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();
    }
}