能够从Android的Firebase数据库中检索数据，但仅限于单个用户 [英] able to retrieve data from Firebase database in android but for single user only

问题描述

由于我能够从Firebase中检索数据，但是只能在android中使用单一用户数据库。
我只是想从数据库中获取数据库中的数据，而且没有用户身份验证登录信息就插入了firebase。

我在logcat中得到错误：

  java.lang.NullPointerException：试图调用虚函数java.lang.String java.lang.Object.toString（）'null对象引用
  

以下是我的代码：

  import android.os.Bundle; 
 
导入android.support.annotation.Nullable; 
导入android.support.v7.app.AppCompatActivity; 
导入android.util.Log; 
导入android.widget.ArrayAdapter; 
导入android.widget.ListView; 
 import com.google.firebase.database.DataSnapshot; 
 import com.google.firebase.database.DatabaseError; 
 import com.google.firebase.database.DatabaseReference; 
 import com.google.firebase.database.FirebaseDatabase; 
 import com.google.firebase.database.ValueEventListener; 
 
 import java.util.ArrayList; 
 
 public class ViewDatabase extends AppCompatActivity {
 private static final String TAG =ViewDatabase; 
 
 private DatabaseReference databaseReference; 
 private String userID; 
 
私人ListView mListView; 
 
 @Override 
 protected void onCreate（@Nullable Bundle savedInstanceState）{
 super.onCreate（savedInstanceState）; 
 setContentView（R.layout.view_database_layout）; 
 
 mListView =（ListView）findViewById（R.id.listview）; 
 databaseReference = FirebaseDatabase.getInstance（）。getReference（）; 
 
 
 $ b databaseReference.addValueEventListener（new ValueEventListener（）{
 @Override 
 public void onDataChange（DataSnapshot dataSnapshot）{
 showData（dataSnapshot ）; 
} 
 
 @Override 
 public void onCancelled（DatabaseError databaseError）{
 
 
}）; 
 
 
 $ private void showData（DataSnapshot dataSnapshot）{
 userinfo user = dataSnapshot.getValue（userinfo.class）; 
 
 Log.d（TAG，showData：name：+ user.getName（））; 
 Log.d（TAG，showData：Mobile：+ user.getMob（））; 
 Log.d（TAG，showData：Vehicle：+ user.getVehicle（））; 
 Log.d（TAG，showData：Address：+ user.getaddress（））; 
 
 ArrayList<字符串> array = new ArrayList< > （）; 
 array.add（user.getName（））; 
 array.add（user.getMob（））; 
 array.add（user.getVehicle（））; 
 array.add（user.getaddress（））; 
 ArrayAdapter adapter = new $ b $ ArrayAdapter（this，android.R.layout.simple_list_item_1，array）; 
 mListView.setAdapter（adapter）; 
 
 
 
 $ b $ / code $ / pre 
 $ b 
这是我的用户信息代码：
  public class userinfo {
 
 public String name; 
 public String mob; 
 public String vehicle; 
公共字符串地址; 
 
 public userinfo（）{
 
} 
 
 public userinfo（String name，String mob，String vehicle，String address）{
 this.name = name; 
 this.mob = mob; 
 this.vehicle =车辆; 
 this.address = address; 
} 
 public String getName（）{
 return name; 
} 
 public String getMob（）{
 return mob; 
} 
 public String getVehicle（）{
 return vehicle; 
 
 public String getaddress（）{
 return address; 
 
 
 
 
 $ div class =h2_lin>解决方案

$ p
$ b pre $ public $ onDataChange（DataSnapshot dataSnapshot）{$ b $ （DataSnapshot data：dataSnapshot.getChildren（））{
userinfo user = data.getValue（userinfo.class）;
Log.d（TAG，User name：+ user.getName（）+，mobile：
+ user.getMob（）+，Vehicle+ user.getVehicle（）+ ，地址+
user.getAdd（））;}
}


As I am able to retrieve data from Firebase, but for single user only database in android. I just want to get the data from database which I have inserted in firebase without user authentication login.

I am getting error in logcat:

java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.Object.toString()' on a null object reference

Here is my code:

import android.os.Bundle;

import android.support.annotation.Nullable;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;

import java.util.ArrayList;

public class ViewDatabase extends AppCompatActivity {
 private static final String TAG = "ViewDatabase";

 private DatabaseReference databaseReference;
 private String userID;

 private ListView mListView;

 @Override
 protected void onCreate(@Nullable Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.view_database_layout);

  mListView = (ListView) findViewById(R.id.listview);
  databaseReference = FirebaseDatabase.getInstance().getReference();



  databaseReference.addValueEventListener(new ValueEventListener() {
   @Override
   public void onDataChange(DataSnapshot dataSnapshot) {
    showData(dataSnapshot);
   }

   @Override
   public void onCancelled(DatabaseError databaseError) {

   }
  });

 }

 private void showData(DataSnapshot dataSnapshot) {
  userinfo user = dataSnapshot.getValue(userinfo.class);

  Log.d(TAG, "showData: name: " + user.getName());
  Log.d(TAG, "showData: Mobile: " + user.getMob());
  Log.d(TAG, "showData: Vehicle: " + user.getVehicle());
  Log.d(TAG, "showData: Address: " + user.getaddress());

  ArrayList < String > array = new ArrayList < > ();
  array.add(user.getName());
  array.add(user.getMob());
  array.add(user.getVehicle());
  array.add(user.getaddress());
  ArrayAdapter adapter = new
  ArrayAdapter(this, android.R.layout.simple_list_item_1, array);
  mListView.setAdapter(adapter);
 }


}

Here is my userinfo code:

  public class userinfo {

   public String name;
   public String mob;
   public String vehicle;
   public String address;

   public userinfo() {

   }

   public userinfo(String name, String mob, String vehicle, String address) {
    this.name = name;
    this.mob = mob;
    this.vehicle = vehicle;
    this.address = address;
   }
   public String getName() {
    return name;
   }
   public String getMob() {
    return mob;
   }
   public String getVehicle() {
    return vehicle;
   }
   public String getaddress() {
    return address;
   }
  }
  }

解决方案

Using an iterator you will be able to accomplish this.

 public void onDataChange(DataSnapshot dataSnapshot) {
                  for(DataSnapshot data: dataSnapshot.getChildren()){
                userinfo  user = data.getValue(userinfo.class);
                Log.d(TAG, "User name: " + user.getName() + ", mobile: " 
     + user.getMob() + ", Vehicle " + user.getVehicle() + ", Address " + 
     user.getAdd());}
            }

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