docs: public synchronized void addItem(long weight, T item) {
          WeightBucket<T> bucket = new WeightBucket<T>(weight);
          int index = indexedSearch(buckets, bucket);
          if (index > buckets.size() - 1) {// 先加一个bucket
              bucket.addLastItem(item);
              buckets.add(index, bucket);
          } else if (buckets.get(index).getWeight() != weight) {// 不匹配的
              bucket.addLastItem(item);
              buckets.add(index, bucket);
          } else {
              buckets.get(index).addLastItem(item);// 添加到已有的bucket上
          }
  
      }