Java 从Firebase获取最后（5或10）个值

我是

Android

新手，我想从

FirebaseDatabase

获取最后

5

或

10

消息

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• 但结果它给了我

  NullPointerException

• Logcat

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• 我的

  数据库结构是这样的吗

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• 更新后

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• Log的结果

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我认为你应该用Query来表示这个和那个

    @Override
public Query getQuery(DatabaseReference databaseReference) {
    // [START recent_posts_query]
    // Last 1000 posts, these are automatically the 1000 most recent
    // due to sorting by push() keys
    Query recentPostsQuery = databaseReference.child("posts")
            .limitToLast(number of post you want);
    // [END recent_posts_query]

    return recentPostsQuery;
}

这将给你的职位数量有限，你给。 您可以从Firebase官方文档或其github repo获取更多信息

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希望这会有所帮助……

我认为你应该使用查询来查询这个和那个

    @Override
public Query getQuery(DatabaseReference databaseReference) {
    // [START recent_posts_query]
    // Last 1000 posts, these are automatically the 1000 most recent
    // due to sorting by push() keys
    Query recentPostsQuery = databaseReference.child("posts")
            .limitToLast(number of post you want);
    // [END recent_posts_query]

    return recentPostsQuery;
}

DatabaseReference messagesReference = FirebaseDatabase.getInstance().getReference().child("chat_rooms").child("tKyhXXAf6TQnQgKyAf23RJcwhsn1_S5ClzMeWUEXR9evLJO9CBd9ABCH3");  
 Query chatQuery = messagesReference.orderByChild("your key").limitToLast(5);
     chatQuery.addValueEventListener(new ValueEventListener() {
                @Override
                public void onDataChange(DataSnapshot dataSnapshot) {
                    if (dataSnapshot.getValue() != null) {
    //do what ever required 
    }

这将给你的职位数量有限，你给。 您可以从Firebase官方文档或其github repo获取更多信息

希望这对……有帮助

DatabaseReference messagesReference = FirebaseDatabase.getInstance().getReference().child("chat_rooms").child("tKyhXXAf6TQnQgKyAf23RJcwhsn1_S5ClzMeWUEXR9evLJO9CBd9ABCH3");  
 Query chatQuery = messagesReference.orderByChild("your key").limitToLast(5);
     chatQuery.addValueEventListener(new ValueEventListener() {
                @Override
                public void onDataChange(DataSnapshot dataSnapshot) {
                    if (dataSnapshot.getValue() != null) {
    //do what ever required 
    }

希望这有帮助。请试一试

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希望这有帮助。请尝试。

更新您的代码，如下所示：

 DatabaseReference databaseReference= FirebaseDatabase.getInstance().getReference();
    Query query=databaseReference.child(Constants.ARG_CHAT_ROOMS).child("tKyhXXAf6TQnQgKyAf23RJcwhsn1_S5ClzMeWUEXR9evLJO9CBd9ABCH3").orderByChild().limitToLast("Number of last messages you want to show");
    query.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {

            for(DataSnapshot snapshot:dataSnapshot.getChildren()){
                Chat chat=snapshot.getValue(Chat.class);
                Log.e("163","ACV"+chat.fromEmail);
            }
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });

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更新您的代码，如下所示：

 DatabaseReference databaseReference= FirebaseDatabase.getInstance().getReference();
    Query query=databaseReference.child(Constants.ARG_CHAT_ROOMS).child("tKyhXXAf6TQnQgKyAf23RJcwhsn1_S5ClzMeWUEXR9evLJO9CBd9ABCH3").orderByChild().limitToLast("Number of last messages you want to show");
    query.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {

            for(DataSnapshot snapshot:dataSnapshot.getChildren()){
                Chat chat=snapshot.getValue(Chat.class);
                Log.e("163","ACV"+chat.fromEmail);
            }
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });

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firebase中有limitTo你试过了吗？firebase中有limitTo你试过了吗？我也试过使用它，但它

返回null

@yashaswi我也更新了我的代码你能查看我的代码吗code@VishalYadav请检查编辑后的答案是否有空异常。我也尝试过使用这个，但是它

返回null

@yashaswi我也更新了我的代码你能查看我的代码吗code@VishalYadav请检查编辑后的答案，查看您得到的空异常。已尝试，但给了我

null

我不知道我错过了什么，但给了我

null

我不知道我错过了什么

 DatabaseReference databaseReference= FirebaseDatabase.getInstance().getReference();
    Query query=databaseReference.child(Constants.ARG_CHAT_ROOMS).child("tKyhXXAf6TQnQgKyAf23RJcwhsn1_S5ClzMeWUEXR9evLJO9CBd9ABCH3").orderByChild().limitToLast("Number of last messages you want to show");
    query.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {

            for(DataSnapshot snapshot:dataSnapshot.getChildren()){
                Chat chat=snapshot.getValue(Chat.class);
                Log.e("163","ACV"+chat.fromEmail);
            }
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });