java - NullPointerException:尝试对空对象引用调用接口(interface)方法 'int java.util.List.size()'

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 4 年前。

我的代码有问题。实际上，这里是一个供玩家输入的名字。它使用类“player input”，该类引用类“nameListAdapter”。这里似乎是错误的谎言，但不幸的是我没有看到问题

一切顺利。然后我尝试使用“可序列化”来做一些事情。这不起作用，当我回到原来的代码时，它不再起作用了。

import android.app.Activity;
import android.app.AlertDialog;
import android.content.DialogInterface;
import android.content.Intent;
import android.graphics.Canvas;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.support.v7.widget.helper.ItemTouchHelper;
import android.view.LayoutInflater;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.RadioGroup;
import android.widget.Toast;
import android.widget.Toolbar;

import com.example.matthias.startanimation.spieler.Spieler;

import java.util.List;

public class Spielereingabe extends Activity {
    List<Spieler> spielerList;

    private Button button;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_spielereingabe);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setActionBar(toolbar);


        //Button
        button = (Button) findViewById(R.id.zukategorieButton);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                openKategorien_n();

            }
        });


        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                showDialog();
            }
        });

        RecyclerView recyclerView = findViewById(R.id.recycler_namensListe);

        final NamensListeAdapter namensListeAdapter = new NamensListeAdapter(this, spielerList);

        recyclerView.setLayoutManager(new LinearLayoutManager(Spielereingabe.this));

        recyclerView.setAdapter(namensListeAdapter);

        ItemTouchHelper.SimpleCallback itemTouchHelperCallback = new ItemTouchHelper.SimpleCallback(0, ItemTouchHelper.LEFT) {
            @Override
            public boolean onMove(RecyclerView recyclerView, RecyclerView.ViewHolder viewHolder, RecyclerView.ViewHolder target) {
                return false;
            }

            @Override
            public void onSwiped(RecyclerView.ViewHolder viewHolder, int direction) {
                namensListeAdapter.removeItem(viewHolder.getAdapterPosition());

            }

            @Override
            public void onChildDraw(Canvas c, RecyclerView recyclerView, RecyclerView.ViewHolder viewHolder, float dX, float dY, int actionState, boolean isCurrentlyActive) {
                // view the background view
            }
        };

        // attaching the touch helper to recycler view
        new ItemTouchHelper(itemTouchHelperCallback).attachToRecyclerView(recyclerView);
    }


    //mathode für button
    public void openKategorien_n() {
        Intent intent = new Intent(this, Kategorien_n.class);
        startActivity(intent);

    }


    //pop-up Dialog
    private void showDialog() {
        View ppview = LayoutInflater.from(getApplicationContext()).inflate(R.layout.popup_enter_name, null);
        final EditText editText = ppview.findViewById(R.id.name_edit);
        final RadioGroup radioGroup = ppview.findViewById(R.id.name_radiogroup);


        final AlertDialog.Builder builder = new AlertDialog.Builder(Spielereingabe.this);
        builder.setView(ppview)
                .setTitle("Namen Eingeben")

                .setPositiveButton("Speicher", new DialogInterface.OnClickListener() {
                    @Override
                    public void onClick(DialogInterface dialogInterface, int i) {
                        if (!editText.getText().toString().isEmpty())
                            spielerList.add(new Spieler()
                                    .setName(editText.getText().toString())
                                    .setGeschlecht(radioGroup.getCheckedRadioButtonId() == R.id.radio_male ? Gender.MALE : Gender.FEMALE));

                        Toast.makeText(Spielereingabe.this, editText.getText().toString() + " wurde hinzugefügt", Toast.LENGTH_SHORT).show();
                    }
                })
                .setNegativeButton("Abbrechen", new DialogInterface.OnClickListener() {
                    @Override
                    public void onClick(DialogInterface dialogInterface, int i) {
                        dialogInterface.cancel();
                    }
                })
                .show();


    }


}

import android.content.Context;
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

import com.example.matthias.startanimation.spieler.Spieler;

import java.util.List;

public class NamensListeAdapter extends RecyclerView.Adapter<NamensListeAdapter.NameViewHolder> {
    private List<Spieler> spielerList;
    private LayoutInflater inflater;

    public NamensListeAdapter(Context context, List<Spieler> namensList) {
        this.spielerList = namensList;
        this.inflater = LayoutInflater.from(context);
    }

    @Override
    public NameViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        return new NameViewHolder(inflater.inflate(R.layout.name_recyclerview, parent, false));
    }

    @Override
    public void onBindViewHolder(NameViewHolder holder, int position) {
        Spieler spieler = spielerList.get(position);

        String text = spieler.getName() + " (" + spieler.getGeschlecht().getCaption() + ")";

        holder.textView_name.setText(text);
    }

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

    public void removeItem(int itemIndex) {
        spielerList.remove(itemIndex);
        notifyItemRemoved(itemIndex);
    }


    public class NameViewHolder extends RecyclerView.ViewHolder {
        TextView textView_name;

        public NameViewHolder(View itemView) {
            super(itemView);
            textView_name = itemView.findViewById(R.id.nameview_text);

        }


    }

}

错误消息

java.lang.NullPointerException: Attempt to invoke interface method 'int java.util.List.size()' on a null object reference
        at com.example.matthias.startanimation.NamensListeAdapter.getItemCount(NamensListeAdapter.java:39)

看到有人犯错误了吗？

Answer 1

spielerList 尚未初始化。在 Spielereingabe.java 类中初始化列表:

private final List<Spieler> spielerList = new Arraylist<>();

最好在 NamensListeAdapter.class 中执行相同的操作，但在此实例中它不会是 final 变量。