public String replace(char oldChar, char newChar) {
    if (oldChar != newChar) {
        int len = count;
        int i = -1;
        char[] val = value; /* avoid getfield opcode */
        int off = offset;   /* avoid getfield opcode */

        while (++i < len) {
        if (val[off + i] == oldChar) {
            break;
        }
        }
        if (i < len) {
        char buf[] = new char[len];
        for (int j = 0 ; j < i ; j++) {
            buf[j] = val[off+j];
        }
        while (i < len) {
            char c = val[off + i];
            buf[i] = (c == oldChar) ? newChar : c;
            i++;
        }
        return new String(0, len, buf);
        }
    }
    return this;
    }

public class Test {

    public static void main(String[] args) {
        String str = "123aa123aaa123bbb123ccc123ddd123aa123";
        System.out.println(str);
        System.out.println(replace(str, "123", "***"));
    }
    
    public static String replace(String str, String old, String replacement) {
        StringBuffer sb = new StringBuffer();
        int index = str.indexOf(old);
        if(index < 0) {
            return str;
        }
        int start = 0;
        int len = old.length();
        do {
            sb.append(str.substring(start, index))
              .append(replacement);
            start = index + len;
            index = str.indexOf(old, start);
        } while (index >= 0);
        sb.append(str.substring(start));
        return sb.toString();
    }
}