Character Extraction

The String class provides a number of ways in which characters can be extracted from
a String object. Each is examined here. Although the characters that comprise a string
within a String object cannot be indexed as if they were a character array, many of the
String methods employ an index (or offset) into the string for their operation. Like
arrays, the string indexes begin at zero.

charAt( )
To extract a single character from a String, you can refer directly to an individual
character via the charAt( ) method. It has this general form:

char charAt(int where)

Here, where is the index of the character that you want to obtain. The value of where
must be nonnegative and specify a location within the string. charAt( ) returns the
character at the specified location. For example,

char ch;
ch = "abc".charAt(1);
assigns the value “b” to ch.


getChars( )
If you need to extract more than one character at a time, you can 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. Thus, 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.

The following program demonstrates getChars( ):

class getCharsDemo {
public static void main(String args[]) {
String s = "This is a demo of the getChars method.";
int start = 10;
int end = 14;
char buf[] = new char[end - start];
s.getChars(start, end, buf, 0);
System.out.println(buf);
}
}

Here is the output of this program:
demo

getBytes( )
There is an alternative to getChars( ) that stores the characters in an array of bytes. This
method is called getBytes( ), and it uses the default character-to-byte conversions
provided by the platform. Here is its simplest form:

byte[ ] getBytes( )

Other forms of getBytes( ) are also available. getBytes( ) is most useful when you
are exporting a String value into an environment that does not support 16-bit Unicode
characters. For example, most Internet protocols and text file formats use 8-bit ASCII
for all text interchange.

toCharArray( )
If you want to convert all the characters in a String object into a character array, the
easiest way is to call toCharArray( ). It returns an array of characters for the entire
string. It has this general form:

char[ ] toCharArray( )
This function is provided as a convenience, since it is possible to use getChars( ) to
achieve the same result.