是啥导致了这个 NullPointerException？ [复制]

Posted 2023-04-13

【中文标题】是啥导致了这个 NullPointerException？ [复制]

【英文标题】：What makes this NullPointerException? [duplicate]是什么导致了这个 NullPointerException？ [复制]

【发布时间】：2019-11-01 01:49:21

【问题描述】：

我从源中收到一个空错误，以显示我的关注和关注列表。 以前它工作，但由于某种原因突然发生错误。 不知为何找了几天。 帮帮我..

我已经检查了我的 firebase 数据列表，但我不知道这不是 firebase 问题

这是我的错误

2019-06-18 21:19:51.482 1875-1894/? E/memtrack: Couldn't load memtrack module
2019-06-18 21:19:54.043 14937-14937/com.example.blogapp E/result  db user: 2okCgbdTqlVgK6oDqPiYPwyM0kD3
2019-06-18 21:19:54.044 14937-14937/com.example.blogapp E/result  db user: 2zUL5Bv7CcPnBENXuWZrr1bxbgs1
2019-06-18 21:19:54.044 14937-14937/com.example.blogapp E/result  db user: 6YZdQrf5RycKO1kwvze0vCXL8V83
2019-06-18 21:19:54.045 14937-14937/com.example.blogapp E/id: 6YZdQrf5RycKO1kwvze0vCXL8V83
2019-06-18 21:19:54.045 14937-14937/com.example.blogapp E/arrayList: 7
2019-06-18 21:19:54.045 14937-14937/com.example.blogapp E/result  db user: 8wYJrIJ1brPTFvY1GdH2cxv9W4v1
2019-06-18 21:19:54.045 14937-14937/com.example.blogapp E/result  db user: BCka8CPfqhfyWZlhgVAwJXDA8fi1
2019-06-18 21:19:54.045 14937-14937/com.example.blogapp E/id: BCka8CPfqhfyWZlhgVAwJXDA8fi1
2019-06-18 21:19:54.045 14937-14937/com.example.blogapp E/arrayList: 7
2019-06-18 21:19:54.046 14937-14937/com.example.blogapp E/result  db user: BcNxFCVZZrgxC0pp48G5LQqEP2B2
2019-06-18 21:19:54.046 14937-14937/com.example.blogapp E/result  db user: Bj6OGUpKgTYq4JQ2ZtlmTNmb6xj2
2019-06-18 21:19:54.046 14937-14937/com.example.blogapp E/result  db user: DAiGCMPqcyOdjIAU7RvHWC0xfbG2
2019-06-18 21:19:54.046 14937-14937/com.example.blogapp E/id: DAiGCMPqcyOdjIAU7RvHWC0xfbG2
2019-06-18 21:19:54.046 14937-14937/com.example.blogapp E/arrayList: 7
2019-06-18 21:19:54.046 14937-14937/com.example.blogapp E/result  db user: J9H2hS8jBbcuDhDBdI9ENkCy3Qp1
2019-06-18 21:19:54.047 14937-14937/com.example.blogapp E/result  db user: Lo4XZ1pMXGac6BXHEBc7gWz1Lbf2
2019-06-18 21:19:54.047 14937-14937/com.example.blogapp E/result  db user: Meqo3PIqWrVv66kmGtVDWXknHbh1
2019-06-18 21:19:54.047 14937-14937/com.example.blogapp E/result  db user: NTq9UMZTBefE5Ze2lZdc9hGGhtZ2
2019-06-18 21:19:54.047 14937-14937/com.example.blogapp E/id: NTq9UMZTBefE5Ze2lZdc9hGGhtZ2
2019-06-18 21:19:54.047 14937-14937/com.example.blogapp E/arrayList: 7
2019-06-18 21:19:54.050 14937-14937/com.example.blogapp E/result  db user: NmrCWzy7aTXSTMSCnwGRGcm417u1
2019-06-18 21:19:54.050 14937-14937/com.example.blogapp E/result  db user: P8uBLb9Ge5MPRbcgrOkl1xJMymI3
2019-06-18 21:19:54.050 14937-14937/com.example.blogapp E/result  db user: Sjzjl1pD2PQBoCcc1x0Ji4gaFkI3
2019-06-18 21:19:54.050 14937-14937/com.example.blogapp E/id: Sjzjl1pD2PQBoCcc1x0Ji4gaFkI3
2019-06-18 21:19:54.050 14937-14937/com.example.blogapp E/arrayList: 7
2019-06-18 21:19:54.051 14937-14937/com.example.blogapp E/result  db user: VxLgeLvRV9RZO8yg5MfYM2CrsfJ3
2019-06-18 21:19:54.051 14937-14937/com.example.blogapp E/result  db user: bazmIYkmqwSdrkNbX6SIFlXuzO22
2019-06-18 21:19:54.051 14937-14937/com.example.blogapp E/result  db user: dJCOwYuLAgfEEfhKyxT0bon8zfv2
2019-06-18 21:19:54.051 14937-14937/com.example.blogapp E/result  db user: fm6XldWYbOdVQYOuiTVLTRouN573
2019-06-18 21:19:54.052 14937-14937/com.example.blogapp E/id: fm6XldWYbOdVQYOuiTVLTRouN573
2019-06-18 21:19:54.052 14937-14937/com.example.blogapp E/arrayList: 7
2019-06-18 21:19:54.052 14937-14937/com.example.blogapp E/result  db user: lPKdx41KR2R3PsZh62ZM3QuyuS93
2019-06-18 21:19:54.052 14937-14937/com.example.blogapp E/result  db user: m8sTZfxm3xSCysNTLdNYOabXRU23
2019-06-18 21:19:54.052 14937-14937/com.example.blogapp E/result  db user: oT1sKEE9ILZjNn2B5O4Yf7gjNqV2
2019-06-18 21:19:54.057 14937-14937/com.example.blogapp E/androidRuntime: FATAL EXCEPTION: main
    Process: com.example.blogapp, PID: 14937
    java.lang.NullPointerException: println needs a message
        at android.util.Log.println_native(Native Method)
        at android.util.Log.e(Log.java:240)
        at com.example.together.Activities.FollowersActivity$5.onDataChange(FollowersActivity.java:158)
        at com.google.firebase.database.Query$1.onDataChange(com.google.firebase:firebase-database@@17.0.0:184)
        at com.google.firebase.database.core.ValueEventRegistration.fireEvent(com.google.firebase:firebase-database@@17.0.0:75)
        at com.google.firebase.database.core.view.DataEvent.fire(com.google.firebase:firebase-database@@17.0.0:63)
        at com.google.firebase.database.core.view.EventRaiser$1.run(com.google.firebase:firebase-database@@17.0.0:55)
        at android.os.Handler.handleCallback(Handler.java:873)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:6669)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)

这是我的完整代码，显示关注和关注者列表

private void getFollowing()

        DatabaseReference reference = FirebaseDatabase.getInstance().getReference("Follow")
                .child(id).child("following");
        reference.addListenerForSingleValueEvent(new ValueEventListener() 
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) 

                idList.clear();
                for (DataSnapshot snapshot : dataSnapshot.getChildren())
                    idList.add(snapshot.getKey());
                
                showUsers();
            

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) 

            
        );

    


    private void getFollowers()
        DatabaseReference reference = FirebaseDatabase.getInstance().getReference("Follow")
                .child(id).child("followers");
        reference.addListenerForSingleValueEvent(new ValueEventListener() 
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) 

                idList.clear();
                for (DataSnapshot snapshot : dataSnapshot.getChildren())
                    idList.add(snapshot.getKey());
                
                showUsers();
            

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) 

            
        );
    


    private void showUsers()
        DatabaseReference reference = FirebaseDatabase.getInstance().getReference("Users");
        reference.addListenerForSingleValueEvent(new ValueEventListener() 
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) 
                userList.clear();
                for (DataSnapshot snapshot : dataSnapshot.getChildren())
                    User user = snapshot.getValue(User.class);
                    System.out.println(user+"TEST");
                    Log.e("result  db user",user.getId());
                        for (String id : idList)
                            if (user.getId().equals(id))
                                Log.e("id",user.getId());
                                Log.e("arrayList",idList.size()+"");
                                userList.add(user);
                            
                        
                

                userAdapter.notifyDataSetChanged();
            

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) 

            
        );
    

这是我的按钮，当我想检查我的关注和关注者列表时

      followers.setOnClickListener(new View.OnClickListener() 
            @Override
            public void onClick(View v) 
                Intent intent = new Intent(getContext(), FollowersActivity.class);
                intent.putExtra("id", profileid);
                intent.putExtra("title","followers");
                startActivity(intent);
            
        );





        following.setOnClickListener(new View.OnClickListener() 
            @Override
            public void onClick(View v) 
                Intent intent = new Intent(getContext(), FollowersActivity.class);
                intent.putExtra("id", profileid);
                intent.putExtra("title","following");
                startActivity(intent);
            
        );

我想知道是什么让它为空......

【问题讨论】：

user+"TEST" 我的日志中没有显示任何内容

user+"TEST" 已写入 System.out，可能不会出现在您的日志中。此外，从堆栈跟踪中可以明显看出 NPE 是在 LOG.e() 中引起的。如果您要传递一些具有空值的变量，请检查您出现的 LOG.e()。

java.lang.NullPointerException: println needs a message 的可能重复项（TL;DR : user.getId() 为空，这使得 Log.e("result db user",user.getId()); 因您遇到的错误而失败）

【参考方案1】：

总是像下面的代码一样记录错误（附加空白字符串），

   Log.e("id",user.getId()+"");

不是这样的，

   Log.e("id",user.getId());