Question

我正在尝试通过简单的游标适配器将我的数据库填充到listview，意图是新活动。不知道为什么会崩溃？

这是我的数据库助手类：

public class DBhelper extends SQLiteOpenHelper {

    public static final String DATABASE_NAME = "Mydatabase.db";
    public static final String TABLE_NAME = "contacts";
    public static final String _ID = "_ID";
    public static final String COL_2 = "NAME";
    public static final String COL_3 = "PHONE";
    public static final String COL_4 = "EMAIL";
    public static final String COL_5 = "HQ";
    public static final String COL_6 = "ADDRESS";


    public DBhelper(Context context) {
        super(context, DATABASE_NAME, null, 1);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL("create table contacts"+ "(_id integer primary key autoincrement, name text, phone text, email text, hq text, address text)");

    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        db.execSQL("DROP TABLE IF EXISTS contacts");
        onCreate(db);
    }

    public boolean insertContact (String name, String phone, String email, String hq, String address){

        SQLiteDatabase db = this.getWritableDatabase();
        ContentValues contentValues = new ContentValues();
        contentValues.put(COL_2,name);
        contentValues.put(COL_3,phone);
        contentValues.put(COL_4,email);
        contentValues.put(COL_5,hq);
        contentValues.put(COL_6,address);
        db.insert(TABLE_NAME,null,contentValues);
        return true;
    }

    public Cursor getAllDAta(){
        Cursor cursor;
        String[] columns = {DBhelper._ID, DBhelper.COL_2, DBhelper.COL_3, DBhelper.COL_4, DBhelper.COL_5, DBhelper.COL_6};
        SQLiteDatabase db = this.getWritableDatabase();
        cursor = db.query(DBhelper.TABLE_NAME,columns, null, null, null, null, null);
        return cursor;
    } 

    DBhelper dbHelper;
    SQLiteDatabase database;
    Context context;
    public DBhelper open() throws SQLException {
    dbHelper = new DBhelper(context);
    database = dbHelper.getWritableDatabase();
    return this;
}

}

MainActivity.java

public class MainActivity extends AppCompatActivity {

    DBhelper mydb;
    EditText name, phone, email, hq, address;
    Button addData, viewData;

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

        mydb = new DBhelper(this);
        name = (EditText) findViewById(R.id.editName);
        phone = (EditText) findViewById(R.id.editPhone);
        email = (EditText) findViewById(R.id.editEmail);
        hq = (EditText) findViewById(R.id.editHQ);
        address = (EditText) findViewById(R.id.editAddress);
        addData = (Button) findViewById(R.id.addData);
        viewData = (Button) findViewById(R.id.viewData);
    }

    public void addClick(View view){

        String nm, ph, em, h, as;
        nm = name.getText().toString();
        ph = phone.getText().toString();
        em = email.getText().toString();
        h = hq.getText().toString();
        as = address.getText().toString();
        boolean checkk = mydb.insertContact(nm,ph,em,h,as);
        if (checkk = true) {
            Toast.makeText(MainActivity.this, "Data Added", Toast.LENGTH_LONG).show();
        }else {
            Toast.makeText(MainActivity.this,"Data Not Added",Toast.LENGTH_LONG).show();

        }
    }

    public void showClick(View v1){
        Intent intent = new Intent(MainActivity.this,Displayc.class);
        startActivity(intent);
        finish();
    }
}

我的展示活动

public class Displayc extends AppCompatActivity {
    ListView listView;
    DBhelper mydb1;
    SimpleCursorAdapter adapter;
    final String[] from = new String[] {DBhelper._ID, DBhelper.COL_2, DBhelper.COL_3, DBhelper.COL_4, DBhelper.COL_5, DBhelper.COL_6};
    final int[] to = new int[] {R.id.idv, R.id.namev, R.id.phonev, R.id.emailv, R.id.hqv, R.id.addressv };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_displayc);
        listView = (ListView) findViewById(R.id.list_sq);
        mydb1 = new DBhelper(this.getBaseContext());
        mydb1.open();
        Cursor c = mydb1.getAllDAta();

        adapter = new SimpleCursorAdapter(getBaseContext(),R.layout.template,c,from,to,0);
        listView.setAdapter(adapter);
    }
}

错误讯息：

[java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.bibulkumar.sqliteapp/com.example.bibulkumar.sqliteapp.Displayc}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.database.sqlite.SQLiteDatabase android.content.Context.openOrCreateDatabase(java.lang.String, int, android.database.sqlite.SQLiteDatabase$CursorFactory, android.database.DatabaseErrorHandler)' on a null object reference
                                                                             [Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.database.sqlite.SQLiteDatabase android.content.Context.openOrCreateDatabase(java.lang.String, int, android.database.sqlite.SQLiteDatabase$CursorFactory, android.database.DatabaseErrorHandler)' on a null object reference

Answer 1

Context context;
public DBhelper open() throws SQLException {
    dbHelper = new DBhelper(context);
    database = dbHelper.getWritableDatabase();
    return this;

您在此处使用的context未初始化，因此null。因此，当你打电话给getWritableDatabase()时，就是NPE。

删除open()以及对它的任何调用 - 您实际上并不需要它。

关于通过SimpleCursorAdapter检索SQlite数据到listiew的Android NullPointerException

1 个答案: