charAt( ) and setCharAt( ) and getChars( ) in java

The value of a single character can be obtained from a StringBuffer via the charAt( ) method. You can set the value of a character within a StringBuffer using setCharAt( ). Their general forms are shown here: 

•  char charAt(int where)
• void setCharAt(int where, char ch)

For charAt( ), where specifies the index of the character being obtained. 
For setCharAt( ), where specifies the index of the character being set, and ch specifies the
new value of that character. For both methods, where must be nonnegative and must
not specify a location beyond the end of the buffer.
The following example demonstrates charAt( ) and setCharAt( ):

 

// Demonstrate charAt() and setCharAt().
 class setCharAtDemo { public static void main(String args[]) { StringBuffer sb = new 

StringBuffer("Hello"); System.out.println("buffer before = " + sb);
System.out.println("charAt(1) before = " + sb.charAt(1)); sb.setCharAt(1, 'i');

sb.setLength(2); System.out.println("buffer after = " + sb); System.out.println("charAt(1) after = " + sb.charAt(1));
}
}

Here is the output generated by this program: buffer before = Hello charAt(1) before = e buffer after = Hi charAt(1) after = i

getChars( ) 

To copy a substring of a StringBuffer into an array, use the getChars( ) method. It has this general form: 
void getChars(int sourceStart, int sourceEnd, char target[ ], int targetStart) 
Here, sourceStart specifies the index of the beginning of the substring, and sourceEnd specifies an index that is one past the end of the desired substring. This means that the substring contains the characters from sourceStart through sourceEnd–1. The array that will receive the characters is specified by target. The index within target at which the substring will be copied is passed in targetStart. Care must be taken to assure that the target array is large enough to hold the number of characters in the specified substring.