空指针异常：尝试在空对象引用上调用虚方法 Android

Question

我正在尝试在 android 中创建一个数据库，并且正在从另一个文件中的 onCreateInputView() 调用 DBManager 中定义的创建函数。

以下是需要的文件。 android 的新手，不太确定发生了什么。我必须做什么？这个错误是什么意思？

DBManager.java

package com.project.pesit.finaltelim;



import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.List;

import android.app.LauncherActivity;
import android.content.Context;
import android.content.res.Resources;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.util.Log;

public class DBManager extends SQLiteOpenHelper {

    private SQLiteDatabase db;

    private final Context context;

    private static final String DATABASE_NAME = "gredict";
    private static final int DATABASE_VERSION = 1;

    private static final String DATABASE_CREATE1 ="CREATE TABLE IF NOT EXISTS wordindex (wid INTEGER(4), word VARCHAR(256), gremeaning VARCHAR(1024), isFav TINYINT(1));";
    private static final String DATABASE_CREATE2 ="CREATE TABLE IF NOT EXISTS lists (wid INTEGER(4), sid INTEGER(8), isDefault TINYINT(1));";
    private static final String DATABASE_CREATE3 ="CREATE TABLE IF NOT EXISTS gloss (sid INTEGER(8), meaning VARCHAR(1024), pos INTEGER(1));";

    public DBManager(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
        this.context=context;
    }


    @Override
    public void onCreate(SQLiteDatabase db)
    {

        db.execSQL(DATABASE_CREATE1);
        db.execSQL(DATABASE_CREATE2);
        db.execSQL(DATABASE_CREATE3);

        BufferedReader br = null;
        String sCurrentLine;

        //Populate word index table
        try
        {
            Resources res=context.getResources();
            InputStream is = res.openRawResource(R.raw.wordindex);
            br = new BufferedReader(new InputStreamReader(is));

            while ((sCurrentLine = br.readLine()) != null) {
                db.execSQL(sCurrentLine);
            }

        }

        catch
                (Exception e)
        {
            e.printStackTrace();
        }

        finally
        {
            try
            {
                if (br != null)br.close();
            }
            catch (IOException ex)
            {
                ex.printStackTrace();
            }
        }

        //Populate lists table
        try
        {

            Resources res=context.getResources();
            InputStream is = res.openRawResource(R.raw.list);
            br = new BufferedReader(new InputStreamReader(is));

            while ((sCurrentLine = br.readLine()) != null) {
                db.execSQL(sCurrentLine);
            }
        }

        catch
                (Exception e)
        {
            e.printStackTrace();
        }

        finally
        {
            try
            {
                if (br != null)br.close();
            }
            catch (IOException ex)
            {
                ex.printStackTrace();
            }
        }

        //Populate gloss table
        try
        {
            Resources res=context.getResources();
            InputStream is = res.openRawResource(R.raw.gloss);
            br = new BufferedReader(new InputStreamReader(is));

            while ((sCurrentLine = br.readLine()) != null) {
                db.execSQL(sCurrentLine);
            }
        }

        catch
                (Exception e)
        {
            e.printStackTrace();
        }

        finally
        {
            try
            {
                if (br != null)br.close();
            }
            catch (IOException ex)
            {
                ex.printStackTrace();
            }
        }




    }

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

        onCreate(db);
    }


    public Boolean open() throws SQLException
    {
        db = this.getWritableDatabase();
        return true;
    }

    public void close()
    {
        db.close();
    }


    public List<String> read(String box)
    {

        List<String> wordList = new ArrayList<String>();
        String query;
        Cursor cursor=null;
        if(box.equals("HF"))
        {
            query = "SELECT distinct word FROM wordindex WHERE wid<335";
        }
        else
        {
            query = "SELECT distinct word FROM wordindex WHERE word LIKE '"+box+"%' and wid>334";
        }
        cursor = db.rawQuery(query, null);

        if (cursor.moveToFirst()) {
            do {
                String word;
                word=cursor.getString(cursor.getColumnIndexOrThrow("word"));
                wordList.add(word);
            } while (cursor.moveToNext());
        }

        cursor.close();
        return wordList;
    }

    public List<String[]> getWord(String name){
        // TODO Auto-generated method stub
        //String[] cols=new String []{WORD,MEAN};
        //String sid=String.valueOf(id);
        List<String[]> meanings = new ArrayList<String[]>();

        Cursor c1=db.rawQuery("SELECT DISTINCT (sid) FROM lists WHERE wid = (SELECT wid FROM wordindex where word='"+name+"')",null);
        if (c1.moveToFirst()) {
            do {

                int sid=c1.getColumnIndex("sid");
                Cursor c2=db.rawQuery("SELECT pos, meaning FROM gloss WHERE sid = "+c1.getString(sid),null);

                if (c2.moveToFirst()) {
                    do {

                        int ipos=c2.getColumnIndex("pos");
                        int imean=c2.getColumnIndex("meaning");
                        String pos=null;
                        if(c2.getInt(ipos)==0)
                            pos="(n)";
                        else if(c2.getInt(ipos)==1)
                            pos="(v)";
                        else if(c2.getInt(ipos)==2)
                            pos="(adj)";
                        String mean[] = {pos,c2.getString(imean)};

                        meanings.add(mean);
                    } while (c2.moveToNext());
                }

            } while (c1.moveToNext());
        }


        return meanings;
    }

    public String getGREMeaning(String name){

        String meaning=null;
        Cursor c=db.rawQuery("SELECT gremeaning FROM wordindex where word='"+name+"'",null);
        c.moveToFirst();

        int imean=c.getColumnIndex("gremeaning");
        meaning=c.getString(imean);

        return meaning;
    }

    public String getGREMeaningWID(int wid){

        String meaning=null;
        Cursor c=db.rawQuery("SELECT gremeaning FROM wordindex where wid='"+wid+"'",null);
        c.moveToFirst();

        int imean=c.getColumnIndex("gremeaning");
        meaning=c.getString(imean);

        return meaning;
    }

    public List<ListItem> getList(String word) {
        List<ListItem> l=new ArrayList<ListItem>();
        int groupcount;
        Cursor c=db.rawQuery("SELECT * FROM lists where wid in(SELECT wid from wordindex where word='"+word+"')",null);
        int imean=c.getColumnIndex("sid");
        for(c.moveToFirst();!c.isAfterLast();c.moveToNext())
        {   Cursor s=db.rawQuery("SELECT distinct wid FROM lists where sid="+c.getString(imean),null);
            int index=s.getColumnIndexOrThrow("wid");
            groupcount=s.getCount();
            if(groupcount>1){
                ListItem li=new ListItem();
                li.setSID(c.getString(imean));
                for(s.moveToFirst();!s.isAfterLast();s.moveToNext())
                {
                    li.addWID(s.getString(index));

                }
                l.add(li);
            }
        }
        return l;
    }

    public List<ListItem> getGroupList() {
        List<ListItem> l=new ArrayList<ListItem>();

        Cursor c=db.rawQuery("SELECT sid,meaning FROM v2",null);
        for(c.moveToFirst();!c.isAfterLast();c.moveToNext())
        {
            ListItem li=new ListItem();
            li.setSID(c.getString(c.getColumnIndex("sid")));
            li.setGloss(c.getString(c.getColumnIndex("meaning")));
            l.add(li);
        }

        /*
        Cursor c=db.rawQuery("SELECT sid FROM  `v` WHERE  `count` >1",null);
        int imean=c.getColumnIndex("sid");
        for(c.moveToFirst();!c.isAfterLast();c.moveToNext())
        {
            Cursor s=db.rawQuery("SELECT sid,meaning FROM gloss where sid="+c.getString(imean),null);
            s.moveToFirst();
            ListItem li=new ListItem();
            li.setSID(s.getString(s.getColumnIndex("sid")));
            li.setGloss(s.getString(s.getColumnIndex("meaning")));
        l.add(li);
        }
        */
        return l;
    }

    public String getGloss(String sid)
    {
        Cursor c=db.rawQuery("select meaning from gloss where sid="+sid,null);
        int i=c.getColumnIndex("meaning");
        if(c.moveToFirst())
        {
            return c.getString(i);
        }
        return "null";
    }
    public String getwd(String wid)
    {
        Cursor c=db.rawQuery("select word from wordindex where wid="+wid,null);
        int i=c.getColumnIndex("word");
        if(c.moveToFirst())
        {
            return c.getString(i);
        }
        return "null";
    }

    public List<String> getGroupWords(String sid)
    {
        List<String> wordList = new ArrayList<String>();
        String query;
        Cursor cursor=null;
        query = "SELECT wid,isDefault FROM lists WHERE sid="+sid;
        cursor = db.rawQuery(query, null);
        int iwid=cursor.getColumnIndexOrThrow("wid");
        int defid=cursor.getColumnIndexOrThrow("isDefault");
        if (cursor.moveToFirst()) {
            do {
                Cursor c=db.rawQuery("SELECT word FROM wordindex WHERE wid="+cursor.getString(iwid),null);
                c.moveToFirst();
                String word;
                word=c.getString(c.getColumnIndexOrThrow("word"));

                wordList.add(word+":"+cursor.getString(defid));
            } while (cursor.moveToNext());
        }

        cursor.close();
        return wordList;
    }

    public String addToGroup(String sid, String word)
    {
        String wid=null;
        Cursor cursor=null;
        String query = "SELECT wid FROM wordindex WHERE word='"+word+"'";
        cursor = db.rawQuery(query, null);
        String result=null;
        if (cursor.moveToFirst()) {
            wid=cursor.getString(cursor.getColumnIndexOrThrow("wid"));

            cursor.close();

            query = "SELECT * FROM lists WHERE wid="+wid+" AND sid="+sid;
            cursor = db.rawQuery(query, null);
            if(cursor.moveToFirst())
            {
                result="Word already in the group.";
            }
            else
            {
                query = "INSERT INTO lists VALUES("+wid+", "+sid+",0);";
                try{
                    db.execSQL(query);
                    result="Added successfully";
                }
                catch(Exception e)
                {
                    result="Could not add the word.";
                }
            }
        }

        return result;
    }

    public String newGroup(String meaning, String pos, String word)
    {
        if(pos.equals("Noun"))
            pos="0";
        else if(pos.equals("Verb"))
            pos="1";
        else if(pos.equals("Adjective"))
            pos="2";

        String wid;
        int max;
        String query = "SELECT max(sid) FROM lists;";
        Cursor cursor = db.rawQuery(query, null);
        String result=null;
        cursor.moveToFirst();
        max=Integer.parseInt(cursor.getString(cursor.getColumnIndexOrThrow("max(sid)")));
        cursor.close();

        query = "SELECT wid FROM wordindex WHERE word='"+word+"'";
        cursor = db.rawQuery(query, null);
        cursor.moveToFirst();
        wid=cursor.getString(cursor.getColumnIndexOrThrow("wid"));
        cursor.close();

        try{
            query = "INSERT INTO lists VALUES ("+wid+", "+(max+1)+",0);";
            db.execSQL(query);
            query = "INSERT INTO gloss VALUES ("+(max+1)+", '"+meaning+"',"+pos+");";
            db.execSQL(query);

            db.execSQL("DROP VIEW v1;");
            db.execSQL("CREATE  VIEW v1 AS SELECT sid, isDefault, COUNT(sid) as count FROM lists GROUP BY sid;");
            db.execSQL("DROP VIEW v2;");
            db.execSQL("CREATE  VIEW v2 AS SELECT sid,meaning FROM gloss where sid IN (SELECT sid FROM v1 WHERE count>1);");

            result="Created successfully";
        }
        catch(Exception e)
        {
            result="Could not create group.";
        }

        return result;
    }

    public List<String> search(String text,String box)
    {

        List<String> wordList = new ArrayList<String>();
        String query;
        Cursor cursor=null;
        if(box.equals("HF"))
        {
            query = "SELECT distinct word FROM wordindex WHERE wid<335 and word LIKE '"+text+"%'";
        }
        else
        {
            query = "SELECT distinct word FROM wordindex WHERE word LIKE '"+box+"%' and wid>334 and word LIKE'"+text+"%'";
        }
        cursor = db.rawQuery(query, null);

        if (cursor.moveToFirst()) {
            do {
                String word;
                word=cursor.getString(cursor.getColumnIndexOrThrow("word"));
                // Adding word to list
                //  Toast.makeText(getApplicationContext(), word, Toast.LENGTH_LONG).show();
                wordList.add(word);
            } while (cursor.moveToNext());
        }

        cursor.close();
        return wordList;
    }
    public List<ListItem> searchGroup(String text)
    {
        List<ListItem> l=new ArrayList<ListItem>();
        //List<String> wordList = new ArrayList<String>();
        String query;
        Cursor c=null;
        query = "SELECT * FROM wordindex WHERE word LIKE'"+text+"%'";

        c = db.rawQuery(query, null);

        for(c.moveToFirst();!c.isAfterLast();c.moveToNext())
        {
            Cursor s=db.rawQuery("SELECT sid,meaning FROM gloss where sid in(SELECT sid from lists where wid="+c.getString(c.getColumnIndexOrThrow("wid"))+")",null);
            for(s.moveToFirst();!s.isAfterLast();s.moveToNext())
            {
                //s.moveToFirst();
                ListItem li=new ListItem();
                li.setSID(s.getString(s.getColumnIndex("sid")));
                li.setGloss(s.getString(s.getColumnIndex("meaning")));
                l.add(li);
            }

        }
        c.close();
        return l;
    }
    public String getID(String word1)
    {
        Cursor c=db.rawQuery("select wid from wordindex where word='"+word1+"'",null);
        int i=c.getColumnIndex("wid");
        if(c.moveToFirst())
        {
            return c.getString(i);
        }
        return "null";
    }

    public List<String> getFavList()
    {

        List<String> wordList = new ArrayList<String>();
        String query;
        Cursor cursor=null;
        query = "SELECT distinct word FROM wordindex WHERE isFav=1";
        cursor = db.rawQuery(query, null);

        if (cursor.moveToFirst()) {
            do {
                String word;
                word=cursor.getString(cursor.getColumnIndexOrThrow("word"));
                wordList.add(word);
            } while (cursor.moveToNext());
        }

        cursor.close();
        return wordList;
    }

    public Boolean isFav(String word)
    {
        Cursor c=db.rawQuery("SELECT isFav FROM wordindex WHERe word='"+word+"'",null);
        int i=c.getColumnIndex("isFav");
        if(c.moveToFirst())
        {
            if(c.getString(i).equals("1"))
            {
                return true;
            }

            else
            {
                return false;
            }
        }
        return false;
    }

    public void toggleFav(String word, Boolean isFav)
    {
        String fav;
        if(isFav)
        {
            fav="1";
        }

        else
        {
            fav="0";
        }
        db.execSQL("UPDATE wordindex SET isFav="+fav+" WHERE word='"+word+"'");


    }

    public int getWordCount()
    {
        Cursor c=db.rawQuery("SELECT MAX(wid) FROM wordindex",null);
        int i=c.getColumnIndex("MAX(wid)");
        if(c.moveToFirst())
        {
            return c.getInt(i);
        }

        else
        {
            return 0;
        }
    }
    public String getBoxCount(String box)
    {

        //List<String> wordList = new ArrayList<String>();
        String query;
        String query1;
        String result="";
        Cursor cursor=null;
        Cursor cursor1=null;

        if(box.equals("HF"))
        {
            return "1@334";
        }
        else
        {   query1 = "SELECT MIN(wid) AS count FROM wordindex WHERE word LIKE '"+box+"%' and wid>334";
            query = "SELECT MAX(wid) AS count FROM wordindex WHERE word LIKE '"+box+"%' and wid>334";
        }
        cursor = db.rawQuery(query, null);
        cursor1 = db.rawQuery(query1, null);

        if (cursor1.moveToFirst()) {
            result+=cursor1.getString(cursor1.getColumnIndexOrThrow("count"));
        }
        if (cursor.moveToFirst()) {
            result+="@"+cursor.getString(cursor.getColumnIndexOrThrow("count"));
        }

        cursor.close();
        return result;
    }
    public String removefromgroup(String item,String sid)
    {
        String result;
        String query;
        Log.d("ITEM",item);
        Log.d("MEANING",getGloss(sid));
        String wid=getID(item);
        query="DELETE FROM lists WHERE wid="+wid+" AND sid="+sid;
        try{
            db.execSQL(query);
            result=item+" removed successfully";
        }
        catch(Exception e)
        {
            e.printStackTrace();
            result="Could not delete the word.";
        }
        return result;
    }}

onCreateInputView.java

@Override public View onCreateInputView() {
        mInputView = (KeyboardView) getLayoutInflater().inflate(
                R.layout.input, null);
        mInputView.setOnKeyboardActionListener(this);
        mInputView.setKeyboard(mQwertyKeyboard);
        DBManager mDBAdapter = new DBManager(getApplicationContext());
        //  boolean a=mDBAdapter.checkDatabase(getApplicationContext());
        //if (!mDBAdapter.checkDatabase(getApplicationContext())) {
//              try{
//                  sampleDB=getApplicationContext().openOrCreateDatabase(DB_NAME, MODE_PRIVATE, null);
//
//              }catch(SQLiteException e){
//                  Toast.makeText(getApplicationContext(), "Could not create database", Toast.LENGTH_SHORT).show();
//              }sampleDB.close();


            mDBAdapter.onCreate(sampleDB);
            Toast.makeText(getApplicationContext(), "Database Created", Toast.LENGTH_SHORT).show();
       // }else{

           //Toast.makeText(getApplicationContext(), "Available", Toast.LENGTH_SHORT).show();
        //}
        return mInputView;

    }

错误日志如下：

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.database.sqlite.SQLiteDatabase.execSQL(java.lang.String)' on a null object reference
            at com.project.pesit.finaltelim.DBManager.onCreate(DBManager.java:44)
            at com.project.pesit.finaltelim.SoftKeyboard.onCreateInputView(SoftKeyboard.java:118)
            at android.inputmethodservice.InputMethodService.updateInputViewShown(InputMethodService.java:1108)
            at android.inputmethodservice.InputMethodService.showWindowInner(InputMethodService.java:1493)
            at android.inputmethodservice.InputMethodService.showWindow(InputMethodService.java:1468)
            at android.inputmethodservice.InputMethodService$InputMethodImpl.showSoftInput(InputMethodService.java:433)
            at android.inputmethodservice.IInputMethodWrapper.executeMessage(IInputMethodWrapper.java:203)
            at com.android.internal.os.HandlerCaller$MyHandler.handleMessage(HandlerCaller.java:37)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5312)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:901)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:696)

Answer 1

SQLiteOpenHelper.onCreate()的参数(db)是null；这就是抛出 NullPointerException 的原因。

Answer 2

您永远不应该自己调用 onCreate 方法，SQLiteOpenHelper class 会自动为您执行此操作。

从您位于 onCreateInputView.java 的代码中删除此行：

mDBAdapter.onCreate(sampleDB);

空指针异常：尝试在空对象引用上调用虚方法 Android

Null pointer exception: Attempt to invoke virtual method on a null object reference Android

java

android