RecyclerView 适配器 Android Studio

Question

当我将RecyclerView 设置为我的适配器时，我缺少一些东西。应用程序在运行时崩溃。谁能帮我确定问题？

// My adapter
public class Adapter extends RecyclerView.Adapter<Adapter.myViewHolder> {

    private Context mContext;
    private List<item> mData;

    public Adapter(Context mContext, List<item> mData) {
        this.mContext = mContext;
        this.mData = mData;
    }

    @Override
    public myViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        LayoutInflater inflater = LayoutInflater.from(mContext);
        View v = inflater.inflate(R.layout.card_item, parent, false);
        return new myViewHolder(v);
    }

    @Override
    public void onBindViewHolder(myViewHolder holder, int position) {
        holder.background_img.setImageResource(mData.get(position).getBackground());
        holder.profilePhoto.setImageResource(mData.get(position).getProfilePhoto());
        holder.tv_title.setText(mData.get(position).getProfileName());
        holder.tv_nbFollowers.setText(mData.get(position).getNbFollower() + " Followers");
    }

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

    public static class myViewHolder extends RecyclerView.ViewHolder {

        ImageView profilePhoto, background_img;
        TextView tv_title, tv_nbFollowers;

        public myViewHolder(View itemView) {
            super(itemView);
            profilePhoto = itemView.findViewById(R.id.profile_img);
            background_img = itemView.findViewById(R.id.card_background);
            tv_title = itemView.findViewById(R.id.card_title);
            tv_nbFollowers = itemView.findViewById(R.id.card_nb_follower);
        }
    }
}

这就是我使用适配器设置RecyclerView 的方式

RecyclerView recList = findViewById(R.id.rv_list);
recList.setHasFixedSize(true);
List<item> mlist = new ArrayList<>();
mlist.add(new item(R.drawable.fish0, "Bass", R.drawable.profile0, 2500));
mlist.add(new item(R.drawable.fish1, "Mondo Bass", R.drawable.profile1, 3500));
mlist.add(new item(R.drawable.fish2, "Large Mouth Bass", R.drawable.profile2, 5500));
mlist.add(new item(R.drawable.fish3, "Bass", R.drawable.profile3, 10500));
Adapter adapter = new Adapter(this, mlist);
recList.setAdapter(adapter);
recList.setLayoutManager(new LinearLayoutManager(this));

这是 Logcat 错误

2018-10-19 19:17:20.479 10635-10635/fishingfreaks.ffapp E/AndroidRuntime: FATAL EXCEPTION: main
    Process: fishingfreaks.ffapp, PID: 10635
    java.lang.RuntimeException: Unable to start activity ComponentInfo{fishingfreaks.ffapp/fishingfreaks.ffapp.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.widget.RecyclerView.setHasFixedSize(boolean)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2913)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:6669)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.widget.RecyclerView.setHasFixedSize(boolean)' on a null object reference
        at fishingfreaks.ffapp.MainActivity.onCreate(MainActivity.java:31)
        at android.app.Activity.performCreate(Activity.java:7136)
        at android.app.Activity.performCreate(Activity.java:7127)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2893)

Answer 1

您将收到NullPointerException。因此，当您尝试对其调用 setHasFixedSize 函数时，找不到您所指的布局项。

这可能有两个原因。

• 您可能引用了错误的布局项 ID。仔细检查 id rv_list。它在您的布局中是否也有相同的拼写？
• 如果上述条件正常（即您的布局确实有rv_list 作为RecyclerView 的ID，那么您可能需要检查您是否在活动的onCreate 函数中设置了内容视图。做您的onCreate 函数中有setContentView 函数调用吗？如果没有，则在super 调用onCreate 函数之后添加setContentView(R.layout.your_layout_with_recyclerview) 作为第一条语句。

Answer 2

原因：java.lang.NullPointerException：尝试在fishingfreaks.ffapp.MainActivity.onCreate(MainActivity. java:31) 在

从日志中可以清楚地看出，您将recyclerView.setHasFixedSize() 设置在您的recylerview 实例为空 的位置。请参阅 MainActivity 的第 31 行。