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：
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)
主要片段：
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();
}
}
}
数据库管理器类：
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，我在MainAcitivity中使用了关于DBManager的代码，并且成功了。当我将代码复制到上面的片段后，它失败了。我该怎么办？
在
MainActivity parent=(MainActivity) getActivity();onAttach()onDetach()因为，你是在片段实例化时做的。方法
getActivityActivityFragment尽可能使用
getActivity()getContext()更改您的
ToastToast.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();
我也经历过同样的错误！它在 MainActivity 中运行良好，但在我的 Fragment 类中运行不佳。
我通过执行以下两件事解决了这个问题：
1）在 Fragment 类中，不要像在 Activity 类中那样全局声明任何内容。
例如：
public class HomeFragment extends Fragment {
// DON'T WRITE THIS HERE LIKE YOU DO IN ACTIVITY
String myArray[] = getResources().getStringArray();
@Nullable
@Override
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
...
}
相反，在里面写任何东西
onCreateView()
方法。
2）使用
获取活动（）
先方法，后
getResource()方法
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
//WRITE LIKE THIS
String myArray[] = getActivity().getResources().getStringArray(R.array.itemsList);
...
}
我也遇到这个问题，花了一整天的时间来调试。 mediaPlayer = MediaPlayer.create(this, R.raw.sound); 只需将“this”关键字替换为来自 unity 的活动引用（unityActivity）..（如果它是一个 unity 插件）我是如何解决它的。
我在 Android 活动中遇到了这个问题。错误是我的 Toast 没有获取上下文，或者我更愿意将其称为活动的引用。
之前的代码：-
Toast.makeText(context, "Session Expire..!", Toast.LENGTH_SHORT).show();
更正代码：-
Toast.makeText(activityname.this, "Session Expire..!", Toast.LENGTH_SHORT).show();
在这种情况下你所要做的就是；
创建上下文； 将其分配给片段上下文，然后在使用创建的上下文之前检查是否为非空。
像这样...
public class MyHome extends Fragment {
**Context context;**
public MyHome() {
}
public static MyHome newInstance() {
return new MyHome();
}
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
**this.context = getContext();**
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.fragment_my_home, container, false);
if(context !=null){
Toast.makeText(context,"Hello homepage",Toast.LENGHT_LONG).show();
}
}
}
声明一个这样的方法：
private boolean checkContext(Context context) {
if(context != null)
return true;
else
return false;
}
现在，如果活动执行此方法 onCreate() 或 onStart()，如下所示：
if(checkContext(getApplicationContext() or NameActivity.this))
{
/// do stuff that hang from context
}
如果在片段中，则在 onCreateView() 或 onStart 中执行，如下所示：
if(checkContext(getContext()))
{
//dostuf that hang from context
}
我吃过几次了。看起来像是一个随机错误。与系统中的其他错误有关。