尝试插入 SQLite 时出现 NullPointerException

本文介绍了尝试插入 SQLite 时出现 NullPointerException - Android的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述

任何关注 Android 标签的人可能都熟悉我.我最难为我的高分实现 SQLite 数据库.这也是我第一次使用 SQLite.

Anyone who follows Android tags are probably familiar with me. I am having the hardest time implementing a SQLite database for my highscores. This is also my first time working with SQLite.

我试图只插入两个变量——int 和 long.我的 insert() 方法工作不正常.除了上面提到的内容之外的任何建议也很感激.

I am trying to insert just two variables -- int and long. My insert() method is not working correctly. Any advice other than the stuff talked about above is also appreciated.

预先感谢您的帮助.

Highscores.java

Highscores.java

public class Highscores extends Activity { DatabaseHelper dh; SQLiteDatabase db; @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.highscoresmain); dh = new DatabaseHelper(this); dh.openDB(db); long x = 11; int y = 22; TextView rank = (TextView)findViewById(R.id.rank); TextView percentage = (TextView)findViewById(R.id.percentage); TextView score = (TextView)findViewById(R.id.score); TextView r1r = (TextView)findViewById(R.id.r1r); TextView r1p = (TextView)findViewById(R.id.r1p); TextView r1s = (TextView)findViewById(R.id.r1s); dh.insert(x, y, db); //Line 55 scores = dh.getScore(db); percentages = dh.getPercentage(db); rank.setText("Rank Column - TEST"); percentage.setText("Percentage Column - TEST "); score.setText("Score Column - Test"); r1r.setText("test..rank"); r1p.setText("" + percentages); r1s.setText("test..score"); table = (TableLayout)findViewById(R.id.tableLayout); dh.closeDB(db); } }

DatabaseHelper.java

DatabaseHelper.java

public class DatabaseHelper extends SQLiteOpenHelper { SQLiteDatabase db; // Table columns names. private static final String RANK = "_id"; private static final String SCORE = "score"; private static final String PERCENTAGE = "percentage"; public DatabaseHelper(Context context) { super(context, DB_NAME, null, DATABASE_VERSION); } public SQLiteDatabase openDB(SQLiteDatabase db) { db = this.getWritableDatabase(); return db; } public int getPercentage(SQLiteDatabase db) { //Cursor c = db.rawQuery("SELECT " + PERCENTAGE + " FROM " + TABLE + " WHERE " + PERCENTAGE + " = " + 22 + ";", null); Cursor c = db.rawQuery("SELECT " + PERCENTAGE + " FROM " + TABLE + ";", null); int i = 0; if(c.getCount() == 0) { i = 333; } else if (c.getCount() == 1) { i = 444; } else { i = 888; /*c.moveToFirst(); int columnIndex = c.getInt(c.getColumnIndex("PERCENTAGE")); if(columnIndex != -1) { i = c.getInt(columnIndex); } else { i = 999; }*/ } c.close(); return i; } //Insert new record. public long insert(long score, int percentage, SQLiteDatabase db) { ContentValues values = new ContentValues(); values.put(SCORE, score); values.put(PERCENTAGE, percentage); return db.insert(TABLE, null, values); //Line 120 } public synchronized void closeDB(SQLiteDatabase db) { if(db != null) { db.close(); } super.close(); } public void onCreate(SQLiteDatabase db) { db.execSQL("CREATE TABLE " + TABLE + " (" + RANK + " INTEGER PRIMARY KEY AUTOINCREMENT," + SCORE + " LONG," + PERCENTAGE + " INTEGER" + ");"); }

LogCat 输出

01-03 17:09:18.232: E/AndroidRuntime(1712): FATAL EXCEPTION: main 01-03 17:09:18.232: E/AndroidRuntime(1712): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.test/com.example.test.Highscores}: java.lang.NullPointerException 01-03 17:09:18.232: E/AndroidRuntime(1712): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180) 01-03 17:09:18.232: E/AndroidRuntime(1712): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230) 01-03 17:09:18.232: E/AndroidRuntime(1712): at android.app.ActivityThread.access$600(ActivityThread.java:141) 01-03 17:09:18.232: E/AndroidRuntime(1712): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234) 01-03 17:09:18.232: E/AndroidRuntime(1712): at android.os.Handler.dispatchMessage(Handler.java:99) 01-03 17:09:18.232: E/AndroidRuntime(1712): at android.os.Looper.loop(Looper.java:137) 01-03 17:09:18.232: E/AndroidRuntime(1712): at android.app.ActivityThread.main(ActivityThread.java:5039) 01-03 17:09:18.232: E/AndroidRuntime(1712): at java.lang.reflect.Method.invokeNative(Native Method) 01-03 17:09:18.232: E/AndroidRuntime(1712): at java.lang.reflect.Method.invoke(Method.java:511) 01-03 17:09:18.232: E/AndroidRuntime(1712): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793) 01-03 17:09:18.232: E/AndroidRuntime(1712): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560) 01-03 17:09:18.232: E/AndroidRuntime(1712): at dalvik.system.NativeStart.main(Native Method) 01-03 17:09:18.232: E/AndroidRuntime(1712): Caused by: java.lang.NullPointerException 01-03 17:09:18.232: E/AndroidRuntime(1712): at com.example.test.DatabaseHelper.insert(DatabaseHelper.java:120) 01-03 17:09:18.232: E/AndroidRuntime(1712): at com.example.test.Highscores.onCreate(Highscores.java:55) 01-03 17:09:18.232: E/AndroidRuntime(1712): at android.app.Activity.performCreate(Activity.java:5104) 01-03 17:09:18.232: E/AndroidRuntime(1712): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080) 01-03 17:09:18.232: E/AndroidRuntime(1712): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144) 01-03 17:09:18.232: E/AndroidRuntime(1712): ... 11 more

推荐答案

在这一行，你打开数据库:

On this line, you open the DB:

dh.openDB(db);

这将数据库的成员变量存储在 DatabaseHelper.db 中(但没有其他地方，尤其是在 Highscores.db 中).然后，您调用:

This stores the member variable of the database in DatabaseHelper.db (but nowhere else, and particularly not in Highscores.db). Then, you call:

dh.insert(x, y, db);

使用来自 Highscores 的 null db 对象.最后，在 insert() 中，你使用这个 null db 对象:

Using a null db object from Highscores. Lastly, within insert(), you use this null db object:

return db.insert(TABLE, null, values);

你应该做的是使用以下作为你的 insert() 方法:

What you should do instead is use the following as your insert() method:

public long insert(long score, int percentage) { // Remove db parameter ContentValues values = new ContentValues(); values.put(SCORE, score); values.put(PERCENTAGE, percentage); return db.insert(TABLE, null, values); // This will use the member variable }

然后，改为调用 dh.insert(x, y);.

您还应该将 openDB(SQLiteDatabase db) 更改为 openDB() 以便它写入正确的数据库；就目前而言，它只是覆盖了局部变量的值(一旦函数的作用域完成，它就不会改变).

You should also change openDB(SQLiteDatabase db) to openDB() so that it is writing the correct DB; as it stands, it just overwrites the local variable's value (which changes nothing once the function's scope is done).