java.lang.NullPointerException : Can't pass null for argument 'pathString' in child() 异常

Question

Structure of database

起初，在测试用户登录或完全注册时没有异常或错误，但当用户注销并重新打开应用程序时，应用程序崩溃了。 logcat 基本上指出了这个错误:

 UsersRef.child(currentUserID).addValueEventListener(new ValueEventListener()

但是如果我将 currentUserID 更改为 Users .. 应用程序停止崩溃并平稳运行，但它不会加载用户名和位于抽屉导航。

ma​​inactivity.java

public class MainActivity extends AppCompatActivity {
    private NavigationView navigationView;
    private DrawerLayout drawerLayout;
    private RecyclerView postlist;
    private Toolbar mToolbar;
    private ActionBarDrawerToggle actionBarDrawerToggle;
    private FirebaseAuth mAuth;
    private FirebaseUser FirebaseUser;
    private DatabaseReference  UsersRef;
    private CircleImageView NavProfileImage;
    private ImageButton AddNewPostButton;
    private TextView NavProfileUserName;
    String currentUserID;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mAuth = FirebaseAuth.getInstance();
        final FirebaseUser mFirebaseUser = mAuth.getCurrentUser();
        if (mFirebaseUser != null) {
            currentUserID = mFirebaseUser.getUid();
        }




        mToolbar =(Toolbar) findViewById(R.id.main_page_toolbar);
        setSupportActionBar(mToolbar);
        getSupportActionBar().setTitle("Home");

        drawerLayout = (DrawerLayout) findViewById(R.id.drawable_layout);
       actionBarDrawerToggle =  new ActionBarDrawerToggle(MainActivity.this,drawerLayout,R.string.drawer_open, R.string.drawer_close);
        navigationView = (NavigationView)findViewById(R.id.navigation_view);
       AddNewPostButton = (ImageButton)findViewById(R.id.add_new_post_button);

        drawerLayout.addDrawerListener(actionBarDrawerToggle);
       actionBarDrawerToggle.syncState();
       getSupportActionBar().setDisplayHomeAsUpEnabled(true);


        View navView = navigationView.inflateHeaderView(R.layout.nav_header);
        NavProfileImage = (CircleImageView)navView.findViewById(R.id.nav_profile_image);
        NavProfileUserName = (TextView) navView.findViewById(R.id.nav_user_full_name);

        UsersRef = FirebaseDatabase.getInstance().getReference().child("Users");
        UsersRef.child(currentUserID).addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot)
            {
                if(dataSnapshot.exists())
                {   if (dataSnapshot.hasChild("fullname")){
                    String fullname = dataSnapshot.child("fullname").getValue().toString();
                    NavProfileUserName.setText(fullname);
                }if (dataSnapshot.hasChild("profileimage")) {
                    String image = dataSnapshot.child("profileimage").getValue().toString();
                    Picasso.with(MainActivity.this).load(image).placeholder(R.drawable.profile).into(NavProfileImage);

                }else {
                    Toast.makeText(MainActivity.this, "Profile name do not exists...", Toast.LENGTH_SHORT).show();
                }




                }
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {

            }
        });

        navigationView.setNavigationItemSelectedListener(new NavigationView.OnNavigationItemSelectedListener() {
            @Override
            public boolean onNavigationItemSelected(@NonNull MenuItem item)
            { UserMenuSelector(item);

                return false;
            }
        });
        AddNewPostButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                SendUserToPostActivity();

            }
        });
    }

    private void SendUserToPostActivity() {
        Intent addNewPostIntent = new Intent (MainActivity.this,PostActivity.class);
        startActivity(addNewPostIntent);
    }

    @Override
    protected void onStart() {
        super.onStart();
        FirebaseUser currentUser = mAuth.getCurrentUser();
        if (currentUser == null)
        {
            sendUserToLoginActivity();
        }else{
            CheckUserExistance();
        }
    }

    private void CheckUserExistance()
    {
        final String current_user_id = mAuth.getCurrentUser().getUid();
          UsersRef.addValueEventListener(new ValueEventListener() {
              @Override
              public void onDataChange(DataSnapshot dataSnapshot) {
                  if (!dataSnapshot.hasChild(current_user_id)){
                      sendUserToSetupActivity();
                  }
              }

              @Override
              public void onCancelled(DatabaseError databaseError) {

              }
          });
    }

    private void sendUserToSetupActivity() {
        Intent setupIntent = new Intent(MainActivity.this, SetupActivity.class);
        setupIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);
        startActivity(setupIntent);
        finish();

    }

    private void sendUserToLoginActivity()
    {
        Intent loginIntent = new Intent(MainActivity.this, LoginActivity.class);
         loginIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);
         startActivity(loginIntent);
         finish();

    }

Answer 1

注销后 mFirebaseUser 获取空值，因此此 currentUserID 值也获取 null 以避免 nullpointerexception 在使用 currentUserID 之前执行空检查，如下面的代码

public class MainActivity extends AppCompatActivity {
private NavigationView navigationView;
private DrawerLayout drawerLayout;
private RecyclerView postlist;
private Toolbar mToolbar;
private ActionBarDrawerToggle actionBarDrawerToggle;
private FirebaseAuth mAuth;
private FirebaseUser FirebaseUser;
private DatabaseReference  UsersRef;
private CircleImageView NavProfileImage;
private ImageButton AddNewPostButton;
private TextView NavProfileUserName;
String currentUserID;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    mAuth = FirebaseAuth.getInstance();
    final FirebaseUser mFirebaseUser = mAuth.getCurrentUser();
    if (mFirebaseUser != null) {
        currentUserID = mFirebaseUser.getUid();
    }




    mToolbar =(Toolbar) findViewById(R.id.main_page_toolbar);
    setSupportActionBar(mToolbar);
    getSupportActionBar().setTitle("Home");

    drawerLayout = (DrawerLayout) findViewById(R.id.drawable_layout);
   actionBarDrawerToggle =  new ActionBarDrawerToggle(MainActivity.this,drawerLayout,R.string.drawer_open, R.string.drawer_close);
    navigationView = (NavigationView)findViewById(R.id.navigation_view);
   AddNewPostButton = (ImageButton)findViewById(R.id.add_new_post_button);

    drawerLayout.addDrawerListener(actionBarDrawerToggle);
   actionBarDrawerToggle.syncState();
   getSupportActionBar().setDisplayHomeAsUpEnabled(true);


    View navView = navigationView.inflateHeaderView(R.layout.nav_header);
    NavProfileImage = (CircleImageView)navView.findViewById(R.id.nav_profile_image);
    NavProfileUserName = (TextView) navView.findViewById(R.id.nav_user_full_name);

    UsersRef = FirebaseDatabase.getInstance().getReference().child("Users");
      // replace below code
     if (mFirebaseUser != null) {

    UsersRef.child(currentUserID).addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot)
        {
            if(dataSnapshot.exists())
            {   if (dataSnapshot.hasChild("fullname")){
                String fullname = dataSnapshot.child("fullname").getValue().toString();
                NavProfileUserName.setText(fullname);
            }if (dataSnapshot.hasChild("profileimage")) {
                String image = dataSnapshot.child("profileimage").getValue().toString();
                Picasso.with(MainActivity.this).load(image).placeholder(R.drawable.profile).into(NavProfileImage);

            }else {
                Toast.makeText(MainActivity.this, "Profile name do not exists...", Toast.LENGTH_SHORT).show();
            }




            }
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });

    }else{
     sendUserToLoginActivity();
    }

    navigationView.setNavigationItemSelectedListener(new NavigationView.OnNavigationItemSelectedListener() {
        @Override
        public boolean onNavigationItemSelected(@NonNull MenuItem item)
        { UserMenuSelector(item);

            return false;
        }
    });
    AddNewPostButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            SendUserToPostActivity();

        }
    });
}

private void SendUserToPostActivity() {
    Intent addNewPostIntent = new Intent (MainActivity.this,PostActivity.class);
    startActivity(addNewPostIntent);
}

@Override
protected void onStart() {
    super.onStart();
    FirebaseUser currentUser = mAuth.getCurrentUser();
    if (currentUser == null)
    {
        sendUserToLoginActivity();
    }else{
        CheckUserExistance();
    }
}

private void CheckUserExistance()
{
    final String current_user_id = mAuth.getCurrentUser().getUid();
      UsersRef.addValueEventListener(new ValueEventListener() {
          @Override
          public void onDataChange(DataSnapshot dataSnapshot) {
              if (!dataSnapshot.hasChild(current_user_id)){
                  sendUserToSetupActivity();
              }
          }

          @Override
          public void onCancelled(DatabaseError databaseError) {

          }
      });
}

private void sendUserToSetupActivity() {
    Intent setupIntent = new Intent(MainActivity.this, SetupActivity.class);
    setupIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);
    startActivity(setupIntent);
    finish();

}

private void sendUserToLoginActivity()
{
    Intent loginIntent = new Intent(MainActivity.this, LoginActivity.class);
     loginIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);
     startActivity(loginIntent);
     finish();

}

或者您也可以直接在这里查看

 if (mFirebaseUser != null) {
        currentUserID = mFirebaseUser.getUid();
    }else{
        sendUserToLoginActivity();
    }