尋找空指針異常的原因

Question

我確定我得到的錯誤Null Pont Exception錯誤背后有一些簡單的東西，但是我看不到。 以下是相關代碼：

package com.example.gameexpensetracker;

import android.app.Activity;
import android.database.Cursor;
import android.os.Bundle;
import android.widget.Toast;

public class Chart_screen_activity extends Activity {       

    double strategyTotalDB;
    DBAdapter db = new DBAdapter(this); 



    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.chart_screen_activity);
        db.open();  
    pieGraphData();
    }  

       public void pieGraphData() {
                    Cursor cursor17 = db.rawQuery("SELECT (strategy_total) FROM strategyTotal WHERE _id=(SELECT max(_id) FROM strategyTotal)",
                    null);
        if (cursor17.moveToFirst()){ 
        strategyTotalDB = cursor17.getDouble(0);
        cursor17.close();
        Toast.makeText(this, "New entry"+strategyTotalDB+ "added!", Toast.LENGTH_LONG).show();
        db.close();
        }}}

LogCat說第23和32行引起了該問題：

12-29 10:46:39.979: D/AndroidRuntime(1754): Shutting down VM
12-29 10:46:39.979: W/dalvikvm(1754): threadid=1: thread exiting with uncaught exception (group=0x40015560)
12-29 10:46:39.989: E/AndroidRuntime(1754): FATAL EXCEPTION: main
12-29 10:46:39.989: E/AndroidRuntime(1754): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.gameexpensetracker/com.example.gameexpensetracker.Chart_screen_activity}: java.lang.NullPointerException
12-29 10:46:39.989: E/AndroidRuntime(1754):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1647)
12-29 10:46:39.989: E/AndroidRuntime(1754):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1663)
12-29 10:46:39.989: E/AndroidRuntime(1754):     at android.app.ActivityThread.access$1500(ActivityThread.java:117)
12-29 10:46:39.989: E/AndroidRuntime(1754):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:931)
12-29 10:46:39.989: E/AndroidRuntime(1754):     at android.os.Handler.dispatchMessage(Handler.java:99)
12-29 10:46:39.989: E/AndroidRuntime(1754):     at android.os.Looper.loop(Looper.java:123)
12-29 10:46:39.989: E/AndroidRuntime(1754):     at android.app.ActivityThread.main(ActivityThread.java:3683)
12-29 10:46:39.989: E/AndroidRuntime(1754):     at java.lang.reflect.Method.invokeNative(Native Method)
12-29 10:46:39.989: E/AndroidRuntime(1754):     at java.lang.reflect.Method.invoke(Method.java:507)
12-29 10:46:39.989: E/AndroidRuntime(1754):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
12-29 10:46:39.989: E/AndroidRuntime(1754):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
12-29 10:46:39.989: E/AndroidRuntime(1754):     at dalvik.system.NativeStart.main(Native Method)
12-29 10:46:39.989: E/AndroidRuntime(1754): Caused by: java.lang.NullPointerException
12-29 10:46:39.989: E/AndroidRuntime(1754):     at com.example.gameexpensetracker.Chart_screen_activity.pieGraphData(Chart_screen_activity.java:32)
12-29 10:46:39.989: E/AndroidRuntime(1754):     at com.example.gameexpensetracker.Chart_screen_activity.onCreate(Chart_screen_activity.java:23)
12-29 10:46:39.989: E/AndroidRuntime(1754):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
12-29 10:46:39.989: E/AndroidRuntime(1754):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1611)
12-29 10:46:39.989: E/AndroidRuntime(1754):     ... 11 more

這是我的DBAdapter：

import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;

public class DBAdapter {
    static final String KEY_ROWID = "_id";
    static final String KEY_TITLE = "title";
    static final String KEY_PLATFORM = "platform";
    static final String KEY_GENRE = "genre";
    static final String KEY_DATE = "date";
    static final String KEY_PRICE = "price";
    static final String KEY_TOTAL = "total";
    static final String KEY_WHEREBOUGHT = "wherebought";
    static final String TAG = "DBAdapter";

    static final String DATABASE_NAME = "gamesDB";
    static final String DATABASE_TABLE = "purchases";
    static final int DATABASE_VERSION = 1;

    static final String DATABASE_CREATE = "CREATE TABLE IF NOT EXISTS purchases(_id integer primary key autoincrement, "
            + "title text , platform text, genre text , date text , price double , wherebought text );";

    static final String DATABASE_CREATE2 = "CREATE TABLE IF NOT EXISTS grandTotal(_id integer primary key autoincrement, "
            + "total double);";

    static final String DATABASE_CREATE3 = "CREATE TABLE IF NOT EXISTS strategyTotal(_id integer primary key autoincrement, "
            + "strategy_prices double, strategy_total double);";

    static final String DATABASE_CREATE4 = "CREATE TABLE IF NOT EXISTS adventureTotal(_id integer primary key autoincrement, "
            + "adventure_prices double, adventure_total double);";

    static final String DATABASE_CREATE5 = "CREATE TABLE IF NOT EXISTS actionTotal(_id integer primary key autoincrement, "
            + "action_prices double, action_total double);";

    static final String DATABASE_CREATE6 = "CREATE TABLE IF NOT EXISTS MMOTotal(_id integer primary key autoincrement, "
            + "mmo_prices double, mmo_total double);";

    static final String DATABASE_CREATE7 = "CREATE TABLE IF NOT EXISTS racingTotal(_id integer primary key autoincrement, "
            + "racing_prices double, racing_total double);";

    static final String DATABASE_CREATE8 = "CREATE TABLE IF NOT EXISTS rpgTotal(_id integer primary key autoincrement, "
            + "rpg_prices double, rpg_total double);";

    static final String DATABASE_CREATE9 = "CREATE TABLE IF NOT EXISTS simulationTotal(_id integer primary key autoincrement, "
            + "simulation_prices double, simulation_total double);";

    static final String DATABASE_CREATE10 = "CREATE TABLE IF NOT EXISTS sportsTotal(_id integer primary key autoincrement, "
            + "sports_prices double, sports_total double);";

    static final String DATABASE_CREATE11 = "CREATE TABLE IF NOT EXISTS otherTotal(_id integer primary key autoincrement, "
            + "other_prices double, other_total double);";

    static final String DATABASE_CREATE12 = "CREATE TABLE IF NOT EXISTS puzzleTotal(_id integer primary key autoincrement, "
            + "puzzle_prices double, puzzle_total double);";


    double columntotal, strategyPriceInput, actionPriceInput, adventurePriceInput, MMOPriceInput, racingPriceInput, rpgPriceInput, simulationPriceInput, 
    sportsPriceInput, puzzlePriceInput, otherPriceInput;



    final Context context;

    DatabaseHelper DBHelper;
    SQLiteDatabase db;

    public DBAdapter(Context ctx) {
        this.context = ctx;
        DBHelper = new DatabaseHelper(context);
    }

    private static class DatabaseHelper extends SQLiteOpenHelper {
        DatabaseHelper(Context context) {
            super(context, DATABASE_NAME, null, DATABASE_VERSION);
        }

        @Override
        public void onCreate(SQLiteDatabase db) {
            try {
                db.execSQL(DATABASE_CREATE);
                db.execSQL(DATABASE_CREATE2);
                db.execSQL(DATABASE_CREATE3);
                db.execSQL(DATABASE_CREATE4);
                db.execSQL(DATABASE_CREATE5);
                db.execSQL(DATABASE_CREATE6);
                db.execSQL(DATABASE_CREATE7);
                db.execSQL(DATABASE_CREATE8);
                db.execSQL(DATABASE_CREATE9);
                db.execSQL(DATABASE_CREATE10);
                db.execSQL(DATABASE_CREATE11);
                db.execSQL(DATABASE_CREATE12);
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }

        @Override
        public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
            db.execSQL(DATABASE_CREATE2);
        }
    }

    // ---opens the database---
    public DBAdapter open() throws SQLException {
        db = DBHelper.getWritableDatabase();
        return this;
    }

    // ---closes the database---
    public void close() {
        DBHelper.close();
    }

    // ---insert a record into the database---
    public long insertRecord(String title, String platform, String genre,
            String date, double price, String wherebought) {
        ContentValues initialValues = new ContentValues();
        initialValues.put(KEY_TITLE, title);
        initialValues.put(KEY_PLATFORM, platform);
        initialValues.put(KEY_GENRE, genre);
        initialValues.put(KEY_DATE, date);
        initialValues.put(KEY_PRICE, price);
        initialValues.put(KEY_WHEREBOUGHT, wherebought);
        return db.insert(DATABASE_TABLE, null, initialValues);

    }





    // delete all records

    public void deleteAll() {
        context.deleteDatabase(DATABASE_NAME);

    }

    // ---deletes a particular record---
    public boolean deleteRecord(long rowId) {
        return db.delete(DATABASE_TABLE, KEY_ROWID + "=" + rowId, null) > 0;
    }

    // ---retrieves all the records---
    public Cursor getAllRecords() {
        return db.query(DATABASE_TABLE, new String[] { KEY_ROWID, KEY_TITLE,
                KEY_GENRE, KEY_DATE, KEY_PRICE, KEY_WHEREBOUGHT }, null, null,
                null, null, null);
    }

    // ---retrieves a particular record---
    public Cursor getRecord(long rowId) throws SQLException {
        Cursor mCursor = db.query(true, DATABASE_TABLE, new String[] {
                KEY_ROWID, KEY_TITLE, KEY_PLATFORM, KEY_GENRE, KEY_DATE,
                KEY_PRICE, KEY_WHEREBOUGHT }, KEY_ROWID + "=" + rowId, null,
                null, null, null, null);
        if (mCursor != null) {
            mCursor.moveToFirst();
        }
        return mCursor;
    }

    // ---updates a record---
    public boolean updateRecord(long rowId, String title, String platform,
            String genre, String date, double price, String wherebought) {
        ContentValues args = new ContentValues();
        args.put(KEY_TITLE, title);
        args.put(KEY_PLATFORM, platform);
        args.put(KEY_GENRE, genre);
        args.put(KEY_DATE, date);
        args.put(KEY_PRICE, price);
        args.put(KEY_WHEREBOUGHT, wherebought);
        return db.update(DATABASE_TABLE, args, KEY_ROWID + "=" + rowId, null) > 0;
    }

    public Cursor rawQuery(String string, Object object) {
        // TODO Auto-generated method stub
        return null;
    }




}

Answer 1

您的DBAdapter.rawQuery()未實現，只是一個返回null的存根。 嘗試在此null引用上調用moveToFirst()會導致NPE。

也就是說，改變

public Cursor rawQuery(String string, Object object) {
    // TODO Auto-generated method stub
    return null;
}

像

public Cursor rawQuery(String sql, String[] args) {
    return db.rawQuery(sql, args);
}

Answer 2

將數據庫初始化移到onCreate

DBAdapter db; 
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.chart_screen_activity);
db = new DBAdapter(this); 

Answer 3

看來變量cursor17為空。 也許在您的情況下，如果條件有所更改，則可能會有所幫助：

if (cursor17 != null && cursor17.moveToFirst()) {

我不知道您的DBAdapter類，但是rawQuery（）方法的第二個參數是否未設置為null，這意味着如果表strategyTotal為空，則您將獲得null？ 我將查看該表是否為空，以及查詢的原因和結果然后將交付。