Replace duplicate characters from a string and leave the first occurrence

ReplaceDuplicate.java

package kata.string;

import java.util.Set;
import java.util.HashSet;

public class ReplaceDuplicates {

    /*
     * @param   input     char array with duplicated characters for example, google
     * @return  output    char array with duplicated characters. After removal of duplicated characters output will be gogle
    */
    public static char[] replace(char[] input) {
        if (null == input) {
            return null;
        }

        Set<Character> characters = new HashSet<>();
        int j = 0;

        for (int i = 0; i < input.length; i++) {
            if (!characters.contains(input[i])) {
                characters.add(input[i]);
                input[j++] = input[i];
            }
        }

        for (; j < input.length; j++) {
            input[j] = '\0';
        }

        return input;
    }
}