Android Studio - 将数据从数据库显示到TextView

Question

我试图通过onMarkerClick将我插入数据库的数据显示到TextView。我还检查了this question以及其他问题和教程视频的解决方案，但我总是会收到此错误：

01-27 10:33:49.747    8287-8287/com.example.diana.ippv2 D/DatabaseHandler﹕ Database created
01-27 10:33:49.747    8287-8287/com.example.diana.ippv2 D/AndroidRuntime﹕ Shutting down VM
01-27 10:33:49.747    8287-8287/com.example.diana.ippv2 W/dalvikvm﹕ threadid=1: thread exiting with uncaught exception (group=0x41d2d2a0)
01-27 10:33:49.757    8287-8287/com.example.diana.ippv2 E/AndroidRuntime﹕ FATAL EXCEPTION: main
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.diana.ippv2/com.example.diana.ippv2.MainActivity}: java.lang.NullPointerException
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2016)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2117)
            at android.app.ActivityThread.access$700(ActivityThread.java:134)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1218)
            at android.os.Handler.dispatchMessage(Handler.java:99)
            at android.os.Looper.loop(Looper.java:137)
            at android.app.ActivityThread.main(ActivityThread.java:4867)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:511)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1007)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:774)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.NullPointerException
            at android.app.Activity.findViewById(Activity.java:1859)
            at com.example.diana.ippv2.MapFragment.<init>(MapFragment.java:20)
            at com.example.diana.ippv2.MainActivity.<init>(MainActivity.java:10)
            at java.lang.Class.newInstanceImpl(Native Method)
            at java.lang.Class.newInstance(Class.java:1319)
            at android.app.Instrumentation.newActivity(Instrumentation.java:1068)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2007)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2117)
            at android.app.ActivityThread.access$700(ActivityThread.java:134)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1218)
            at android.os.Handler.dispatchMessage(Handler.java:99)
            at android.os.Looper.loop(Looper.java:137)
            at android.app.ActivityThread.main(ActivityThread.java:4867)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:511)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1007)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:774)
            at dalvik.system.NativeStart.main(Native Method)

这是我的LotData课程

public class LotData
{
    private String _address, _area, _utilities, _accessibility, _price;
    private int _id;

    public LotData(int id, String address, String area, String utilities, String accessibility, String price)
    {
        _id = id;
        _address = address;
        _area = area;
        _utilities = utilities;
        _accessibility = accessibility;
        _price = price;
    }

    public int getId() { return _id; }
    public String getAddress() { return _address; }
    public String getArea() { return _area; }
    public String getUtilities() { return _utilities; }
    public String getAccessibility() { return _accessibility; }
    public String getPrice() { return _price; }
}

DatabaseHandler类：

public class DatabaseHandler extends SQLiteOpenHelper
{
    public static final int DATABASE_VERSION = 1;
    public static final String DATABASE_NAME = "lotManager", TABLE_NAME = "lot_data", LOT_ID = "lot_id", LOT_ADDRESS = "lot_address", LOT_AREA = "lot_area", LOT_UTILITIES = "lot_utilities", LOT_ACCESSIBILITY = "lot_accessibility", LOT_PRICE = "lot_price";

    public DatabaseHandler(Context context)
    {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);

        Log.d("DatabaseHandler", "Database created");
    }

    //Creating tables
    @Override
    public void onCreate(SQLiteDatabase db)
    {
        db.execSQL("CREATE TABLE" + TABLE_NAME + "(" + LOT_ID + "INTEGER PRIMARY KEY AUTOINCREMENT," + LOT_ADDRESS + "TEXT," + LOT_AREA + "TEXT," + LOT_UTILITIES + "TEXT," + LOT_ACCESSIBILITY + "TEXT," + LOT_PRICE + "TEXT");

        Log.d("DatabaseHandler", "Table created");
    }

    //Upgrading database
    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion)
    {
        db.execSQL("DROP TABLE IF EXISTS" + TABLE_NAME);

        onCreate(db);
    }

    //Adding new data
    public void addData(LotData lotData)
    {
        SQLiteDatabase db = this.getWritableDatabase();
        ContentValues values = new ContentValues();

        values.put(LOT_ADDRESS, lotData.getAddress());
        values.put(LOT_AREA, lotData.getArea());
        values.put(LOT_UTILITIES, lotData.getUtilities());
        values.put(LOT_ACCESSIBILITY, lotData.getAccessibility());
        values.put(LOT_PRICE, lotData.getPrice());

        db.insert(TABLE_NAME, null, values);
        db.close();
    }

    //Retrieving data
    public LotData getData(int id)
    {
        SQLiteDatabase db = this.getReadableDatabase();
        Cursor cursor = db.query(TABLE_NAME, new String[] { LOT_ID, LOT_ADDRESS, LOT_AREA, LOT_UTILITIES, LOT_ACCESSIBILITY, LOT_PRICE }, LOT_ID + "=?", new String[]{ String.valueOf(id) }, null, null, null, null);

        if (cursor != null)
            cursor.moveToFirst();

        LotData data = new LotData(Integer.parseInt(cursor.getString(0)), cursor.getString(1), cursor.getString(2), cursor.getString(3), cursor.getString(4), cursor.getString(5));

        db.close();
        cursor.close();
        return data;
    }

    //Deleting single data
    public void deleteData (LotData lotData)
    {
        SQLiteDatabase db = getReadableDatabase();
        db.delete(TABLE_NAME, LOT_ID + "=?", new String[] { String.valueOf(lotData.getId()) });
        db.close();
    }

    //Updating single data from database
    public int updateData(LotData lotData)
    {
        SQLiteDatabase db = getReadableDatabase();
        ContentValues values = new ContentValues();

        values.put(LOT_ADDRESS, lotData.getAddress());
        values.put(LOT_AREA, lotData.getArea());
        values.put(LOT_UTILITIES, lotData.getUtilities());
        values.put(LOT_ACCESSIBILITY, lotData.getAccessibility());
        values.put(LOT_PRICE, lotData.getPrice());

        return db.update(TABLE_NAME, values, LOT_ID + "=?", new String[] { String.valueOf(lotData.getId()) });
    }

    public int getDataCount()
    {
        SQLiteDatabase db = getReadableDatabase();
        Cursor cursor = db.rawQuery("SELECT * FROM " + TABLE_NAME, null);
        int count = cursor.getCount();
        db.close();
        cursor.close();

        return count;
    }

    public List<LotData> getAllDatas()
    {
        List<LotData> dataList = new ArrayList<LotData>();

        SQLiteDatabase db = getWritableDatabase();
        Cursor cursor = db.rawQuery("SELECT * FROM " + TABLE_NAME, null);

        if (cursor.moveToFirst())
        {
            do
            {
                LotData data = new LotData(Integer.parseInt(cursor.getString(0)), cursor.getString(1), cursor.getString(2), cursor.getString(3), cursor.getString(4), cursor.getString(5)); dataList.add(data);
            }

            while (cursor.moveToNext());
        }
        return dataList;
    }
}

MapFragment类：

public class MapFragment extends FragmentActivity implements OnMapReadyCallback, OnMarkerClickListener
{
    List<LotData> dataList;
    DatabaseHandler dbHandler = new DatabaseHandler(this);
    TextView lotAddress = (TextView) findViewById(R.id.addressTV);
    TextView lotArea = (TextView) findViewById(R.id.areaTV);
    TextView lotUtilities = (TextView) findViewById(R.id.utilTV);
    TextView lotAccessibility = (TextView) findViewById(R.id.accsTV);

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

        SupportMapFragment sMapFragment = (SupportMapFragment) getSupportFragmentManager().findFragmentById(R.id.map);
        sMapFragment.getMapAsync(this);

        Log.d("Insert: ", "Inserting data..");
        dbHandler.addData(new LotData(1, "Jaro Iloilo", "24", "Water and Electrical Supply", "Market, Hospital, Police Station," + "School", "Unknown"));

        Log.d("Reading: ", "Reading all lot data..");
        dataList = dbHandler.getAllDatas();

        for (LotData lotData : dataList)
        {
            String log = "ID: " + lotData.getId() + "Address: " + lotData.getAddress() + "Area: " + lotData.getArea() + "Utilities: " + lotData.getUtilities() + "Accessibility: " + lotData.getAccessibility() + "Price: " + lotData.getPrice();

            //Writing to log
            Log.d("Address: ", log);
        }
    }

    @Override
    public void onMapReady(GoogleMap mMap)
    {
        LatLng one = new LatLng(10.7187530, 122.5611620);
        LatLng ILOILO = new LatLng(10.730278, 122.548889);

        mMap.setBuildingsEnabled(true);
        mMap.setMyLocationEnabled(true);
        mMap.setMapType(GoogleMap.MAP_TYPE_TERRAIN);
        mMap.moveCamera(CameraUpdateFactory.newLatLngZoom(ILOILO, 12));

        Marker oneJRD = mMap.addMarker(new MarkerOptions()
                            .title("Lot ID: 0115-0001JRD")
                            .snippet("Click for more info")
                            .position(one)
                            .icon(BitmapDescriptorFactory.defaultMarker(BitmapDescriptorFactory.HUE_AZURE)));

        oneJRD.showInfoWindow();
        mMap.setOnMarkerClickListener(this);
    }

    @Override
    public boolean onMarkerClick (Marker oneJRD)
    {
        setContentView(R.layout.info_layout);

        LotData lotData = dbHandler.getData(0);

        lotAddress.setText("Address: " + lotData.getAddress());
        lotArea.setText("Area: " + lotData.getArea());
        lotUtilities.setText("Utilities: " + lotData.getUtilities());
        lotAccessibility.setText("Accessibility: " + lotData.getAccessibility());

        return false;
    }
}

Answer 1

在设置活动视图之前使用findviewbyid。如果你希望那些变量是全局变量，你必须这样做。

public class MapFragment extends FragmentActivity implements OnMapReadyCallback, OnMarkerClickListener
{
    List<LotData> dataList;
    DatabaseHandler dbHandler = new DatabaseHandler(this);
    TextView lotAddress;
    TextView lotArea;
    TextView lotUtilities;
    TextView lotAccessibility;

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

    lotAddress = (TextView) findViewById(R.id.addressTV);
    lotArea = (TextView) findViewById(R.id.areaTV);
    lotUtilities = (TextView) findViewById(R.id.utilTV);
    lotAccessibility = (TextView) findViewById(R.id.accsTV);

    SupportMapFragment sMapFragment = (SupportMapFragment) getSupportFragmentManager().findFragmentById(R.id.map);
    sMapFragment.getMapAsync(this);

    Log.d("Insert: ", "Inserting data..");
    dbHandler.addData(new LotData(1, "Jaro Iloilo", "24", "Water and Electrical Supply", "Market, Hospital, Police Station," + "School", "Unknown"));

    Log.d("Reading: ", "Reading all lot data..");
    dataList = dbHandler.getAllDatas();

    for (LotData lotData : dataList)
    {
        String log = "ID: " + lotData.getId() + "Address: " + lotData.getAddress() + "Area: " + lotData.getArea() + "Utilities: " + lotData.getUtilities() + "Accessibility: " + lotData.getAccessibility() + "Price: " + lotData.getPrice();

        //Writing to log
        Log.d("Address: ", log);
    }
}

@Override
public void onMapReady(GoogleMap mMap)
{
    LatLng one = new LatLng(10.7187530, 122.5611620);
    LatLng ILOILO = new LatLng(10.730278, 122.548889);

    mMap.setBuildingsEnabled(true);
    mMap.setMyLocationEnabled(true);
    mMap.setMapType(GoogleMap.MAP_TYPE_TERRAIN);
    mMap.moveCamera(CameraUpdateFactory.newLatLngZoom(ILOILO, 12));

    Marker oneJRD = mMap.addMarker(new MarkerOptions()
                        .title("Lot ID: 0115-0001JRD")
                        .snippet("Click for more info")
                        .position(one)
                        .icon(BitmapDescriptorFactory.defaultMarker(BitmapDescriptorFactory.HUE_AZURE)));

    oneJRD.showInfoWindow();
    mMap.setOnMarkerClickListener(this);
}

@Override
public boolean onMarkerClick (Marker oneJRD)
{
    setContentView(R.layout.info_layout);

    LotData lotData = dbHandler.getData(0);

    lotAddress.setText("Address: " + lotData.getAddress());
    lotArea.setText("Area: " + lotData.getArea());
    lotUtilities.setText("Utilities: " + lotData.getUtilities());
    lotAccessibility.setText("Accessibility: " + lotData.getAccessibility());

    return false;
}
}

Answer 2

在设置活动布局之前无法使用某些方法。其中一种方法是findViewById。这需要在setContentView()

之后使用

像这样改变

public class MapFragment extends FragmentActivity implements OnMapReadyCallback, OnMarkerClickListener
{
List<LotData> dataList;
DatabaseHandler dbHandler = null;
TextView lotAddress;
TextView lotArea;
TextView lotUtilities;
TextView lotAccessibility;

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

    dbHandler = new DatabaseHandler(this);
    lotAddress = (TextView) findViewById(R.id.addressTV);
    lotArea = (TextView) findViewById(R.id.areaTV);
    lotUtilities = (TextView) findViewById(R.id.utilTV);
    lotAccessibility = (TextView) findViewById(R.id.accsTV);

    SupportMapFragment sMapFragment = (SupportMapFragment) getSupportFragmentManager().findFragmentById(R.id.map);
    sMapFragment.getMapAsync(this);

    Log.d("Insert: ", "Inserting data..");
    dbHandler.addData(new LotData(1, "Jaro Iloilo", "24", "Water and Electrical Supply", "Market, Hospital, Police Station," + "School", "Unknown"));

    Log.d("Reading: ", "Reading all lot data..");
    dataList = dbHandler.getAllDatas();

    for (LotData lotData : dataList)
    {
        String log = "ID: " + lotData.getId() + "Address: " + lotData.getAddress() + "Area: " + lotData.getArea() + "Utilities: " + lotData.getUtilities() + "Accessibility: " + lotData.getAccessibility() + "Price: " + lotData.getPrice();

        //Writing to log
        Log.d("Address: ", log);
    }
}

@Override
public void onMapReady(GoogleMap mMap)
{
    LatLng one = new LatLng(10.7187530, 122.5611620);
    LatLng ILOILO = new LatLng(10.730278, 122.548889);

    mMap.setBuildingsEnabled(true);
    mMap.setMyLocationEnabled(true);
    mMap.setMapType(GoogleMap.MAP_TYPE_TERRAIN);
    mMap.moveCamera(CameraUpdateFactory.newLatLngZoom(ILOILO, 12));

    Marker oneJRD = mMap.addMarker(new MarkerOptions()
                        .title("Lot ID: 0115-0001JRD")
                        .snippet("Click for more info")
                        .position(one)
                        .icon(BitmapDescriptorFactory.defaultMarker(BitmapDescriptorFactory.HUE_AZURE)));

    oneJRD.showInfoWindow();
    mMap.setOnMarkerClickListener(this);
}

@Override
public boolean onMarkerClick (Marker oneJRD)
{
    setContentView(R.layout.info_layout);

    LotData lotData = dbHandler.getData(0);

    lotAddress.setText("Address: " + lotData.getAddress());
    lotArea.setText("Area: " + lotData.getArea());
    lotUtilities.setText("Utilities: " + lotData.getUtilities());
    lotAccessibility.setText("Accessibility: " + lotData.getAccessibility());

    return false;
}
}