android - 在我的应用程序中使用按钮会导致它崩溃

Question

所以我和我的 friend 们正在开发一个带有 firebase 数据库的应用程序。我们是实现和读取数据库的新手，所以我想测试一下，看看我的方法是否可行。我创建了一个 ViewDatabase 类来尝试它，我想有意地开始这个 Activity 。带有 Intent 的按钮是从一个 fragment 开始的。我可以启动该应用程序，但是当我按下按钮时它就崩溃了。我找不到让它工作的方法，所以我希望这里有人对我的问题有所了解。

这是 ViewDatabase 类。

public class ViewDatabase extends AppCompatActivity {
private static final String TAG = "ViewDatabase";
private DatabaseReference mFirebaseDatabase;
private FirebaseAuth mAuth;
private FirebaseAuth.AuthStateListener mAuthListener;
private DatabaseReference myRef;
private String recipes;

private ListView listView_test;

@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.view_database_layout);
    FirebaseApp.initializeApp(this);


    //declare the database reference object. This is what we use to access the database.
    //NOTE: Unless you are signed in, this will not be useable.
    mAuth = FirebaseAuth.getInstance();
    mFirebaseDatabase = FirebaseDatabase.getInstance().getReference().child("category");
    FirebaseUser user = mAuth.getCurrentUser();
    recipes = user.getUid();



    myRef.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
            showData(dataSnapshot);
        }

        @Override
        public void onCancelled(@NonNull DatabaseError databaseError) {

        }
    });

}


private void showData(DataSnapshot dataSnapshot){
    for (DataSnapshot ds : dataSnapshot.getChildren()){
        recipe recipe = new recipe();
        recipe.setNameOfRecipe(ds.child(recipes).getValue(com.example.myapplicationteeeeeeeeeest.recipe.class).getNameOfRecipe());
        recipe.setCategoryOfRecipe(ds.child(recipes).getValue(com.example.myapplicationteeeeeeeeeest.recipe.class).getCategoryOfRecipe());
        recipe.setDescriptionOfRecipe(ds.child(recipes).getValue(com.example.myapplicationteeeeeeeeeest.recipe.class).getDescriptionOfRecipe());
        recipe.setIDOfRecipe(ds.child(recipes).getValue(com.example.myapplicationteeeeeeeeeest.recipe.class).getIDOfRecipe());


        ArrayList<String> array = new ArrayList<>();
        array.add(recipe.getNameOfRecipe());
        array.add(recipe.getDescriptionOfRecipe());
        array.add(recipe.getCategoryOfRecipe());
        ArrayAdapter adapter = new ArrayAdapter(this, R.layout.shopping_list_item, array);
        listView_test.setAdapter(adapter);
    }
}

@Override
public void onStart() {
    super.onStart();
    mAuth.addAuthStateListener(mAuthListener);
}

@Override
public void onStop() {
    super.onStop();
    if (mAuthListener != null) {
        mAuth.removeAuthStateListener(mAuthListener);
    }
}


/**
 * customizable toast
 * @param message
 */
private void toastMessage(String message){
    Toast.makeText(this,message,Toast.LENGTH_SHORT).show();
}

这是 intentbutton 所在的 fragment 。

public class HomeFragment extends Fragment {

private HomeViewModel homeViewModel;
private Button categorie1;
private Button categorie2;
private Button categorie3;
private Button categorie4;
private Button database_button;


public View onCreateView(@NonNull LayoutInflater inflater,
                         ViewGroup container, Bundle savedInstanceState) {
    homeViewModel =
            ViewModelProviders.of( this ).get( HomeViewModel.class );

    View root = inflater.inflate( R.layout.fragment_home, container, false );
    categorie1 = (Button) root.findViewById(R.id.button_category_1);
    categorie2 = (Button) root.findViewById(R.id.button_category_2);
    categorie3 = (Button) root.findViewById(R.id.button_category_3);
    categorie4 = (Button) root.findViewById(R.id.button_category_4);
    database_button = (Button) root.findViewById(R.id.button_database);
    initButtons();

    final TextView textView = root.findViewById( R.id.text_home );
    homeViewModel.getText().observe( this, new Observer<String>() {
        @Override
        public void onChanged(@Nullable String s) {
            textView.setText( s );
        }
    } );
    return root;
}

public void initButtons(){

    database_button.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Intent intent = new Intent(getActivity(), ViewDatabase.class);
            startActivity(intent);
        }
    });
}
}
}

编辑:对不起，崩溃日志丢失了，我是编程新手，我的错

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.myapplicationteeeeeeeeeest, PID: 3799
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.myapplicationteeeeeeeeeest/com.example.myapplicationteeeeeeeeeest.ViewDatabase}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.google.firebase.auth.FirebaseUser.getUid()' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2913)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:6669)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.google.firebase.auth.FirebaseUser.getUid()' on a null object reference
        at com.example.myapplicationteeeeeeeeeest.ViewDatabase.onCreate(ViewDatabase.java:50)
        at android.app.Activity.performCreate(Activity.java:7136)
        at android.app.Activity.performCreate(Activity.java:7127)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2893)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:193) 
        at android.app.ActivityThread.main(ActivityThread.java:6669) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)

Answer 1

处理没有用户帐户的可能性，将他们引导至登录屏幕。

FirebaseUser user = mAuth.getCurrentUser();
if (user != null) {
    recipes = user.getUid();
} else {
    // Ask user to login and repeat the logic to getUid();
}