Programming Tutorials

Extract characters in Java

By: Hong in Java Tutorials on 2007-09-02  

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.


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 v0alue "b" to ch.


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. 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);

Here is the output of this program:



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.


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.

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