Question

我是Android开发新手，在尝试获取可写数据库时遇到NullPointerException。这是代码：

主要活动

public class MainActivity extends AppCompatActivity {

    MyDatabase myDatabase;
    SQLiteDatabase sqLiteDatabase;
    EditText textOne;
    EditText textTwo;
    Context context;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        textOne = (EditText) findViewById(R.id.textOne);
        textTwo = (EditText) findViewById(R.id.textTwo);

    }

    public void addInfo(View view) {
        String name = textOne.getText().toString();
        String address = textTwo.getText().toString();
        myDatabase = new MyDatabase(context);
        sqLiteDatabase = myDatabase.getWritableDatabase();
        myDatabase.addData(name, address, sqLiteDatabase);
        myDatabase.close();

        textOne.setText("");
        textTwo.setText("");

    }
}

我的数据库代码：

public class MyDatabase extends SQLiteOpenHelper {

    private static final String DATABASE_NAME = "info.db";
    private static final String TABLE_NAME = "Personal_Info";
    private static final String NAME_COLUMN = "names";
    private static final String ADDRESS_COLUMN = "address";
    private static final String ROW_IDENTIFIER = "_id";
    private static int DATABSE_VERSION = 1;
    private static String query = "CREATE TABLE " + TABLE_NAME + "(" + ROW_IDENTIFIER + " INTEGER PRIMARY KEY AUTOINCREMENT, " + NAME_COLUMN + " TEXT, " + ADDRESS_COLUMN + " TEXT);";

    public MyDatabase(Context context) {
        super(context, DATABASE_NAME, null, DATABSE_VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {

        try {
            db.execSQL(query);
        } catch (SQLException e) {
            e.printStackTrace();
        }

    }

    public void addData(String name, String address, SQLiteDatabase db) {

        ContentValues contentValues = new ContentValues();
        contentValues.put(NAME_COLUMN,name);
        contentValues.put(ADDRESS_COLUMN,address);
        db.insert(TABLE_NAME,null,contentValues);

    }

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

        try {
            db.execSQL("DROP TABLE IF EXISTS " + TABLE_NAME );
        } catch (SQLException e) {
            e.printStackTrace();
        }

        onCreate(db);
    }
}

MainActivity中的这一行抛出错误：

myDatabase = new MyDatabase(context);

欢迎所有输入。

Answer 1

您从未初始化变量myDatabase = new MyDatabase(this);，因此它为null。既然你有一个Activity，你也可以这样做：

this

其中onCreate()指的是Activity实例。如果您愿意，可以在addItem()内执行此操作，或将其保留在$('#estimate').validator({ custom: { 'distance': function() { if( $("#distance-group").hasClass("invalid")){ return false; console.log("ERROR") }else{ return true; console.log("NO ERROR") } } }, errors: { 'distance': "Nope" } })内的位置。

与数据库通信时，Android Studio中的NPE

1 个答案: