android - 带 fragment 的 RecyclerView

Question

我正在尝试在 Fragment 中使用新的 RecyclerView 小部件，但出现此错误:

Unable to start activity ComponentInfo{com.example.myapplication/com.example.myapplication.MyActivity}: java.lang.NullPointerException: Attempt to invoke interface method 'boolean java.util.List.add(java.lang.Object)' on a null object reference

我做错了什么？

这是我的 fragment :

import android.app.Fragment;
import android.os.Bundle;
import android.support.annotation.Nullable;
import android.support.v7.widget.DefaultItemAnimator;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

import java.util.List;

public class RecyclerviewFragment extends Fragment {

    private RecyclerView mRecyclerView;
    private RecyclerviewAdapter mRecyclerviewAdapter;
    private LinearLayoutManager mLinearLayoutManager;

    private List<ViewModel> viewModel;

    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_recyclerview, container, false);
        mRecyclerView = (RecyclerView) rootView.findViewById(R.id.recycler_view);

        return rootView;
    }

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);

        mRecyclerviewAdapter = new RecyclerviewAdapter(viewModel);
        mLinearLayoutManager = new LinearLayoutManager(getActivity());
        mRecyclerView.setAdapter(mRecyclerviewAdapter);
        mRecyclerView.setLayoutManager(mLinearLayoutManager);
        mRecyclerView.setItemAnimator(new DefaultItemAnimator());
    }
}

还有我的 Activity :

import android.app.Activity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;

import java.util.List;

public class MyActivity extends Activity {

    private List<ViewModel> viewModel;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_my);

        RecyclerviewFragment recyclerviewFragment = new RecyclerviewFragment();
        getFragmentManager().beginTransaction().add(android.R.id.content, recyclerviewFragment).commit();

        viewModel.add(new ViewModel("View"));



    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.my, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }
}

谢谢!

Answer 1

你没有初始化 private List<ViewModel> viewModel