Firebase数据库数据检索-Java [英] Firebase database data retrieving -Java

问题描述

这是我的数据库结构：

我用来读取数据的代码：

  user =hrcj7; 
 mDatabase = FirebaseDatabase.getInstance（）。getReference（）。child（User）; 
查询phoneQuery = mDatabase.orderByChild（user）; 
 phoneQuery.addChildEventListener（new ChildEventListener（）{
 @Override $ b $ public void onChildAdded（DataSnapshot dataSnapshot，String prevChildKey）{
 User dinosaur = dataSnapshot.getValue（User.class）; 
 System.out.println（dataSnapshot.getKey（）+was+ dinosaur.getEmail（）+meters tall。）; 
} 
 
 @Override 
 public void onChildChanged（DataSnapshot dataSnapshot，String s）{
 
} 
 $ b $ @覆盖
 public void onChildRemoved（DataSnapshot dataSnapshot）{
 
 
 $ b @Override 
 public void onChildMoved（DataSnapshot dataSnapshot，String s）{
 
} 
 
 
 @Override 
 public void onCancelled（DatabaseError databaseError）{
 Log.e（App，onCancelled，databaseError.toException（））; 
} 
}）; 
  

这是检索后的dataSnapsshot值：

  DataSnapshot {
 key = hrcj7，
 value = {
 -Kh2-jOeGXCOr-VE3uD5 = {
 username = hrcj7，
 email=[e-mail]，
 imageurl = https：//firebasestorage.googleapis.com/v0/b/freelancer-33195.appspot.com/o/Blog_Images%2Fcropped933315999.jpg？alt = media& token = 7890f05f-87db-4a9d-9534-02da00225470 
} 
} 
} 
  

这是模型类：

  public class User {
 
 
 public String email; 
 public String imageurl; 
 public String username; 
 $ b public public User（String email，String imageurl，String username）{
 
 this.email = email; 
 this.imageurl = imageurl; 
 this.username = username; 
} 
 
 public String getEmail（）{
 return email; 
} 
 
 public void setEmail（String email）{
 this.email = email; 
} 
 
 public String getImageurl（）{
 return imageurl; 
 
 
 public void setImageurl（String imageurl）{
 this.imageurl = imageurl; 
} 
 
 public String getUsername（）{
 return username; 
 
 
 public void setUsername（String username）{
 this.username = username; 
 
 public User（）{
 //调用DataSnapshot.getValue（User.class）所需的默认构造函数
} 
 
} 
  

我的问题是 dinosaur.getEmail（） get检索值后返回null。可能是什么问题？提前致谢。

解决方案

这里我找到了解决方案。 行：

  User dinosaur = dataSnapshot.getValue（User.class）; 
  

应替换为以下内容：

  String key = mDatabase.child（User）。child（user）.push（）。getKey（）; 
 User dinosaur = dataSnapshot.child（key）.getValue（User.class）; 
  

Here is my DB structure:

Code which I used for data reading:

user="hrcj7";
mDatabase = FirebaseDatabase.getInstance().getReference().child("User");
                                        Query phoneQuery = mDatabase.orderByChild(user);
                                        phoneQuery.addChildEventListener(new ChildEventListener() {
                                            @Override
                                            public void onChildAdded(DataSnapshot dataSnapshot, String prevChildKey) {
                                                User dinosaur = dataSnapshot.getValue(User.class);
                                                System.out.println(dataSnapshot.getKey() + " was " + dinosaur.getEmail() + " meters tall.");
                                            }

                                            @Override
                                            public void onChildChanged(DataSnapshot dataSnapshot, String s) {

                                            }

                                            @Override
                                            public void onChildRemoved(DataSnapshot dataSnapshot) {

                                            }

                                            @Override
                                            public void onChildMoved(DataSnapshot dataSnapshot, String s) {

                                            }


                                            @Override
                                            public void onCancelled(DatabaseError databaseError) {
                                                Log.e("App", "onCancelled", databaseError.toException());
                                            }
                                        });

This is the dataSnapsshot value after retrieving:

DataSnapshot {
    key = hrcj7,
    value = {
        -Kh2-jOeGXCOr-VE3uD5={
            username=hrcj7,
            email=[e-mail],
            imageurl=https://firebasestorage.googleapis.com/v0/b/freelancer-33195.appspot.com/o/Blog_Images%2Fcropped933315999.jpg?alt=media&token=7890f05f-87db-4a9d-9534-02da00225470
        }
    }
}

This is the model class:

public class User {


            public String email;
            public String imageurl;
            public String username;

            public User(String email,String imageurl,String username) {

                this.email = email;
                this.imageurl=imageurl;
                this.username=username;
            }

            public String getEmail() {
                return email;
            }

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

            public String getImageurl() {
                return imageurl;
            }

            public void setImageurl(String imageurl) {
                this.imageurl = imageurl;
            }

            public String getUsername() {
                return username;
            }

            public void setUsername(String username) {
                this.username = username;
            }
     public User() {
            // Default constructor required for calls to DataSnapshot.getValue(User.class)
        }

        }

My problem is that dinosaur.getEmail() get null after retrieving the value. What can be the issue? Thanks in advance.

解决方案

Here I found the solution .

In line:

User dinosaur = dataSnapshot.getValue(User.class);

should be replaced as following:

String key = mDatabase.child("User").child(user).push().getKey();
User dinosaur = dataSnapshot.child(key).getValue(User.class);

这篇关于Firebase数据库数据检索-Java的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！