code: public synchronized void put(ApnsConnection connection) {
          if (connection == null) {
              throw new IllegalArgumentException("connection must not be null");
          }
          if (mPool.size() < mCapacity) {
              mPool.add(connection);
          }
      }
docs: If and only if size of the pool is less than its capacity then
  connection is put into the pool and size of the pool incremented by one;
  otherwise method argument is ignored and pool is not changed.
  
  @param connection that you want to put into the pool