Android工作室“尝试在空对象引用上调用虚拟方法android.database.Cursor”问题[重复]

Question

我用 SQLite 数据库构建了一个网格视图，并使用光标来更新 Imageview。它在我上一个项目中运行良好，但是当我将它与自定义工具栏结合使用时，出现错误

java.lang.RuntimeException：无法启动活动 组件信息{com.playah.medcare01/com.playah.medcare01.MainActivity}： java.lang.NullPointerException：尝试调用虚拟方法 'android.database.Cursor com.playah.medcare01.SQLiteHelper.getData(java.lang.String)' 为空 对象引用”

MainActivity

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

            toolbar = findViewById(R.id.toolBar);
            setSupportActionBar(toolbar);

            //grid view
            gridView = (GridView) findViewById(R.id.gridView);
            list = new ArrayList<>();
            adapter = new MedicineListAdapter(this, R.layout.medicine_items, list);
            gridView.setAdapter(adapter);

            // get all data from sqlite
            Cursor cursor = AddNewMedicine.sqLiteHelper.getData("SELECT * FROM MEDICINE");
            list.clear();
            while (cursor.moveToNext()) {
                int id = cursor.getInt(0);
                String name = cursor.getString(1);
                String dose = cursor.getString(2);
                byte[] image = cursor.getBlob(3);

                list.add(new Medicine(name, dose, image, id));
            }
            adapter.notifyDataSetChanged();
 public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {

        if(requestCode == 888){
            if(grantResults.length >0 && grantResults[0] == PackageManager.PERMISSION_GRANTED){
                Intent intent = new Intent(Intent.ACTION_PICK);
                intent.setType("image/*");
                startActivityForResult(intent, 888);
            }
            else {
                Toast.makeText(getApplicationContext(), "You don't have permission to access file location!", Toast.LENGTH_SHORT).show();
            }
            return;
        }
        super.onRequestPermissionsResult(requestCode, permissions, grantResults);
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {

        if(requestCode == 888 && resultCode == RESULT_OK && data != null){
            Uri uri = data.getData();
            try {
                InputStream inputStream = getContentResolver().openInputStream(uri);
                Bitmap bitmap = BitmapFactory.decodeStream(inputStream);
                imageViewMedicine.setImageBitmap(bitmap);

            } catch (FileNotFoundException e) {
                e.printStackTrace();
            }
        }

        super.onActivityResult(requestCode, resultCode, data);
    }

SQLiteHelper

public class SQLiteHelper extends SQLiteOpenHelper{

    public SQLiteHelper(Context context, String name, SQLiteDatabase.CursorFactory factory, int version) {
        super(context, name, factory, version);
    }

    public void queryData(String sql){
        SQLiteDatabase database = getWritableDatabase();
        database.execSQL(sql);
    }

    public void insertData(String name, String dose, byte[] image){
        SQLiteDatabase database = getWritableDatabase();
        String sql = "INSERT INTO MEDICINE VALUES (NULL, ?, ?, ?)";

        SQLiteStatement statement = database.compileStatement(sql);
        statement.clearBindings();

        statement.bindString(1, name);
        statement.bindString(2, dose);
        statement.bindBlob(3, image);

        statement.executeInsert();
    }

    public void updateData(String name, String dose, byte[] image, int id) {
        SQLiteDatabase database = getWritableDatabase();

        String sql = "UPDATE MEDICINE SET name = ?, dose = ?, image = ? WHERE id = ?";
        SQLiteStatement statement = database.compileStatement(sql);

        statement.bindString(1, name);
        statement.bindString(2, dose);
        statement.bindBlob(3, image);
        statement.bindDouble(4, (double)id);

        statement.execute();
        database.close();
    }

    public  void deleteData(int id) {
        SQLiteDatabase database = getWritableDatabase();

        String sql = "DELETE FROM MEDICINE WHERE id = ?";
        SQLiteStatement statement = database.compileStatement(sql);
        statement.clearBindings();
        statement.bindDouble(1, (double)id);

        statement.execute();
        database.close();
    }

    public Cursor getData(String sql){
        SQLiteDatabase database = getReadableDatabase();
        return database.rawQuery(sql, null);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {


    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int i, int i1) {

    } }

添加新药

public class AddNewMedicine extends AppCompatActivity{
    EditText editName, editDose;
    ImageButton bn_photo;
    Button bn_Save;
    ImageView imageView;

    final int REQUEST_CODE_GALLERY = 999;

    public static SQLiteHelper sqLiteHelper;

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

        init();


        sqLiteHelper = new SQLiteHelper(this, "MedicineDB.sqlite", null, 1);

        sqLiteHelper.queryData("CREATE TABLE IF NOT EXISTS MEDICINE(Id INTEGER PRIMARY KEY AUTOINCREMENT, name VARCHAR, dose VARCHAR, image BLOB)");

        bn_photo.setOnClickListener(new View.OnClickListener() {
            //Select picture button
            @Override
            public void onClick(View view) {
                ActivityCompat.requestPermissions(
                        AddNewMedicine.this,
                        new String[]{Manifest.permission.READ_EXTERNAL_STORAGE},
                        REQUEST_CODE_GALLERY
                );
            }
        });

        bn_Save.setOnClickListener(new View.OnClickListener() {
            //Add picture button
            @Override
            public void onClick(View view) {
                try{
                    sqLiteHelper.insertData(
                            editName.getText().toString().trim(),
                            editDose.getText().toString().trim(),
                            imageViewToByte(imageView)
                    );
                    Toast.makeText(getApplicationContext(), "Added successfully!", Toast.LENGTH_SHORT).show();
                    editName.setText("");
                    editDose.setText("");
                    imageView.setImageResource(R.mipmap.ic_launcher);
                }
                catch (Exception e){
                    e.printStackTrace();
                }

                //Go to the home page

                Intent to_intent = new Intent(AddNewMedicine.this, Alarm.class);
                startActivity(to_intent);
            }
        });
    }

    //Implement image insertion

    public static byte[] imageViewToByte(ImageView image) {
        Bitmap bitmap = ((BitmapDrawable)image.getDrawable()).getBitmap();
        ByteArrayOutputStream stream = new ByteArrayOutputStream();
        bitmap.compress(Bitmap.CompressFormat.PNG, 100, stream);
        byte[] byteArray = stream.toByteArray();
        return byteArray;
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {

        //Permission confirmation

        if(requestCode == REQUEST_CODE_GALLERY){
            if(grantResults.length >0 && grantResults[0] == PackageManager.PERMISSION_GRANTED){
                Intent intent = new Intent(Intent.ACTION_PICK);
                intent.setType("image/*");
                startActivityForResult(intent, REQUEST_CODE_GALLERY);
            }
            else {
                Toast.makeText(getApplicationContext(), "You don't have permission to access file location!", Toast.LENGTH_SHORT).show();
            }
            return;
        }

        super.onRequestPermissionsResult(requestCode, permissions, grantResults);
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {

        if(requestCode == REQUEST_CODE_GALLERY && resultCode == RESULT_OK && data != null){
            Uri uri = data.getData();

            try {
                InputStream inputStream = getContentResolver().openInputStream(uri);

                Bitmap bitmap = BitmapFactory.decodeStream(inputStream);
                imageView.setImageBitmap(bitmap);

            } catch (FileNotFoundException e) {
                e.printStackTrace();
            }
        }

        super.onActivityResult(requestCode, resultCode, data);
    }

    private void init(){
        editName = (EditText) findViewById(R.id.editName);
        editDose = (EditText) findViewById(R.id.editDose);
        bn_photo = (ImageButton) findViewById(R.id.bn_photo);
        bn_Save = (Button) findViewById(R.id.bn_Save);
        imageView = (ImageView) findViewById(R.id.imageView);
    }
}

Answer 1

您的 AddNewMedicine.sqLiteHelper 是 null 这就是为什么您会收到 NullPointerException （因为您的 AddNewMedicine 是已经销毁）。

您应该改为在 MainActivity 中创建 SqLiteHelper 的新实例。

所以替换

Cursor cursor = AddNewMedicine.sqLiteHelper.getData("SELECT * FROM MEDICINE");

到

SQLiteHelper sqLiteHelper = new SQLiteHelper(this, "MedicineDB.sqlite", null, 1);
Cursor cursor = sqLiteHelper.getData("SELECT * FROM MEDICINE");