exercises in java

exercises.java

package crackingcodeinterview;

public class Exercise {

	static String textString = "this is a random text of example";

	public static void main(String[] args) {

		filterLetters(0, "");
	}

	public static void filterLetters(int position, String letters) {
		if (position != textString.length()) {

			boolean p = false;

			for (int i = 0; i < letters.length(); i++) {
				if (letters.charAt(i) == textString.charAt(position)) {
					p = true;
					break;
				}
			}

			if (!p) {
				letters += textString.charAt(position);
			}
			position++;
			filterLetters(position, letters);
		} else {
			System.out.println(letters);
		}

	}

}

i need change this solution, becase i cant use the substring because create a new string constantly :(