RecyclerView试图在空对象引用上调用虚拟方法setAdapter

我有一个 BaseActivity . 其中一个常见的是一个菜单按钮,可以打开 MaterialDialog RecyclerView . 当我试图 setAdapter() 在里面 我得到以下错误

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.widget.RecyclerView.setAdapter(android.support.v7.widget.RecyclerView$Adapter)' on a null object reference

如果我没有 setAdapter()

错误输出

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v7.widget.RecyclerView.setAdapter(android.support.v7.widget.RecyclerView$Adapter)' on a null object reference
    at com.app.nex.nex.BaseActivity.onSelectPrinter(BaseActivity.java:445)
    at com.app.nex.nex.BaseActivity$1.onItemClick(BaseActivity.java:206)
    at com.mikepenz.materialdrawer.DrawerBuilder$6.onClick(DrawerBuilder.java:1786)
    at com.mikepenz.materialdrawer.DrawerBuilder$6.onClick(DrawerBuilder.java:1760)
    at com.mikepenz.fastadapter.FastAdapter$1.onClick(FastAdapter.java:608)
    at com.mikepenz.fastadapter.utils.EventHookUtil$1.onClick(EventHookUtil.java:72)
    at android.view.View.performClick(View.java:6306)
    at android.view.View$PerformClick.run(View.java:23962)
    at android.os.Handler.handleCallback(Handler.java:751)
    at android.os.Handler.dispatchMessage(Handler.java:95)
    at android.os.Looper.loop(Looper.java:154)
    at android.app.ActivityThread.main(ActivityThread.java:6823)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1557)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1445)

基本活动

private ArrayList<String> mPrinters = new ArrayList<>();
private AccountHeader headerResult = null;
private Drawer result = null;


@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    context = this;
    this.savedInstanceState = savedInstanceState;

    //Setup DrawerBuilder that has the SetupPrinter button that calls onSelectPrinter()

    new DrawerBuilder().withActivity(this).build();

    headerResult = new AccountHeaderBuilder()
            .withActivity(this)
            .withCompactStyle(true)
            .withHeaderBackground(R.drawable.logo)
            .withSavedInstance(savedInstanceState)
            .build();


    result = new DrawerBuilder()
            .withActivity(this)
            .withAccountHeader(headerResult) 
            .addDrawerItems(
                    new SectionDrawerItem().withName(R.string.side_menu_title_settings),
                    new PrimaryDrawerItem().withName(R.string.side_menu_0_printer).withIcon(R.drawable.fire_smoke).withIdentifier(R.integer.side_menu_0_printer),
                    new SectionDrawerItem().withName("Settings"),
                    new SecondaryDrawerItem().withName("tWO").withIcon(R.drawable.fire)
            )
            .withOnDrawerItemClickListener(new Drawer.OnDrawerItemClickListener() {
                @Override
                public boolean onItemClick(View view, int position, IDrawerItem drawerItem) {
                    int item = (int) drawerItem.getIdentifier();
                    switch (item) {
                        case R.integer.side_menu_0_printer:
                            onSelectPrinter();
                            break;
                        default:
                            break;
                    }
                    System.out.println("DRAWER " + drawerItem.getIdentifier());
                    if (drawerItem != null && drawerItem.getIdentifier() == 1) {
                        //startSupportActionMode(new ActionBarCallBack());
                    }

                    if (drawerItem instanceof Nameable) {

                    }

                    return false;
                }
            })
            .withSavedInstance(savedInstanceState)
            .build();

}


protected void onSelectPrinter() {

        mPrinters.add("Epson 123");
        mPrinters.add("Canon 999");
        mPrinters.add("HP 12322");
        mPrinters.add("Brother 12344");

        TestAdapter adapter = new TestAdapter(mPrinters, this);

        MaterialDialog dialog =
                new MaterialDialog.Builder(this)
                        .title(R.string.title_printer_setup)
                        .customView(R.layout.dialog_select_printer, true)
                        .negativeText(android.R.string.cancel)
                        .build();

        RecyclerView rv = dialog.getRecyclerView();
        rv.setAdapter(adapter);
        dialog.show();
}

测试适配器

public class TestAdapter extends RecyclerView.Adapter<TestAdapter.ViewHolder> {

    private final String TAG = this.getClass().getSimpleName();

    private ArrayList<String> mPrinters;
    private Context mContext;

    public TestAdapter(ArrayList<String> printers, Context context) {
        mContext = context;
        mPrinters = printers;
    }

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

    @Override
    public void onBindViewHolder(ViewHolder holder, int position) {
        Log.d(TAG, "onviewbindholder called");

        holder.tvTxt.setText(mPrinters.get(position));
        holder.rlLayout.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

            }
        });

    }

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

    public class ViewHolder extends RecyclerView.ViewHolder {

        TextView tvTxt;
        RelativeLayout rlLayout;

        public ViewHolder(View itemView) {
            super(itemView);
            rlLayout = itemView.findViewById(R.id.layoutSelectPrinter);
            tvTxt = itemView.findViewById(R.id.tvPrinterName);
        }
    }
}

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent" android:layout_height="match_parent">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textColor="#80000000"
        android:id="@+id/tvPrinterName"
        />

</RelativeLayout>

对话框\u选择\u printer.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/layoutSelectPrinter"
    >

    <android.support.v7.widget.RecyclerView
        android:id="@+id/rvPrinters"
        android:scrollbars="vertical"
        android:layout_width="match_parent"
        android:layout_height="match_parent"/>

</RelativeLayout>