interface that is implemented by the String class. Therefore, you can use a string as an argument for the contains() method.

Replacing Characters and Substrings into a String The String class has very few methods for inserting characters or substrings into a string.

In general, they are not needed: You can create a new string by concatenation of substrings you have removed from a string with the substring that you want to insert.

The String class does have four methods for replacing found characters or substrings, however. They are:

Methods in the `String` Class for Manipulating Strings
Method	Description
`String replace(char oldChar, char newChar)`	Returns a new string resulting from replacing all occurrences of oldChar in this string with newChar.
`String replace(CharSequence target, CharSequence replacement)`	Replaces each substring of this string that matches the literal target sequence with the specified literal replacement sequence.
`String replaceAll(String regex, String replacement)`	Replaces each substring of this string that matches the given regular expression with the given replacement.
`String replaceFirst(String regex, String replacement)`	Replaces the first substring of this string that matches the given regular expression with the given replacement.

An Example The following class, Filename, illustrates the use of lastIndexOf() and substring() to isolate different parts of a file name.

Note: The methods in the following Filename class don't do any error checking and assume that their argument contains a full directory path and a filename with an extension.

If these methods were production code, they would verify that their arguments were properly constructed.

 
public class Filename {
    private String fullPath;
    private char pathSeparator, 
                 extensionSeparator;

    public Filename(String str, char sep, char ext) {
        fullPath = str;
        pathSeparator = sep;
        extensionSeparator = ext;
    }

    public String extension() {
        int dot = fullPath.lastIndexOf(extensionSeparator);
        return fullPath.substring(dot + 1);
    }

    // gets filename without extension
    public String filename() {
        int dot = fullPath.lastIndexOf(extensionSeparator);
        int sep = fullPath.lastIndexOf(pathSeparator);
        return fullPath.substring(sep + 1, dot);
    }

    public String path() {
        int sep = fullPath.lastIndexOf(pathSeparator);
        return fullPath.substring(0, sep);
    }
}

Here is a program, FilenameDemo, that constructs a Filename object and calls all of its methods:

 
public class FilenameDemo {
    public static void main(String[] args) {
        final String FPATH = "/home/user/index.html";
        Filename myHomePage = new Filename(FPATH, '/', '.');
        System.out.println("Extension = " + myHomePage.extension());
        System.out.println("Filename = " + myHomePage.filename());
        System.out.println("Path = " + myHomePage.path());
    }
}
And here's the output from the program:

Extension = html
Filename = index
Path = /home/user

As shown in the following figure, our extension method uses lastIndexOf to locate the last occurrence of the period (.) in the file name.

Then substring uses the return value of lastIndexOf to extract the file name extension — that is, the substring from the period to the end of the string.

This code assumes that the file name has a period in it; if the file name does not have a period, lastIndexOf returns -1, and the substring method throws a StringIndexOutOfBoundsException.

Also, notice that the extension method uses dot + 1 as the argument to substring.

If the period character (.) is the last character of the string, dot + 1 is equal to the length of the string, which is one larger than the largest index into the string (because indices start at 0).

This is a legal argument to substring because that method accepts an index equal to, but not greater than, the length of the string and interprets it to mean "the end of the string."

 

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

    String myString =" I am a computer scientist ";
    System.out.println("Output of charAt ="+myString.charAt(9));
    System.out.println("Output of substring ="+myString.substring(1,5));
    System.out.println("Output of substring ="+myString.substring(5));
    System.out.println("Output of subsequence ="+myString.subSequence(1,6));
    System.out.println("Output of trim ="+myString.trim());
    System.out.println("Output of tolowercase ="+myString.toLowerCase());
    System.out.println("Output of touppercase ="+myString.toUpperCase());
    System.out.println("Output of indexof ="+myString.indexOf('a'));
    System.out.println("Output of lastIndexOf ="+myString.lastIndexOf('i'));
    System.out.println("Output of indexOf ="+myString.indexOf('a',3));
    System.out.println("Output of lastIndexOf ="+myString.lastIndexOf('i',3));
    System.out.println("Output of indexOf ="+myString.indexOf("am"));
    System.out.println("Output of lastIndexOf ="+myString.lastIndexOf("am"));
    System.out.println("Output of indexOf ="+myString.indexOf("am",3));
    System.out.println("Output of lastIndexOf ="+myString.lastIndexOf("am",3));
    System.out.println("Output of contains ="+myString.contains("am"));
    System.out.println("Output of replace ="+myString.replace('I', 'i'));
    System.out.println("Output of replaceAll ="+myString.replaceAll("I", "i"));
    System.out.println("Output of replaceFirst ="+myString.replaceFirst("I", "i"));
    }
}


Output


Output of charAt =o                                                                                  
Output of substring =I am                                                                            
Output of substring = a computer scientist                                                           
Output of subsequence =I am                                                                          
Output of trim =I am a computer scientist                                                            
Output of tolowercase = i am a computer scientist                                                    
Output of touppercase = I AM A COMPUTER SCIENTIST                                                    
Output of indexof =3                                                                                 
Output of lastIndexOf =23                                                                            
Output of indexOf =3                                                                                 
Output of lastIndexOf =-1                                                                            
Output of indexOf =3                                                                                 
Output of lastIndexOf =3                                                                             
Output of indexOf =3                                                                                 
Output of lastIndexOf =3                                                                             
Output of contains =true                                                                             
Output of replace = i am a computer scientist                                                        
Output of replaceAll = i am a computer scientist                                                     
Output of replaceFirst = i am a computer scientist

The String class has a number of methods for comparing strings and portions of strings. The following table lists these methods

Methods for Comparing Strings
Method	Description
`boolean endsWith(String suffix) boolean startsWith(String prefix)`	Returns `true` if this string ends with or begins with the substring specified as an argument to the method.
`boolean startsWith(String prefix, int offset)`	Considers the string beginning at the index `offset`, and returns `true` if it begins with the substring specified as an argument.
`int compareTo(String anotherString)`	Compares two strings lexicographically. Returns an integer indicating whether this string is greater than (result is > 0), equal to (result is = 0), or less than (result is < 0) the argument.
`int compareToIgnoreCase(String str)`	Compares two strings lexicographically, ignoring differences in case. Returns an integer indicating whether this string is greater than (result is > 0), equal to (result is = 0), or less than (result is < 0) the argument.
`boolean equals(Object anObject)`	Returns `true` if and only if the argument is a `String` object that represents the same sequence of characters as this object.
`boolean equalsIgnoreCase(String anotherString)`	Returns `true` if and only if the argument is a `String` object that represents the same sequence of characters as this object, ignoring differences in case.
`boolean regionMatches(int toffset, String other, int ooffset, int len)`	Tests whether the specified region of this string matches the specified region of the String argument. Region is of length `len` and begins at the index `toffset` for this string and `ooffset` for the other string.
`boolean regionMatches(boolean ignoreCase, int toffset, String other, int ooffset, int len)`	Tests whether the specified region of this string matches the specified region of the String argument. Region is of length `len` and begins at the index `toffset` for this string and `ooffset` for the other string. The boolean argument indicates whether case should be ignored; if true, case is ignored when comparing characters.
`boolean matches(String regex)`	Tests whether this string matches the specified regular expression. Regular expressions are discussed in the lesson titled "Regular Expressions."

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

    String myString ="I am a computer scientist";
    
    System.out.println("Output of endsWith = "+myString.endsWith("t"));
    System.out.println("Output of startsWith ="+myString.startsWith("I"));
    System.out.println("Output of startsWith ="+myString.startsWith("i",2));
    System.out.println("Output of compareTo ="+myString.compareTo("I am not scientist"));
    System.out.println("Output of compareToIgnoreCase ="+myString.compareToIgnoreCase("i am a computer scientist"));
    System.out.println("Output of equals ="+myString.equals("I am a computer scientist"));
    System.out.println("Output of equalsIgnoreCase ="+myString.equalsIgnoreCase("I am a computer scientist"));
    
    }
}


Output:
Output of endsWith = true                                                                            
Output of startsWith =true                                                                           
Output of startsWith =false                                                                          
Output of compareTo =-13                                                                             
Output of compareToIgnoreCase =0                                                                     
Output of equals =true                                                                               
Output of equalsIgnoreCase =true

The following program, RegionMatchesDemo, uses the regionMatches method to search for a string within another string:

 
public class RegionMatchesDemo {
    public static void main(String[] args) {
        String searchMe = "Green Eggs and Ham";
        String findMe = "Eggs";
        int searchMeLength = searchMe.length();
        int findMeLength = findMe.length();
        boolean foundIt = false;
        for (int i = 0; 
             i <= (searchMeLength - findMeLength);
             i++) {
           if (searchMe.regionMatches(i, findMe, 0, findMeLength)) {
              foundIt = true;
              System.out.println(searchMe.substring(i, i + findMeLength));
              break;
           }
        }
        if (!foundIt)
            System.out.println("No match found.");
    }
}
The output from this program is Eggs.

The program steps through the string referred to by searchMe one character at a time.

For each character, the program calls the regionMatches method to determine whether the substring beginning with the current character matches the string the program is looking for.

Manipulating Characters in a String

Java Program: Manipulating Characters in a String

Comparing Strings and Portions of Strings