显示java.lang.NullPointerException。为什么？

Question

我有RecyclerView的适配器类，我将它用于多个活动（多个RecyclerView）。

我通过StartActivityForResult（）意图进入这些活动，但当我到达'Clairvoyant'时，它会崩溃，它会给我这个java.lang.NullPointerException，但我不知道为什么，正如它所说的那样错误在这里：

    @Override
public int getItemCount() {
    return mArrayCourses.size();
}

但是这个类对于所有活动都是通用的，前两个显示没有任何错误。

Adapter班级

请注意，此课程不是我设计的，您可以找到原始版本here。

package com.example.leonardo.scurcola;

// Imports Skipped

public class CoursesAdapter extends RecyclerView.Adapter<CoursesAdapter.CoursesViewHolder> {

public class CoursesViewHolder extends RecyclerView.ViewHolder implements View.OnClickListener {
    TextView name;
    TextView card;

    CoursesViewHolder(View itemView) {
        super(itemView);
        itemView.setOnClickListener(this);
        name = (TextView) itemView.findViewById(R.id.textName);
        card = (TextView) itemView.findViewById(R.id.textCard);
    }

    @Override
    public void onClick(View v) {
        // The user may not set a click listener for list items, in which case our listener
        // will be null, so we need to check for this
        if (mOnEntryClickListener != null) {
            mOnEntryClickListener.onEntryClick(v, getLayoutPosition());
        }
    }
}

private ArrayList<Player> mArrayCourses;

public CoursesAdapter(ArrayList<Player> arrayCourses) {
    mArrayCourses = arrayCourses;
}

@Override
public int getItemCount() {
    return mArrayCourses.size();
}

@Override
public CoursesViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
    View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.two_line_list_item_horizontal, parent, false);
    return new CoursesViewHolder(view);
}

@Override
public void onBindViewHolder(CoursesViewHolder holder, int position) {
    Player player = mArrayCourses.get(position);
    holder.name.setText(player.getName());
    holder.card.setText(player.getCardName());
}

@Override
public void onAttachedToRecyclerView(RecyclerView recyclerView) {
    super.onAttachedToRecyclerView(recyclerView);
}


private OnEntryClickListener mOnEntryClickListener;

public interface OnEntryClickListener {
    void onEntryClick(View view, int position);
}

public void setOnEntryClickListener(OnEntryClickListener onEntryClickListener) {
    mOnEntryClickListener = onEntryClickListener;
    }



}

Activity w / RecyclerView

package com.example.leonardo.scurcola;

// Imports Skipped

public class ListPlayersClairvoyant extends Activity {

RecyclerView myList;

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

    // Get the players and remove the Clairvoyant
    Intent intent = this.getIntent();
    Bundle bundle = intent.getExtras();
    ArrayList<Player> playersNoClairvoyant = (ArrayList<Player>) bundle.getSerializable("PLAYERS");
    for(Player player : playersNoClairvoyant){
        if(player.getCardName().equals("Clairvoyant")){
            playersNoClairvoyant.remove(player);
        }
    }

    myList = (RecyclerView) findViewById(R.id.playersNoClairvoyant);
    myList.setLayoutManager(new LinearLayoutManager(this));
    CoursesAdapter adapter = new CoursesAdapter(playersNoClairvoyant);
    myList.setAdapter(adapter);

    // RecyclerView with a click listener

    CoursesAdapter clickAdapter = new CoursesAdapter(playersNoClairvoyant);

    clickAdapter.setOnEntryClickListener(new CoursesAdapter.OnEntryClickListener() {
        @Override
        public void onEntryClick(View view, int position) {
            // Stuff that will happen when a list item is clicked
            Intent intent = new Intent();
            intent.putExtra("pos", position);
            setResult(Activity.RESULT_OK, intent);
            finish();
        }
    });
    myList.setAdapter(clickAdapter);
    }
}

修改

错误发生在这一行，我没有正确更改要启动的Activity，因此在我启动的Activity中没有getter来获取玩家List而是另一个。

case 2:
                    clairvoyant(); // Clairvoyant's turn
                    /* --- Let him choose ---*/
                    Intent intent = new Intent(this, ERROR_HERE.class);
                    Bundle bundle = new Bundle();
                    bundle.putSerializable("PLAYERS", (Serializable) players);
                    intent.putExtras(bundle);
                    startActivityForResult(intent, REQUEST_CLAIRVOYANT);
                    /* --------------------- */
                    nightInsideCounter++;
                    break;

Answer 1

看起来你没有在任何地方设置mArrayCourses，所以调用它的大小会给你一个NullPointerException！尝试确保将其设置为某种东西。