RecyclerView 适配器在尝试删除项目时返回 null

Question

我偶然发现了我的RecyclerView 的一个问题。我目前正在尝试使我的上下文菜单（出现在 Long CLick 上）可以删除我的回收站视图中的项目，但没有运气。 我已经调试了我的应用程序，每次单击应该删除该项目的按钮时，我的应用程序都会崩溃，并且我会收到我的适配器返回 null 的信息。

我的主要活动：

public class MainActivity extends AppCompatActivity {

//Til LongClickListener
CalculationsAdapter adapter;
ArrayList<Calculation> calculations = new ArrayList<>();


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    android.support.v7.widget.Toolbar toolbar = (android.support.v7.widget.Toolbar) findViewById(R.id.app_bar);
    setSupportActionBar(toolbar);

    RecyclerView rvCalculations = (RecyclerView) findViewById(R.id.rvCalculations);
    RecyclerView.ItemDecoration itemDecoration = new DividerItemDecoration(MainActivity.this, DividerItemDecoration.VERTICAL);
    rvCalculations.addItemDecoration(itemDecoration);


    //Til CostumItemDecoration
    //RecyclerView.ItemDecoration CostumItemDecoration = new DividerItemDecoration(MainActivity.this, DividerItemDecoration.VERTICAL);
    //rvCalculations.addItemDecoration(CostumItemDecoration);



}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    MenuInflater inflater = getMenuInflater();
    inflater.inflate(R.menu.menu, menu);
    return super.onCreateOptionsMenu(menu);
}

@Override
public boolean onOptionsItemSelected(final MenuItem item) {

    switch (item.getItemId()){
        case R.id.add_new_calculation:
            /* for at starte newcalculation activiteten, gør det i gråt.
            Intent intent = new Intent(MainActivity.this, newcalculation.class);
            startActivity(intent);
            */

            final AlertDialog.Builder mBuilder = new AlertDialog.Builder(MainActivity.this);
            final View mView = getLayoutInflater().inflate(R.layout.dialog_namecalculation, null);
            final EditText mNameEditText = (EditText) mView.findViewById(R.id.editText_calculationName);
            Button mNameButton = (Button) mView.findViewById(R.id.button_nameCalculation);

            //Viser Alertdialogen
            mBuilder.setView(mView);
            final AlertDialog dialog = mBuilder.create();
            dialog.show();

            mNameButton.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View view) {

                //Her skal der være noget der gør at jeg ikke kan give to af samme navn
                //if (mNameEditText.getText() == .getName) ){
                //}

                if (mNameEditText.getText().toString().isEmpty()) {
                    Toast.makeText(MainActivity.this, R.string.ErrorMessageNameCalculation,Toast.LENGTH_SHORT).show();

                }else{
                    RecyclerView rvCalculations = (RecyclerView) findViewById(R.id.rvCalculations);
                    //calculations = Calculation.createCalculationsList(50);
                    calculations.add(0, new Calculation("  " + mNameEditText.getText()));
                    CalculationsAdapter adapter = new CalculationsAdapter(calculations);
                    rvCalculations.setAdapter(adapter);
                    rvCalculations.setLayoutManager(new LinearLayoutManager(MainActivity.this));
                    adapter.notifyItemInserted(0);

                    rvCalculations.scrollToPosition(0);

                    Toast.makeText(MainActivity.this, R.string.SuccesMessageNameCalculation,Toast.LENGTH_SHORT).show();

                    dialog.dismiss();

                }


                }
            });


    }
    return super.onOptionsItemSelected(item);
}

//Til LongClickListener
@Override
public boolean onContextItemSelected(MenuItem item) {

    //Hvad itemet i LongClicket skal gøre

    switch (item.getItemId()){

        case 1:
            adapter.removeItem(item.getGroupId());
            Toast.makeText(MainActivity.this, "Regningen blev slettet", Toast.LENGTH_SHORT).show();
            return true;

        case 2:
            Toast.makeText(MainActivity.this, "Hello", Toast.LENGTH_SHORT).show();
            return true;

            default:
                return super.onContextItemSelected(item);

    }

}


}

LogCat：

08-03 23:33:44.443 1481-1481/hndvrkerregner.hndvrkerregner E/AndroidRuntime: FATAL EXCEPTION: main
    Process: package, PID: 1481
    java.lang.NullPointerException: Attempt to invoke virtual method 'void package.CalculationsAdapter.removeItem(int)' on a null object reference
        at package.MainActivity.onContextItemSelected(MainActivity.java:128)

Answer 1

您已经在Activity 类“MainActivity”的顶部声明了类变量CalculationsAdapter adapter;。但是在onOptionsItemSelected() 方法中，您将在这行代码中覆盖此声明：

CalculationsAdapter adapter = new CalculationsAdapter(calculations);

这一行应该是：

adapter = new CalculationsAdapter(calculations);

---

解释：

通过这样做：

CalculationsAdapter adapter = new CalculationsAdapter(calculations);

您正在将adapter 变成一个局部变量——仅对方法onOptionsItemSelected() 是局部的，因此，adapter 在onContextItemSelected() 中是null。

为了将adapter 保留为有效的类变量，请改用这行代码：