code: public void set(String key, String value, int ttl, Boolean prevExist) throws EtcdClientException {
          Preconditions.checkNotNull(key);
          Preconditions.checkNotNull(value);
  
          List<BasicNameValuePair> list = Lists.newArrayList();
          list.add(new BasicNameValuePair("value", value));
          if (ttl > 0) {
              list.add(new BasicNameValuePair("ttl", String.valueOf(ttl)));
          }
          if (prevExist != null) {
              list.add(new BasicNameValuePair("prevExist", String.valueOf(prevExist)));
          }
  
          put(key, list, null, new int[]{200, 201});
      }
docs: Setting the value of a key with ttl and optional prevExist(update key ttl)
  @param key the key
  @param value the value
  @param ttl the ttl
  @param prevExist exists before