导入第三方类库却出现the importXXX can not be resolved

在导入下拉刷新的第三方类库时，
import com.handmark.pulltorefcresh.library.PullToRefreshBase;
import com.handmark.pulltorefresh.library.PullToRefreshBase.OnRefreshListener2;
import com.handmark.pulltorefresh.library.PullToRefreshBase.State;
import com.handmark.pulltorefresh.library.PullToRefreshListView;
import com.handmark.pulltorefresh.library.extras.SoundPullEventListener;
这些Import都can not be resolved