Firebase获取最后一个生成null的子项[重复](Firebase get last child producing null [duplicate])

Firebase获取最后一个生成null的子项[重复](Firebase get last child producing null [duplicate])

这个问题在这里已经有了答案：

如何从dataSnapshot获取价值？ 2个答案

我正在使用Firebase，我想获得最后一个密钥（消息）。 但是我的代码只是生产NPE。 fb数据库的结构可以在这里看到：

Query lastQuery = rootRef.child("Chatmessages").child(last).child(userid).orderByKey().limitToLast(1); lastQuery.addListenerForSingleValueEvent(new ValueEventListener() { @Override public void onDataChange(DataSnapshot dataSnapshot) { String message = dataSnapshot.child("message").getValue().toString(); // Error on this line // viewHolder.setUserStatus(message); } @Override public void onCancelled(DatabaseError databaseError) { } });

This question already has an answer here:

How get value from dataSnapshot? 2 answers

I am using Firebase and I would like to get the last key (message). However with my code it is just producing NPE. The structure of the fb database can be seen here:

Query lastQuery = rootRef.child("Chatmessages").child(last).child(userid).orderByKey().limitToLast(1); lastQuery.addListenerForSingleValueEvent(new ValueEventListener() { @Override public void onDataChange(DataSnapshot dataSnapshot) { String message = dataSnapshot.child("message").getValue().toString(); // Error on this line // viewHolder.setUserStatus(message); } @Override public void onCancelled(DatabaseError databaseError) { } });

最满意答案

查询返回一个数组而不是单个对象，因此您应该遍历dataSnapShot以获取它拥有的唯一对象：上次聊天...

@Override public void onDataChange(DataSnapshot dataSnapshot) { for (DataSnapshot child: dataSnapshot.getChildren()) { String message = child.child("message").getValue().toString(); // viewHolder.setUserStatus(message); } }

The query returns an array and not a single object, so you should loop over the dataSnapShot to get the only object it has: the last chat...

@Override public void onDataChange(DataSnapshot dataSnapshot) { for (DataSnapshot child: dataSnapshot.getChildren()) { String message = child.child("message").getValue().toString(); // viewHolder.setUserStatus(message); } }