Question

我将聊天功能放到我使用firebase在android studio中构建的应用程序中。我一直在遵循指南，但是，我一直收到NullPointerException错误。这是因为我从用户适配器传递的意图没有传递额外信息，甚至没有传递意图本身，所以为什么会这样？

我的代码未为我的User对象分配ID。测试应用程序时，我单击一个用户，由于未分配其ID，应用程序崩溃。

我对AS的了解不足以确切地说明为什么会这样。它似乎发生在我的用户适配器和消息传递活动中。

我根据用户列表中的位置分配了用户。

final User user = mUsers.get(position);

然后将其设置为发送到我的消息传递活动中。

holder.itemView.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Intent intent = new Intent(mContext, ActualMessaging.class);
            intent.putExtra("userid,", user.getId());
            mContext.startActivity(intent);
        }
    });

一旦出现，我认为这是在我的useradapter活动中引用代码

final String userid = intent.getStringExtra("userid");

然后我尝试获取该特定的用户ID，但“用户ID”返回null。

reference = FirebaseDatabase.getInstance().getReference("Users").child(userid);

如果有人能让我知道我在做错什么，那太好了，我将在下面放置（我确定是正确的代码位）。

用户片段：

package com.example.testandroidapplication.Fragments;

import android.content.Context;
import android.net.Uri;
import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.v4.app.Fragment;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

import com.example.testandroidapplication.Adapter.UserAdapter;
import com.example.testandroidapplication.Model.User;
import com.example.testandroidapplication.R;
import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.auth.FirebaseUser;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;

import java.util.ArrayList;
import java.util.List;


public class UsersFragment extends Fragment {

    private RecyclerView recyclerView;

    private UserAdapter userAdapter;
    private List<User> mUsers;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {

        View view = inflater.inflate(R.layout.fragment_users, container, false);

        recyclerView = view.findViewById(R.id.recycler_view);
        recyclerView.setHasFixedSize(true);
        recyclerView.setLayoutManager(new LinearLayoutManager(getContext()));

        mUsers = new ArrayList<>();


        readUsers();

        return view;
    }

    private void readUsers() {
        final FirebaseUser firebaseUser = FirebaseAuth.getInstance().getCurrentUser();
        DatabaseReference reference = FirebaseDatabase.getInstance().getReference("Users");

        reference.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                mUsers.clear();
                for(DataSnapshot snapshot : dataSnapshot.getChildren()){
                    User user = snapshot.getValue(User.class);

                    assert user != null;
                    assert firebaseUser != null;
                    if(!user.getId().equals(firebaseUser.getUid())){
                        mUsers.add(user);
                    }
                }

                userAdapter = new UserAdapter(getContext(), mUsers);
                recyclerView.setAdapter(userAdapter);
            }

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

            }
        });
    }

}

用户适配器

package com.example.testandroidapplication.Adapter;

import android.content.Context;
import android.content.Intent;
import android.support.annotation.NonNull;
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ImageView;
import android.widget.TextView;

import com.bumptech.glide.Glide;
import com.example.testandroidapplication.ActualMessaging;
import com.example.testandroidapplication.Model.User;
import com.example.testandroidapplication.R;

import java.util.List;

public class UserAdapter extends RecyclerView.Adapter<UserAdapter.ViewHolder> {

    private Context mContext;
    private List<User> mUsers;

    public UserAdapter(Context mContext, List<User> mUsers){

        this.mUsers = mUsers;
        this.mContext = mContext;
    }

    @NonNull
    @Override
    public ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        View view = LayoutInflater.from(mContext).inflate(R.layout.user_item, parent, false);
        return new UserAdapter.ViewHolder(view);
    }

    @Override
    public void onBindViewHolder(@NonNull ViewHolder holder, int position) {

        //made user final variable, debugging, check back
        final User user = mUsers.get(position);
        holder.username.setText(user.getUsername());

        if(user.getImageURL().equals("default")){
            holder.profile_image.setImageResource(R.mipmap.ic_launcher);
        } else {
            Glide.with(mContext).load(user.getImageURL()).into(holder.profile_image);
        }

        holder.itemView.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent intent = new Intent(mContext, ActualMessaging.class);
                intent.putExtra("userid,", user.getId());
                mContext.startActivity(intent);
            }
        });

    }

    @Override
    public int getItemCount() {
        return mUsers.size();
    }

    public class ViewHolder extends RecyclerView.ViewHolder {

        public TextView username;
        public ImageView profile_image;

        public ViewHolder(View itemView) {
            super(itemView);

            username = itemView.findViewById(R.id.username);
            profile_image = itemView.findViewById(R.id.profile_image);
        }
    }
}

用户类别：

    package com.example.testandroidapplication.Model;

public class User {

    private String id;
    private String username;
    private String imageURL;

    public User(String id, String username, String imageURL) {
        this.id = id;
        this.username = username;
        this.imageURL = imageURL;
    }

    public User(String id, String username) {
        this.id = id;
        this.username = username;
    }

    public User(String id){

        this.id=id;
    }

    public User() {

    }

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getImageURL() {
        return imageURL;
    }

    public void setImageURL(String imageURL) {
        this.imageURL = imageURL;
    }
}

实际消息传递：

package com.example.testandroidapplication;

import android.content.Intent;
import android.support.annotation.NonNull;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.support.v7.widget.Toolbar;
import android.util.Log;
import android.view.View;
import android.widget.EditText;
import android.widget.ImageButton;
import android.widget.LinearLayout;
import android.widget.TextView;
import android.widget.Toast;

import com.bumptech.glide.Glide;
import com.example.testandroidapplication.Adapter.MessageAdapter;
import com.example.testandroidapplication.Model.Chat;
import com.example.testandroidapplication.Model.User;
import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.auth.FirebaseUser;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;

import de.hdodenhof.circleimageview.CircleImageView;

import static com.google.android.gms.common.internal.safeparcel.SafeParcelable.NULL;

public class ActualMessaging extends AppCompatActivity {

    CircleImageView profile_image;
    TextView username;

    FirebaseUser fuser;
    DatabaseReference reference;

    ImageButton btn_send;
    EditText text_send;

    MessageAdapter messageAdapter;
    List<Chat> mchat;

    RecyclerView recyclerView;


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

        Toolbar toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        getSupportActionBar().setTitle("");
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
        toolbar.setNavigationOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                finish();
            }
        });

        recyclerView = findViewById(R.id.recycler_view);
        recyclerView.setHasFixedSize(true);
        LinearLayoutManager linearLayoutManager = new LinearLayoutManager(getApplicationContext());
        linearLayoutManager.setStackFromEnd(true);
        recyclerView.setLayoutManager(linearLayoutManager);

        profile_image = findViewById(R.id.profile_image);
        username = findViewById(R.id.username);
        btn_send = findViewById(R.id.btn_send);
        text_send = findViewById(R.id.text_send);

        Intent intent = getIntent();

        final String userid = intent.getStringExtra("userid");

        fuser = FirebaseAuth.getInstance().getCurrentUser();

        btn_send.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                String msg = text_send.getText().toString();
                if(!msg.equals("")){
                    sendMessage(fuser.getUid(), userid, msg);
                } else {
                    Toast.makeText(ActualMessaging.this, "You can't send empty message", Toast.LENGTH_SHORT).show();
                }
                text_send.setText("");
            }
        });

        fuser = FirebaseAuth.getInstance().getCurrentUser();
        reference = FirebaseDatabase.getInstance().getReference("Users").child(userid);

        reference.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {

                final User user = dataSnapshot.getValue(User.class);

                try {

                    username.setText(user.getUsername());

                    if (user.getImageURL().equals("default")) {
                        profile_image.setImageResource(R.mipmap.ic_launcher);
                    } else {
                        Glide.with(ActualMessaging.this).load(user.getImageURL()).into(profile_image);
                    }

                    readMessages(fuser.getUid(), userid, user.getImageURL());

                } catch (NullPointerException npe){
                    Toast.makeText(ActualMessaging.this, ("userid: " + userid + "username: " + username.toString()), Toast.LENGTH_SHORT).show();
                }
            }

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

            }
        });
    }

    private void sendMessage(String sender, String receiver, String message) {

        DatabaseReference reference = FirebaseDatabase.getInstance().getReference();

        HashMap<String, Object> hashMap = new HashMap<>();
        hashMap.put("sender", sender);
        hashMap.put("receiver", receiver);
        hashMap.put("message", message);

        reference.child("Chats").push().setValue(hashMap);

    }
private void readMessages(final String myid, final String userid, final String imageurl){
    mchat = new ArrayList<>();

    reference = FirebaseDatabase.getInstance().getReference("Chats");
    reference.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
            mchat.clear();
            for(DataSnapshot snapshot : dataSnapshot.getChildren()){
                Chat chat = snapshot.getValue(Chat.class);
                if(chat.getReceiver().equals(myid) && chat.getSender().equals(userid) ||
                        chat.getReceiver().equals(userid) && chat.getSender().equals(myid)) {
                    mchat.add(chat);
                }

                messageAdapter = new MessageAdapter(ActualMessaging.this, mchat, imageurl);
                recyclerView.setAdapter(messageAdapter);

            }
        }

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

        }
    });
}

}

Logcat错误

2019-03-11 22:36:47.920 18284-18284/com.example.testandroidapplication E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.testandroidapplication, PID: 18284
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.testandroidapplication/com.example.testandroidapplication.ActualMessaging}: java.lang.NullPointerException: Can't pass null for argument 'pathString' in child()
        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: Can't pass null for argument 'pathString' in child()
        at com.google.firebase.database.DatabaseReference.child(com.google.firebase:firebase-database@@16.1.0:101)
        at com.example.testandroidapplication.ActualMessaging.onCreate(ActualMessaging.java:109)
        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)

谢谢您的建议。

Answer 1

问题：此行使您的应用无法按预期运行。

intent.putExtra("userid,", user.getId());

如您所见，您必须使用 userid 而不是 userid ，并以逗号结尾。

解决方案：：将代码更改为

intent.putExtra("userid", user.getId());

Answer 2

您要在putExtra中添加“用户ID” ，并获取“用户ID” 。

所以它变成了错误。

更新代码，并在用户适配器中使用此行

intent.putExtra("userid", user.getId());

，您还可以通过此代码访问userid

intent.putExtra("userid", mUsers.get(position).getId());

希望这对您有帮助！

谢谢。

Answer 3

我认为您可以先检查userid是否为null，然后仅应传递child（）在您的情况下，问题在于您传递的密钥是不同的，并且获取也不同，因此需要获取正确的密钥。如果您要传递密钥

intent.putExtra("userid,", user.getId());

获取密钥

String userid = intent.getStringExtra("userid,");

或您还可以将密钥创建为常量

public static final USERID="userid"

intent.putExtra(USERID, user.getId());

获取密钥

String userid = intent.getStringExtra(USERID);

这不会造成拼写错误

为什么我的用户ID不从适配器转移到消息传递活动？

3 个答案: