DataSnapShot对象的值在getvalue(Boolean.class)上返回null [英] DataSnapShot Object's value returning null on getvalue(Boolean.class)

问题描述

我正在从在线教程中进行实时跟踪应用程序，这里我使用firebase设置状态系统.但是它崩溃了:

I'm doing a realtime tracking application from a tutorial online, Here i'm setting the presence system using firebase. But it's crashing with:

/java.lang.NullPointerException:尝试在空对象引用上调用虚拟方法'boolean java.lang.Boolean.booleanValue()'

我不明白编写此代码的人有什么问题，使其工作正常.

I don't understand what's wrong the guy who coded this has it working perfectly.

此行发生异常: if(dataSnapshot.getValue(Boolean.class)){

当我在屏幕上登录时，datasnapshot对象有一个键，但没有值

When i log this on screen the datasnapshot object has a key but no value

帮助！

ListOnline类

//firebase
DatabaseReference onlineRef,currentUserRef,counterRef;
FirebaseRecyclerAdapter<User,ListOnlineViewHolder> adapter;

//View
RecyclerView listOnline;
RecyclerView.LayoutManager layoutManager;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_list_online);

    //setting the recyclerview
    listOnline = (RecyclerView)findViewById(R.id.listOnlineRecyclerview);
    listOnline.setHasFixedSize(true);
    layoutManager = new LinearLayoutManager(this);
    listOnline.setLayoutManager(layoutManager);

    //set toolbar and menu / join,logout
    Toolbar toolbar = (Toolbar)findViewById(R.id.toolbarID);
    toolbar.setTitle("Presence System");
    setSupportActionBar(toolbar);

    //firebase
    onlineRef = FirebaseDatabase.getInstance().getReference().child("info/connected");
    counterRef = FirebaseDatabase.getInstance().getReference("lastOnline"); //create new child name lastOnline
    currentUserRef = FirebaseDatabase.getInstance().getReference().child(FirebaseAuth.getInstance().getCurrentUser().getUid());

    setupSystem();
    //after setup we load all users and display in recyclerview
    //this is online list
    updateList();
}

private void updateList() {
    adapter = new FirebaseRecyclerAdapter<User, ListOnlineViewHolder>(
            User.class,R.layout.user_layout,ListOnlineViewHolder.class,counterRef
    ) {
        @Override
        protected void populateViewHolder(ListOnlineViewHolder viewHolder, User model, int position) {
            viewHolder.emailTextView.setText(model.getEmail());
        }

    };
    adapter.notifyDataSetChanged();
    listOnline.setAdapter(adapter);
}

private void setupSystem() {
    onlineRef.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
                if(dataSnapshot.getValue(Boolean.class)){
                    currentUserRef.onDisconnect().removeValue();
                    //set online user in list
                    counterRef.child(FirebaseAuth.getInstance().getCurrentUser().getUid())
                            .setValue(FirebaseAuth.getInstance().getCurrentUser().getEmail(),"Online");
                    adapter.notifyDataSetChanged();
                }
            }



        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });
    counterRef.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            for(DataSnapshot postSnapshot:dataSnapshot.getChildren()){
                User user = postSnapshot.getValue(User.class);
                Log.d("LOG",""+user.getEmail()+"is "+user.getStatus());
            }
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });

}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    MenuInflater menuInflater = getMenuInflater();
    menuInflater.inflate(R.menu.main_menu,menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    switch (item.getItemId()){
        case R.id.action_join:
            counterRef.child(FirebaseAuth.getInstance().getCurrentUser().getUid())
                    .setValue(FirebaseAuth.getInstance().getCurrentUser().getEmail(),"Online");
            break;
        case R.id.action_logout:
            currentUserRef.removeValue();

    }
    return super.onOptionsItemSelected(item);
}

}

用户类别

public class User {
private String email,status;


public User(String email, String status) {
    this.email = email;
    this.status = status;
}

public User() {

}

public String getEmail() {
    return email;
}

public void setEmail(String email) {
    this.email = email;
}

public String getStatus() {
    return status;
}

public void setStatus(String status) {
    this.status = status;
}}

MainActivity

public class MainActivity extends AppCompatActivity {

Button signInButton;
private final static int LOGIN_PERMISSION = 1000;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    signInButton = (Button) findViewById(R.id.signInButton);
    signInButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            startActivityForResult(AuthUI.getInstance().createSignInIntentBuilder().setAllowNewEmailAccounts(true).build(),LOGIN_PERMISSION);

        }

    });
}

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {

    if(requestCode == LOGIN_PERMISSION){
        startNewActivity(resultCode,data);
    }
}

private void startNewActivity(int resultcode, Intent data) {

    if(resultcode == RESULT_OK){
        Intent intent = new Intent(MainActivity.this,ListOnline.class);
        startActivity(intent);
        finish();

    }
    else{
        Toast.makeText(this,"login failed!!",Toast.LENGTH_SHORT).show();
    }
}}

推荐答案

在您的 setupSystem()方法中，您将监听器附加到 onlineRef (info/connected 节点)，然后将返回的值编组为 Boolean 值.

In your setupSystem() method, you are attaching a listener to onlineRef (the info/connected node) and then marshalling the returned value to a Boolean value.

但是， DataSnapshot#getValue() 将返回 null .如果发生这种情况， dataSnapshot.getValue(Boolean.class)调用将创建一个值为 null 的 Boolean 变量，该变量不能为在您当前的if语句中检查了一个真值(请参见检查null布尔值是否为true会导致异常).

However, DataSnapshot#getValue() will return null if there is no data at the specified location in the database. If this happens, the dataSnapshot.getValue(Boolean.class) call will create a Boolean variable with the value of null, which then cannot be checked for a true value in your current if statement (see Check if null Boolean is true results in exception).

您可以先在您的if语句中添加一个空检查，以检查 getValue()是否不返回 null :

You could check that getValue() does not return null first by adding a null-check to your if statement:

if(dataSnapshot.getValue() != null && dataSnapshot.getValue(Boolean.class)){
    // ...
}

或使用 DataSnapshot#exists() :

if(dataSnapshot.exists() && dataSnapshot.getValue(Boolean.class)){
    // ...
}

但是，如果您要尝试检测到连接状态，您的意思是将侦听器附加到 .info/connected 节点吗?从文档开始:

However, if you're trying to detect connection state, did you mean to attach the listener to the .info/connected node instead? As from the documentation:

对于许多与状态相关的功能，它有助于您的应用了解联机或脱机时.Firebase实时数据库提供了/.info/connected 中的特殊位置，每次Firebase Realtime Database客户端的连接状态更改.这是一个例子:

For many presence-related features, it is useful for your app to know when it is online or offline. Firebase Realtime Database provides a special location at /.info/connected which is updated every time the Firebase Realtime Database client's connection state changes. Here is an example:

DatabaseReference connectedRef = FirebaseDatabase.getInstance().getReference(".info/connected");
connectedRef.addValueEventListener(new ValueEventListener() {
  @Override
  public void onDataChange(DataSnapshot snapshot) {
    boolean connected = snapshot.getValue(Boolean.class);
    if (connected) {
      System.out.println("connected");
    } else {
      System.out.println("not connected");
    }
  }

  @Override
  public void onCancelled(DatabaseError error) {
    System.err.println("Listener was cancelled");
  }
});

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