CharGenerator

package chargenerator;

import java.util.Arrays;

public class CharGenerator {

    private final char minLetter;
    private final char maxLetter;
    private final int maxLength;

    private char[] next;

    public CharGenerator(char minLetter, char maxLetter, int maxLength) {
        if (minLetter > maxLetter) {
            throw new IllegalArgumentException("minLetter > maxLetter");
        }
        if (maxLength <= 0) {
            throw new IllegalArgumentException("maxLength <= 0");
        }
        this.minLetter = minLetter;
        this.maxLetter = maxLetter;
        this.maxLength = maxLength;
        next = new char[]{minLetter};
    }

    public String getNext() {
        String ret = new String(next);
        _generateNext();
        return ret;
    }

    private void _generateNext() {
        for (int i = next.length - 1; i >= 0; --i) {
            char letter = next[i];
            if (letter == maxLetter) {
                next[i] = minLetter;
            } else {
                next[i] = (char) (letter + 1);
                return;
            }
        }
        if (next.length == maxLength) {
            next = new char[]{minLetter};
        } else {
            next = Arrays.copyOf(next, next.length + 1);
            next[next.length - 1] = minLetter;
        }
    }

}