Firebase noob java.lang.NullPointerException： 不能为 child() 中的参数 'pathString' 传递 null

Question

我正在尝试添加一个功能，用户可以点击所有用户的列表并查看所述用户配置文件。添加此代码后，我收到此错误。数据库图像当我点击用户列表以启动意图时，以及当我查看用户设置时，错误发生在以前工作的时候，现在不检索数据。

08-05 15:42:33.372 4697-4697/com.example.android.lapitchat E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.android.lapitchat, PID: 4697
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.android.lapitchat/com.example.android.lapitchat.ProfileActivity}: java.lang.NullPointerException: Can't pass null for argument 'pathString' in child()
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2726)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2787)
at android.app.ActivityThread.-wrap12(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1504)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6247)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:872)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:762)
Caused by: java.lang.NullPointerException: Can't pass null for argument 'pathString' in child()
at com.google.firebase.database.DatabaseReference.child(Unknown Source)
at com.example.android.lapitchat.ProfileActivity.onCreate(ProfileActivity.java:38)
at android.app.Activity.performCreate(Activity.java:6757)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1119)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2679)

这是我的代码的状态

public class ProfileActivity extends AppCompatActivity {
private ImageView mProfileImage;
private TextView mProfileName, mProfileStatus, mProfileFriendsCount;
private Button mProfileSendReqBtn;
private DatabaseReference mUsersDatabase;

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_profile);
String user_id = getIntent().getStringExtra("user id");
mUsersDatabase = FirebaseDatabase.getInstance().getReference().child("Users").child(user_id);
mProfileImage =  findViewById(R.id.profileImage);
mProfileName = findViewById(R.id.profileName);
mProfileStatus = findViewById(R.id.profileStatus);
mProfileFriendsCount = findViewById(R.id.profileFriendsCount);
mProfileSendReqBtn = findViewById(R.id.profileSendReqBtn);
mUsersDatabase.addValueEventListener(new ValueEventListener() {
@Override
public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
String display_name = Objects.requireNonNull(dataSnapshot.child("name").getValue()).toString();
String status = Objects.requireNonNull(dataSnapshot.child("status").getValue()).toString();
String image = Objects.requireNonNull(dataSnapshot.child("image").getValue()).toString();
mProfileName.setText(display_name);
mProfileStatus.setText(status);

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

}

import static com.example.android.lapitchat.R.layout.*;


protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(activity_users);
mToolbar = findViewById(R.id.users_appbar);
mUsersList = findViewById(R.id.users_list);
mUsersList.hasFixedSize();
mUsersList.setLayoutManager(new LinearLayoutManager(this));
mUserDatabase = FirebaseDatabase.getInstance().getReference().child("User");
setSupportActionBar(mToolbar);
getSupportActionBar().setTitle("All Users");
getSupportActionBar().setDisplayHomeAsUpEnabled(true);
}
@Override
protected void onStart() {
super.onStart();
startListening();
}
public void startListening() {
Query query = FirebaseDatabase.getInstance()
.getReference()
.child("Users")
.limitToLast(50);

FirebaseRecyclerOptions<Users> options =
new FirebaseRecyclerOptions.Builder<Users>()
.setQuery(query, Users.class)
.build();
FirebaseRecyclerAdapter adapter = new FirebaseRecyclerAdapter<Users, UserViewHolder>(options) {
@Override
public UserViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
// Create a new instance of the ViewHolder, in this case we are using a custom
// layout called R.layout.message for each item
View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.users_single_layout, parent, false);
return new UserViewHolder(view);
}
@Override
protected void onBindViewHolder(UserViewHolder holder, int position, Users model) {// Bind the Chat object to the ChatHolder
holder.setName(model.name);
holder.setStatus(model.status);
final String user_id = getRef(position).getKey();
holder.mView.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
Intent profileIntent = new Intent(UsersActivity.this, ProfileActivity.class);
profileIntent.putExtra("user_id", user_id);
startActivity(profileIntent);
}
});

}
};
mUsersList.setAdapter(adapter);
adapter.startListening();
}
public static class UserViewHolder extends RecyclerView.ViewHolder {
View mView;
public UserViewHolder(View itemView) {
super(itemView);
mView = itemView;
}
public void setName(String name) {
TextView userNameView = (TextView) mView.findViewById(R.id.user_single_name);
userNameView.setText(name);
}
public void setStatus(String status){
TextView statusTextView = (TextView) mView.findViewById(R.id.user_single_status);
statusTextView.setText(status);
}
}

}

Answer 1

这是代码中唯一可以向child():传递null值的地方

String user_id = getIntent().getStringExtra("user id");
mUsersDatabase = FirebaseDatabase.getInstance().getReference().child("Users").child(user_id);

因此，user_id可能为空。在这样使用它之前，你应该先检查一下。

Answer 2

我假设这来自LapitChatApp教程。如果是，则需要在ProfileActivity.java中验证user_id是通过getIntent()方法传递的，还是通过from_user_id传递的。您可以简单地使用onCreate方法中的if-else来执行此操作。

final String user_id;
String data = getIntent().getStringExtra("user_id");
if (data == null) {
user_id = getIntent().getStringExtra("from_user_id");
} else {
user_id = getIntent().getStringExtra("user_id");
}