尝试删除行时出现NullPointerException

Question

尝试在空对象引用

上调用虚方法int android.database.sqlite.SQLiteDatabase.delete(java.lang.String, java.lang.String, java.lang.String[])

由于我的代码很长，与手头问题无关的部分已被删除（它们工作正常，与数据库无关）

这是我的代码：

protected static final String ACTIVITY_NAME = "ChatWindow";
ListView myDisplay;
EditText myChat;
Button mySend;
ArrayList<String> myList;
ChatAdapter messageAdapter;
public static Cursor cursor;
SQLiteDatabase db;
Boolean isTablet;
FrameLayout fLayout;
ChatDatabaseHelper cdh;


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

    //declarations
    myDisplay = (ListView) findViewById(R.id.display);
    myChat = (EditText) findViewById(R.id.chat);
    mySend = (Button) findViewById(R.id.sndBtn);
    myList = new ArrayList<>();
    //in this case, “this” is the ChatWindow, which is-A Context object

    messageAdapter =new ChatAdapter( this );
    myDisplay.setAdapter (messageAdapter);

    cdh = new ChatDatabaseHelper(this);
    db = cdh.getWritableDatabase();

    cursor = db.rawQuery("SELECT * from ChatDatabaseHelper", null);

    cursor.moveToFirst();
    while(!cursor.isAfterLast() ){
        myList.add(cursor.getString(cursor.getColumnIndex(ChatDatabaseHelper.KEY_MESSAGE)));
        Log.i(ACTIVITY_NAME, "SQL MESASAGE:" + cursor.getString(
                cursor.getColumnIndex(ChatDatabaseHelper.KEY_MESSAGE)));
        Log.i(ACTIVITY_NAME, "Cursor's column count =" + cursor.getColumnCount() );
        cursor.moveToNext();


    //send button actions
    mySend.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            String message;
            message = myChat.getText().toString();
            myList.add(message);
            messageAdapter.notifyDataSetChanged(); //this restarts the   process of getCount()/getView()
            myChat.setText("");

            ContentValues cValues = new ContentValues();
            cValues.put(ChatDatabaseHelper.KEY_MESSAGE, message);
            db.insert(TABLE_NAME, "NullColumnName", cValues);

        }
    });

//This is where I need help
public void deleteEntry (String id) {

    Log.i("deleteRow", id);
//The log prints just fine so this method is being run and I have the correct value for id

    db.delete(ChatDatabaseHelper.TABLE_NAME, ChatDatabaseHelper.KEY_ID + "=" + id, null);
}

//You can ignore anything below
private class ChatAdapter extends ArrayAdapter<String> {
    public ChatAdapter(Context ctx) {
        super(ctx, 0);
    }
    @Override
    public int getCount() {
        return myList.size();
    }
    @Override
    public String getItem(int position) {
        return myList.get(position);
    }
    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        LayoutInflater inflater = ChatWindow.this.getLayoutInflater();
        View result = null ;
        if(position%2 == 0)
            result = inflater.inflate(R.layout.chat_row_incoming, null);
        else
            result = inflater.inflate(R.layout.chat_row_outgoing, null);

        TextView message = (TextView)result.findViewById(R.id.message_text);
        message.setText(   getItem(position)  ); // get the string at position
        return result;
    }
    public long getId(int position) {

        return Long.parseLong(myList.get(position));
    }
    public long getItemId(int position) {

        cursor.moveToPosition(position);
        return cursor.getInt(cursor.getColumnIndex(ChatDatabaseHelper.KEY_ID));
    }
}

protected void onDestroy() {
    super.onDestroy();
    cursor.close();
    db.close();
}

以下是数据库活动的代码：

public class ChatDatabaseHelper extends SQLiteOpenHelper {
    private static int VERSION_NUM = 1;
    private static String DATABASE_NAME = "Messages.db";
    final static String TABLE_NAME = "ChatDatabaseHelper";
    final static String KEY_ID = "id";
    final static String KEY_MESSAGE = "MESSAGE";

    private static final String CREATE_TABLE = "CREATE TABLE " + TABLE_NAME + "( "
            + KEY_ID + " INTEGER PRIMARY KEY AUTOINCREMENT, "
            + KEY_MESSAGE + " text "
            + " ); ";

    public ChatDatabaseHelper(Context ctx) {
        super(ctx, DATABASE_NAME, null, VERSION_NUM);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL(CREATE_TABLE);
        Log.i("ChatDatabaseHelper", "Calling onCreate");
    }

    public void onUpgrade(SQLiteDatabase db) {
        db.execSQL("DROP TABLE IF EXISTS '" + TABLE_NAME + "'");
        onCreate(db);
    }

    public void onUpgrade(SQLiteDatabase db, int oldVer, int newVer) {
        Log.i("ChatDatabaseHelper", "Calling onUpgrade, oldVersion=" + oldVer + " newVersion=" + newVer);
        db.execSQL("DROP TABLE IF EXISTS '" + TABLE_NAME + "'");
        onCreate(db);
    }
}

这是我的堆栈跟踪：

   E/AndroidRuntime: FATAL EXCEPTION: main
              Process: com.example.himanshu.androidlabs, PID: 22613
              java.lang.NullPointerException: Attempt to invoke virtual method 'int android.database.sqlite.SQLiteDatabase.delete(java.lang.String, java.lang.String, java.lang.String[])' on a null object reference
                  at com.example.himanshu.androidlabs.ChatWindow.deleteEntry(ChatWindow.java:159)
                  at com.example.himanshu.androidlabs.MessageFragment$1.onClick(MessageFragment.java:55)
                  at android.view.View.performClick(View.java:6256)
                  at android.view.View$PerformClick.run(View.java:24701)
                  at android.os.Handler.handleCallback(Handler.java:789)
                  at android.os.Handler.dispatchMessage(Handler.java:98)
                  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)