for (int i = 0; i < num; i++) {
            hmap.put(String.valueOf(i), null);
        }
        System.out.println("HashMap.put:" + (System.currentTimeMillis() - start));

        start = System.currentTimeMillis();
        for (int i = 0; i < num; i++) {
            amap.put(String.valueOf(i), null);
        }
        System.out.println("ArrayMap.put:" + (System.currentTimeMillis() - start));