从房间数据库中删除一行时出现错误

Question

在我的应用程序中，每当用户单击回收器视图项时，我都会打开该视图的详细信息视图。

在详细信息视图中，有一个"删除"按钮，用于按 ID 删除项目。

每当我删除一行数据时，我都会收到以下错误

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.almaneakhaled.coutdeplat, PID: 19844
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.almaneakhaled.coutdeplat.rawmaterialsdata.RawMaterialsEntity.getRawMaterialName()' on a null object reference
at com.almaneakhaled.coutdeplat.itemview.MaterialItemView.lambda$onCreate$0$MaterialItemView(MaterialItemView.java:81)
at com.almaneakhaled.coutdeplat.itemview.-$$Lambda$MaterialItemView$_tUa0kDVoDvYdVg5rSyCjITsVqg.onChanged(Unknown Source:4)
at androidx.lifecycle.LiveData.considerNotify(LiveData.java:131)
at androidx.lifecycle.LiveData.dispatchingValue(LiveData.java:149)
at androidx.lifecycle.LiveData.setValue(LiveData.java:307)
at androidx.lifecycle.LiveData$1.run(LiveData.java:91)
at android.os.Handler.handleCallback(Handler.java:873)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:214)
at android.app.ActivityThread.main(ActivityThread.java:7050)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:965)

这是我的详细活动代码：

public class MaterialItemView extends AppCompatActivity {
//define textviews to load strings from room for the intended ID
TextView mMaterialName;
TextView mMaterialBrand;
TextView mMaterialWeight;
TextView mMaterialUnitWeight;
TextView mMaterialUnitCost;
TextView mMaterialCostPerGm;
TextView mMaterialAvailableQuantity;
TextView mMaterialTotalCost;
TextView mMaterialSupplierName;
TextView mMaterialSupplierEmail;
TextView mMaterialSupplierPhone;
int mId;
RawMaterialViewModel mMaterialViewModel;
private static final int UPDATE_MATERIAL_ACTIVITY_REQUEST_CODE = 2;
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.material_item_view);
//connect each textView with its corresponding ID in the Layout material_item_view
mMaterialName = findViewById(R.id.material_name_view);
mMaterialBrand = findViewById(R.id.material_brand_view);
mMaterialWeight = findViewById(R.id.material_weight_view);
mMaterialUnitWeight = findViewById(R.id.material_weight_unit_view);
mMaterialUnitCost = findViewById(R.id.material_unit_cost_view);
mMaterialCostPerGm = findViewById(R.id.material_cost_per_gm_view);
mMaterialAvailableQuantity = findViewById(R.id.material_quantity_view);
mMaterialTotalCost = findViewById(R.id.material_total_cost_view);
mMaterialSupplierName = findViewById(R.id.material_supplier_name_view);
mMaterialSupplierEmail = findViewById(R.id.material_supplier_email_view);
mMaterialSupplierPhone = findViewById(R.id.material_supplier_phone_view);

//get the intent sent from the previous activity
Intent intent = getIntent();
//check if the intent is null
if (intent != null && intent.hasExtra("ID")) {
mId = intent.getIntExtra("ID", -1);
// TODO: get material details based on material id
// Set up the materialViewModel
mMaterialViewModel = new ViewModelProvider(this).get(RawMaterialViewModel.class);
mMaterialViewModel.findMaterialById(mId).observe(this, rawMaterialsEntity -> {
mMaterialName.setText(rawMaterialsEntity.getRawMaterialName());
mMaterialBrand.setText(rawMaterialsEntity.getRawMaterialBrand());
mMaterialWeight.setText(String.valueOf(rawMaterialsEntity.getUnitWeight()));
mMaterialUnitWeight.setText(rawMaterialsEntity.getUOM());
mMaterialUnitCost.setText(String.valueOf(rawMaterialsEntity.getUnitCost()));
mMaterialCostPerGm.setText(String.valueOf(rawMaterialsEntity.getCostPerGm()));
mMaterialAvailableQuantity.setText(String.valueOf(rawMaterialsEntity.getRawMaterialQuantity()));
mMaterialTotalCost.setText(String.valueOf(rawMaterialsEntity.getTotalCost()));
mMaterialSupplierName.setText(rawMaterialsEntity.getSupplierName());
mMaterialSupplierEmail.setText(rawMaterialsEntity.getSupplierEmail());
mMaterialSupplierPhone.setText(rawMaterialsEntity.getSupplierPhone());
});
}
else {
mMaterialName.setText(R.string.material_name_view_na);
mMaterialBrand.setText(R.string.material_brand_view_na);
mMaterialWeight.setText(R.string.material_weight_view_na);
mMaterialUnitWeight.setText(R.string.material_uom_view_na);
mMaterialUnitCost.setText(R.string.material_cost_view_na);
mMaterialCostPerGm.setText(R.string.material_cost_per_gm_view_na);
mMaterialAvailableQuantity.setText(R.string.material_quantity_view_na);
mMaterialTotalCost.setText(R.string.material_total_cost_view_na);
mMaterialSupplierName.setText(R.string.material_s_name_view_na);
mMaterialSupplierEmail.setText(R.string.material_s_email_view_na);
mMaterialSupplierPhone.setText(R.string.material_s_phone_view_na);
}
//define the control buttons
ImageButton cancelButton = findViewById(R.id.materials_cancel_button_view);
ImageButton editButton = findViewById(R.id.materials_edit_button);
ImageButton deleteButton = findViewById(R.id.materials_delete_button);
ImageButton emailButton = findViewById(R.id.materials_email_button);
ImageButton callButton = findViewById(R.id.materials_call_button);
cancelButton.setOnClickListener(view -> finish());
editButton.setOnClickListener(view -> {
Intent intent1 = new Intent(getBaseContext() ,RawMaterialsEditor.class);
intent1.putExtra("ID", mId);
startActivityForResult(intent1, UPDATE_MATERIAL_ACTIVITY_REQUEST_CODE);
});
deleteButton.setOnClickListener(view -> {
mMaterialViewModel.deleteMaterial(mId);
Intent intent1 = new Intent(this, MainActivity.class);
finish();
});
}
public void onActivityResult(int requestCode, int resultCode, Intent data) {
super.onActivityResult(requestCode, resultCode, data);
if (requestCode == UPDATE_MATERIAL_ACTIVITY_REQUEST_CODE && resultCode == RESULT_OK) {
RawMaterialsEntity material = new RawMaterialsEntity(Integer.valueOf(data.
getStringExtra(RawMaterialsEditor.EXTRA_REPLY_id)) ,data
.getStringExtra(RawMaterialsEditor.EXTRA_REPLY_NAME),
data.getStringExtra(RawMaterialsEditor.EXTRA_REPLY_BRAND),
Float.valueOf(data.getStringExtra(RawMaterialsEditor.EXTRA_REPLY_WEIGHT)),
Float.valueOf(data.getStringExtra(RawMaterialsEditor.EXTRA_REPLY_COST)),
Integer.valueOf(data.getStringExtra(RawMaterialsEditor.EXTRA_REPLY_QUANTITY)),
data.getStringExtra(RawMaterialsEditor.EXTRA_REPLY_S_NAME),
data.getStringExtra(RawMaterialsEditor.EXTRA_REPLY_S_EMAIL),
data.getStringExtra(RawMaterialsEditor.EXTRA_REPLY_S_PHONE),
data.getStringExtra(RawMaterialsEditor.EXTRA_REPLY_UOM));
mMaterialViewModel.updateMaterial(material);
mMaterialViewModel.costPerGm();
mMaterialViewModel.totalCost();
}
else {
Toast.makeText(
this.getApplicationContext(),
R.string.editor_insert_rm_failed,
Toast.LENGTH_LONG).show();
}
}
}

请帮助我修复此错误，我调试了应用程序，如果手动结束 viewModel 生命周期，它将解决问题，但我不知道该怎么做。

Answer 1

通过使用 if 语句包装 textviews 数据加载器来检查实体对象是否不为 null，从而解决了此错误。

mMaterialViewModel = new ViewModelProvider(this).get(RawMaterialViewModel.class);
mMaterialViewModel.findMaterialById(mId).observe(this, rawMaterialsEntity -> {
if (rawMaterialsEntity != null) {
mMaterialName.setText(rawMaterialsEntity.getRawMaterialName());
mMaterialBrand.setText(rawMaterialsEntity.getRawMaterialBrand());
mMaterialWeight.setText(String.valueOf(rawMaterialsEntity.getUnitWeight()));
mMaterialUnitWeight.setText(rawMaterialsEntity.getUOM());
mMaterialUnitCost.setText(String.valueOf(rawMaterialsEntity.getUnitCost()));
mMaterialCostPerGm.setText(String.valueOf(rawMaterialsEntity.getCostPerGm()));
mMaterialAvailableQuantity.setText(String.valueOf(rawMaterialsEntity.getRawMaterialQuantity()));
mMaterialTotalCost.setText(String.valueOf(rawMaterialsEntity.getTotalCost()));
mMaterialSupplierName.setText(rawMaterialsEntity.getSupplierName());
mMaterialSupplierEmail.setText(rawMaterialsEntity.getSupplierEmail());
mMaterialSupplierPhone.setText(rawMaterialsEntity.getSupplierPhone());
}