Needing some assistance on a problem, unique characters

public class AllUniqueStrings1 {

	public static boolean allUniqueChars(String s) {
	   boolean[] char_set = new boolean[256];
	    for (int i = 0; i < s.length(); i++) {
	        int val = s.charAt(i);
	        if (char_set[val])
	            return false;
	        char_set[val] = true;
	    }
	    return true;
	}
}