尝试在空对象引用上调用虚方法'android.content.Context.getResources（）' [英] Attempt to invoke virtual method 'android.content.Context.getResources()' on a null object reference

问题描述

我尝试使用片段打开数据库，但是，当我单击按钮开始搜索时，程序意外终止并显示如下错误：

I try to use a fragment to open a database, however, when I click the button to begin searching, the program unexpectedly terminates and it show the error like this:

java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference
     at android.widget.Toast.<init>(Toast.java:102)
     at android.widget.Toast.makeText(Toast.java:259)
     at com.example.nkwpy.myapplication.MainFragment.query(MainFragment.java:176)
     at com.example.nkwpy.myapplication.MainFragment.access$000(MainFragment.java:46)
     at com.example.nkwpy.myapplication.MainFragment$queryListener.onClick(MainFragment.java:161)
     at android.view.View.performClick(View.java:5207)
     at android.view.View$PerformClick.run(View.java:21177)
     at android.os.Handler.handleCallback(Handler.java:739)
     at android.os.Handler.dispatchMessage(Handler.java:95)
     at android.os.Looper.loop(Looper.java:148)
     at android.app.ActivityThread.main(ActivityThread.java:5458)
     at java.lang.reflect.Method.invoke(Native Method)
     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:738)
     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:628)

MainFragment：

MainFragment:

package com.example.nkwpy.myapplication;
import android.content.Context;
import android.net.Uri;
import android.os.Bundle;
import android.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.EditText;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.view.View.OnClickListener;
import android.widget.Toast;



/**
 * A simple {@link Fragment} subclass.
 * Activities that contain this fragment must implement the
 * {@link MainFragment.OnFragmentInteractionListener} interface
 * to handle interaction events.
 * Use the {@link MainFragment#newInstance} factory method to
 * create an instance of this fragment.
 */
public class MainFragment extends Fragment {
    // TODO: Rename parameter arguments, choose names that match
    // the fragment initialization parameters, e.g. ARG_ITEM_NUMBER
    private static final String ARG_PARAM1 = "param1";
    private static final String ARG_PARAM2 = "param2";

    MainActivity parent=(MainActivity) getActivity();
    SQLiteDatabase test;
    DBManager dbHelper;


    // TODO: Rename and change types of parameters
    private String mParam1;
    private String mParam2;

    private EditText N;
    private EditText Z;
    private EditText R_A;
    private Button queryBtn;

    private OnFragmentInteractionListener mListener;

    public MainFragment() {
        // Required empty public constructor
    }

    /**
     * Use this factory method to create a new instance of
     * this fragment using the provided parameters.
     *
     * @param param1 Parameter 1.
     * @param param2 Parameter 2.
     * @return A new instance of fragment MainFragment.
     */
    // TODO: Rename and change types and number of parameters
    public static MainFragment newInstance(String param1, String param2) {
        MainFragment fragment = new MainFragment();
        Bundle args = new Bundle();
        args.putString(ARG_PARAM1, param1);
        args.putString(ARG_PARAM2, param2);
        fragment.setArguments(args);
        return fragment;
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        if (getArguments() != null) {
            mParam1 = getArguments().getString(ARG_PARAM1);
            mParam2 = getArguments().getString(ARG_PARAM2);
        }


    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        View view =inflater.inflate(R.layout.fragment_main, container, false);
        N=(EditText)view.findViewById(R.id.neuton);
        Z=(EditText)view.findViewById(R.id.proton);
        queryBtn = (Button)view.findViewById(R.id.query);
        queryBtn.setOnClickListener(new queryListener());
        R_A=(EditText)view.findViewById(R.id.result);


        dbHelper = new DBManager(getActivity());
        dbHelper.openDatabase();
        dbHelper.closeDatabase();
        return view;
    }

    // TODO: Rename method, update argument and hook method into UI event
    public void onButtonPressed(Uri uri) {
        if (mListener != null) {
            mListener.onFragmentInteraction(uri);
        }
    }

    @Override
    public void onAttach(Context context) {
        super.onAttach(context);
        if (context instanceof OnFragmentInteractionListener) {
            mListener = (OnFragmentInteractionListener) context;
        } else {
            throw new RuntimeException(context.toString()
                    + " must implement OnFragmentInteractionListener");
        }
    }

    @Override
    public void onDetach() {
        super.onDetach();
        mListener = null;
    }

    /**
     * This interface must be implemented by activities that contain this
     * fragment to allow an interaction in this fragment to be communicated
     * to the activity and potentially other fragments contained in that
     * activity.
     * <p/>
     * See the Android Training lesson <a href=
     * "http://developer.android.com/training/basics/fragments/communicating.html"
     * >Communicating with Other Fragments</a> for more information.
     */
    public interface OnFragmentInteractionListener {
        // TODO: Update argument type and name
        void onFragmentInteraction(Uri uri);
    }

    class queryListener implements OnClickListener{

        @Override
        public void onClick(View v) {
            //
            query();
            test.close();
        }
    }
    private void query() {

        try {
            String string1 = N.getText().toString();
            String string2 = Z.getText().toString();
            String sql = "select * from sly4 where N=" + string1 + " and Z=" + string2;
            Cursor cursor =test.rawQuery(sql, null);
            cursor.moveToFirst();
            String r = cursor.getString(cursor.getColumnIndex("value"));
            R_A.setText(r);
        } catch (Exception e) {
            Toast.makeText(parent, "Please check the number you entered", Toast.LENGTH_LONG).show();
        }
    }
}

class DBManager：

class DBManager:

import java.io.File;
 import java.io.FileNotFoundException;
 import java.io.FileOutputStream;
 import java.io.IOException;
 import java.io.InputStream;
 import android.content.Context;
 import android.database.sqlite.SQLiteDatabase;
 import android.os.Environment;
 import android.util.Log;

public class DBManager {
    private final int BUFFER_SIZE = 400000;
    public static final String DB_NAME = "main.db"; 
    public static final String PACKAGE_NAME = "com.example.nkwpy.myapplication";
    public static final String DB_PATH = "/data"
            + Environment.getDataDirectory().getAbsolutePath() + "/"
            + PACKAGE_NAME;

    private SQLiteDatabase database;
    private Context context;

    DBManager(Context context) {
        this.context = context;
    }

    public void openDatabase() {
        this.database = this.openDatabase(DB_PATH + "/" + DB_NAME);
    }

    private SQLiteDatabase openDatabase(String dbfile) {
        try {
            if (!(new File(dbfile).exists())) {
                InputStream is = this.context.getResources().openRawResource(R.raw.main);
                FileOutputStream fos = new FileOutputStream(dbfile);
                byte[] buffer = new byte[BUFFER_SIZE];
                int count = 0;
                while ((count = is.read(buffer)) > 0) {
                    fos.write(buffer, 0, count);
                }
                fos.close();
                is.close();
            }
            SQLiteDatabase db = SQLiteDatabase.openOrCreateDatabase(dbfile,
                    null);
            return db;
        } catch (FileNotFoundException e) {
            Log.e("Database", "File not found");
            e.printStackTrace();
        } catch (IOException e) {
            Log.e("Database", "IO exception");
            e.printStackTrace();
        }
        return null;
    }
    public void closeDatabase() {
        this.database.close();
    }
}

顺便说一句，我使用了关于DBManager的代码MainAcitivity，它成功了。在我将代码复制到上面的片段后，它失败了。我该怎么做？

By the way,I have used code about DBManager in MainAcitivity,and it succeeded.After I copy the code to the fragment like the one above,it failed.How should I do?

推荐答案

你不能 MainActivity parent =（MainActivity）getActivity（）; 在 onAttach（）之前和之后 onDetach（）。

因为，您正在进行片段实例化。方法 getActivity 将始终返回null。另外，尽量不要在片段中保留活动引用。如果引用没有正确无效，这可能会导致内存泄漏。

Since, you are doing at the time of fragment instantiation. The method getActivity will always return null. Also as far as possible don't try to keep your Activity reference in your Fragment. This could cause memory leak if the reference is not nullified properly.

使用 getActivity（）或 getContext（）尽可能。

更改 Toast ，如下所示：

Toast.makeText(getContext(), "Please check the number you entered", 
Toast.LENGTH_LONG).show(); 

或

Toast.makeText(getActivity(), "Please check the number you entered", 
Toast.LENGTH_LONG).show();

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