致命异常： main ，尝试在空对象引用上调用虚拟方法 [重复]

Posted 2023-04-19

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【中文标题】致命异常： main ，尝试在空对象引用上调用虚拟方法 [重复]【英文标题】：Fatal Exception: main , Attempt to invoke Virtual method on a Null Object Reference [duplicate] 【发布时间】：2018-04-11 03:26:43 【问题描述】：

我在启动应用程序时遇到问题。我得到了这个问题，尝试在空对象引用上调用虚拟方法。我认为是因为我没有正确创建 LoadJobList 的实例，但我不完全确定。

主要活动 [已编辑]

公共类 MainActivity 扩展 AppCompatActivity

IntDataBaseHelper intDataBaseHelper;
ArrayAdapter<String> mAdapter;
ListView lstJob;


@Override
protected void onCreate(Bundle savedInstanceState) 
    super.onCreate(savedInstanceState);
    setContentView(R.layout.header);

/// create instance of db helper and jobs

    IntDataBaseHelper myhelper = new IntDataBaseHelper(this);
    lstJob = (ListView)findViewById(R.id.lstJob);
    LoadJobList();

    //  Create the database (only if it doesn't exists)
    //  does so by copying from the assets
    if (CopyDBFromAssets.createDataBase(this,IntDataBaseHelper.DB_NAME)) 


        // Get the data from the database
        ArrayList<String> jobs = intDataBaseHelper.getJobList();
        for (String s : jobs) 
            Log.d("JobList", "Found Job " + s);
        
     else 
        throw new RuntimeException("No Usable Database exists or was copied from the assets.");
    



   // loads job to screen
    private void LoadJobList() 

       ArrayList<String> Joblist = intDataBaseHelper.getJobList();
       if (mAdapter == null) 
           mAdapter = new ArrayAdapter<String>(this,R.layout.header,R.id.header);
           mAdapter = new ArrayAdapter<>(this,R.layout.row,R.id.BtnComplete);
           mAdapter = new ArrayAdapter<>(this, R.layout.row, R.id.Job_name,Joblist);

           lstJob.setAdapter(mAdapter);
        else 
           mAdapter.clear();
           mAdapter.addAll(Joblist);
           mAdapter.notifyDataSetChanged();
       
   


   public void JobComplete(View view)
   View parent = (View)view.getParent();
   TextView taskTextView=(TextView)parent.findViewById(R.id.BtnComplete);
   Log.e("String",(String) taskTextView.getText());

日志猫

[ 10-30 07:24:10.779  1506: 1551 D/]
HostConnection::get() New Host Connection established 0x949247c0, tid 1551
    10-30 07:24:11.120 2774-2774/? E/androidRuntime: FATAL EXCEPTION: main
                                                     Process: com.example.joelg.clapp, PID: 2774
                                                     java.lang.RuntimeException: Unable to start activity 

        ComponentInfocom.example.joelg.clapp/com.example.joelg.clapp.MainActivity: java.lang.NullPointerException: Attempt to invoke virtual method 'java.util.ArrayList com.example.joelg.clapp.IntDataBaseHelper.getJobList()' on a null object reference
                                                             at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2817)
                                                             at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2892)
                                                             at android.app.ActivityThread.-wrap11(Unknown Source:0)
                                                             at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1593)
                                                             at android.os.Handler.dispatchMessage(Handler.java:105)
                                                             at android.os.Looper.loop(Looper.java:164)
                                                             at android.app.ActivityThread.main(ActivityThread.java:6541)
                                                             at java.lang.reflect.Method.invoke(Native Method)
                                                             at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
                                                             at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)
                                                          Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.util.ArrayList com.example.joelg.clapp.IntDataBaseHelper.getJobList()' on a null object reference
                                                             at com.example.joelg.clapp.MainActivity.LoadJobList(MainActivity.java:52)
                                                             at com.example.joelg.clapp.MainActivity.onCreate(MainActivity.java:31)
                                                             at android.app.Activity.performCreate(Activity.java:6975)
                                                             at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1213)
                                                             at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2770)
                                                             at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2892) 
                                                             at android.app.ActivityThread.-wrap11(Unknown Source:0) 
                                                             at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1593) 
                                                             at android.os.Handler.dispatchMessage(Handler.java:105) 
                                                             at android.os.Looper.loop(Looper.java:164) 
                                                             at android.app.ActivityThread.main(ActivityThread.java:6541) 
                                                             at java.lang.reflect.Method.invoke(Native Method) 
                                                             at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240) 
                                                             at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)

getJobList 方法

  public  ArrayList<String> getJobList() 
        ArrayList<String> JobList = new ArrayList<>();
        SQLiteDatabase db = this.getReadableDatabase();
        Cursor cursor =  db.query(DB_TABLE,new String[]
                DB_COLUMN,null,null,null,null,null);
        while(cursor.moveToNext())
            int index = cursor.getColumnIndex(DB_COLUMN);
            JobList.add(cursor.getString(index));
        

        cursor.close();
        db.close();
        return JobList;

数据库助手类

package com.example.joelg.clapp;

import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteException;
import android.database.sqlite.SQLiteOpenHelper;
import android.util.Log;

import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.util.ArrayList;

/**
 * Created by joelg on 22/10/2017.
 */
public class IntDataBaseHelper extends SQLiteOpenHelper


        public static  String DB_PATH ="//data/data/com.example.joelg.clapp/databases";
        public static final String DB_NAME = "JobList";
        private static final String DB_COLUMN = "jobNM";
        private static final String DB_TABLE = "job";
        public static final String DB_JOB_DETAILS = "jobDetails";
        private static final String DB_ISDONE = "jobIsDone";
        private SQLiteDatabase JobListDatabase;
        private final Context jobContext;


        /**
         * constructor t
         */
        public IntDataBaseHelper (Context context) 

            super (context, DB_NAME,null, 1);
            this.jobContext = context;
            DB_PATH = jobContext.getDatabasePath(DB_NAME).getPath();
        


        public void openDataBase() 
            // open the database
            String JobListPath = DB_PATH;
            JobListDatabase = SQLiteDatabase.openDatabase(
                    JobListPath,null,SQLiteDatabase.OPEN_READONLY);
        

        // Getting Job Count
        public  ArrayList<String> getJobList() 
            ArrayList<String> JobList = new ArrayList<>();
            SQLiteDatabase db = this.getReadableDatabase();
            Cursor cursor =  db.query(DB_TABLE,new String[]
                    DB_COLUMN,null,null,null,null,null);
            while(cursor.moveToNext())
                int index = cursor.getColumnIndex(DB_COLUMN);
                JobList.add(cursor.getString(index));
            

            cursor.close();
            db.close();
            return JobList;
        


        // Gets the job state if it has been competed or not
        public ArrayList<String> getIsDone() 
            ArrayList<String>  IsDone = new ArrayList<>();
            SQLiteDatabase db = this.getReadableDatabase();
            Cursor cursor = db.query(DB_TABLE,new String[]DB_ISDONE,null,null,null,null,null);
               while(cursor.moveToFirst())
                   int index = cursor.getColumnIndex(DB_ISDONE);
                   IsDone.add(cursor.getString(index));
               

               cursor.close();
               db.close();
               return IsDone;
         




        @Override
        public synchronized void close()

            if(JobListDatabase !=null)
                JobListDatabase.close();

                super.close();
            
        
        @Override
        public  void onCreate(SQLiteDatabase db) 

        
        @Override
        public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion)

【问题讨论】：

名为intDataBaseHelper的对象从未初始化，所以它是null。 【参考方案1】：

public class MainActivity extends AppCompatActivity 

IntDataBaseHelper intDataBaseHelper;
ArrayAdapter<String> mAdapter;
ListView lstJob;


@Override
protected void onCreate(Bundle savedInstanceState) 
    super.onCreate(savedInstanceState);
    setContentView(R.layout.header);

/// 创建数据库助手和作业的实例

    intDataBaseHelper = new IntDataBaseHelper(this);
    lstJob = (ListView)findViewById(R.id.lstJob);
    LoadJobList();

    //  Create the database (only if it doesn't exists)
    //  does so by copying from the assets
    if (CopyDBFromAssets.createDataBase(this,IntDataBaseHelper.DB_NAME)) 


        // Get the data from the database
        ArrayList<String> jobs = myhelper.getJobList();
        for (String s : jobs) 
            Log.d("JobList", "Found Job " + s);
        
     else 
        throw new RuntimeException("No Usable Database exists or was copied from the assets.");
    



   // loads job to screen
    private void LoadJobList() 

       ArrayList<String> Joblist = intDataBaseHelper.getJobList();
       if (mAdapter == null) 
           mAdapter = new ArrayAdapter<String>(this,R.layout.header,R.id.header);
           mAdapter = new ArrayAdapter<>(this,R.layout.row,R.id.BtnComplete);
           mAdapter = new ArrayAdapter<>(this, R.layout.row, R.id.Job_name,Joblist);

           lstJob.setAdapter(mAdapter);
        else 
           mAdapter.clear();
           mAdapter.addAll(Joblist);
           mAdapter.notifyDataSetChanged();
       
   


   public void JobComplete(View view)
   View parent = (View)view.getParent();
   TextView taskTextView=(TextView)parent.findViewById(R.id.BtnComplete);
   Log.e("String",(String) taskTextView.getText());

名为 intDataBaseHelper 的对象从未初始化，因此为 null。

【讨论】：

这不是什么“intDataBaseHelper = new IntDataBaseHelper(this);”有吗？它初始化了 databaseHelper 因为 ArrayList Joblist = intDataBaseHelper.getJobList();在这一行，您调用尚未初始化的数据库函数。好的，所以我将行更改为“ intDataBaseHelper = new IntDataBaseHelper(this); ”，现在抛出“android.database.sqlite.SQLiteException: no such table: job (code 1):” any想法？我接受了他的回复，它解决了第一个问题，但它导致了这个问题。这几天我不能再问任何问题了，因为我显然问得太多了此问题可能与您尝试访问的数据库中不存在该表的数据库有关。发布 getJobList() 方法的代码。【参考方案2】：

初始化intDataBaseHelper并使用它，如下所示

所以方法将是 .

public class MainActivity extends AppCompatActivity 

    IntDataBaseHelper intDataBaseHelper;
    ArrayAdapter<String> mAdapter;
    ListView lstJob;


    @Override
    protected void onCreate(Bundle savedInstanceState) 
        super.onCreate(savedInstanceState);
        setContentView(R.layout.header);

/// create instance of db helper and jobs

        intDataBaseHelper = new IntDataBaseHelper(this);
        lstJob = (ListView)findViewById(R.id.lstJob);
        LoadJobList();

        //  Create the database (only if it doesn't exists)
        //  does so by copying from the assets
        if (CopyDBFromAssets.createDataBase(this,IntDataBaseHelper.DB_NAME)) 


            // Get the data from the database
            ArrayList<String> jobs = intDataBaseHelper.getJobList();
            for (String s : jobs) 
                Log.d("JobList", "Found Job " + s);
            
         else 
            throw new RuntimeException("No Usable Database exists or was copied from the assets.");
        
    


       // loads job to screen
        private void LoadJobList() 

           ArrayList<String> Joblist = intDataBaseHelper.getJobList();
           if (mAdapter == null) 
               mAdapter = new ArrayAdapter<String>(this,R.layout.header,R.id.header);
               mAdapter = new ArrayAdapter<>(this,R.layout.row,R.id.BtnComplete);
               mAdapter = new ArrayAdapter<>(this, R.layout.row, R.id.Job_name,Joblist);

               lstJob.setAdapter(mAdapter);
            else 
               mAdapter.clear();
               mAdapter.addAll(Joblist);
               mAdapter.notifyDataSetChanged();
           
       


       public void JobComplete(View view)
       View parent = (View)view.getParent();
       TextView taskTextView=(TextView)parent.findViewById(R.id.BtnComplete);
       Log.e("String",(String) taskTextView.getText());

【讨论】：

这会抛出无法解析符号“myhelper” @JoelGeri 检查编辑我已经编辑了我的帖子以显示您的更改，但我仍然遇到同样的错误 @JoelGeri，你做得不对。还有 IntDataBaseHelper myhelper = new IntDataBaseHelper(this); ，应该是 intDataBaseHelper = new IntDataBaseHelper(this);和谢谢

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