android- snapshot.getvalue 指向空指针

Question

我使用一个接口(interface)从数据库中检索数据并使用它，起初当我使用 String 时它可以工作但是在我想带走整个 child 之后我一直得到一个空指针，这是整个类但是空指针在 uti 变量中的 checkUser 方法中的部分

package com.android.pfe.other;

import android.support.annotation.Keep;
import android.util.Log;
import android.widget.Toast;

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.IgnoreExtraProperties;
import com.google.firebase.database.ValueEventListener;

import java.io.Serializable;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Hashtable;
import java.util.List;

/**
 * Created by SADA INFO on 13/04/2018.
 */
@IgnoreExtraProperties
@Keep
public class User implements Serializable {
    private static final String TAG ="UserClass" ;
    public String username;
    public String email;
    public ArrayList<User> contact;
    public String Uid;
    public List article;
    public DatabaseReference mDatabase;
    public ArrayList<User> UserList;
    public User uti;
    public User() {
        // Default constructor required for calls to DataSnapshot.getValue(com.android.pfe.other.User.class)
    }

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



    }

    public User(String username, String email,String uid) {
        this.username = username;
        this.email = email;
        this.contact=new ArrayList<User>();
        this.Uid=Uid;


    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getEmail() {
        return email;
    }

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

    public void addUser(String UserId, String name, String email) {
        mDatabase = FirebaseDatabase.getInstance().getReference("User");
        User user = new User(name, email,UserId);
        mDatabase.child(UserId).setValue(user);

    }
    public void addFriend(String UserId, final String email)
    {
        mDatabase = FirebaseDatabase.getInstance().getReference("User");
        DatabaseReference user = mDatabase.child(UserId);
        final DatabaseReference friendlist = user.child("contact");
        checkUser(email, new ICheckUserListener() {
        @Override
        public void onSuccess(final User value) {
            friendlist.addValueEventListener(new ValueEventListener() {
                @Override
                public void onDataChange(DataSnapshot dataSnapshot) {

                   ArrayList<User> list  =(ArrayList<User>) dataSnapshot.getValue();

                    //this is where value is null
                    if(list==null&&value!=null)
                    {
                        list=new ArrayList<User>();
                        User user=new  User(value.username,email);
                        list.add(user);
                        friendlist.setValue(list);
                    }

                }

                @Override
                public void onCancelled(DatabaseError databaseError) {

                }
            });
        }

        @Override
        public void onError(Exception e) {

        }
    });

    }
    public void checkUser(String email, final ICheckUserListener listener) {
        ValueEventListener mValueEventListener = new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
              //  listener.onSuccess(dataSnapshot.exists() ? dataSnapshot : null);
                //uti is always null
                uti=dataSnapshot.getValue(User.class);
                listener.onSuccess(uti);
                if(uti==null) {
                    Log.w(TAG, "user pas trouvé");
                }
                // addFriend(uti.getEmail(),uti.getUsername());

            }

            @Override
            public void onCancelled(DatabaseError databaseError) {
                Log.w(TAG, "loadUser:onCancelled", databaseError.toException());
                listener.onError(databaseError.toException());
            }
        };
        FirebaseDatabase
                .getInstance()
                .getReference("User")
                .equalTo(email)
                .addListenerForSingleValueEvent(mValueEventListener);
    }


public void getFriends(String UserId){
    mDatabase = FirebaseDatabase.getInstance().getReference("User");
    DatabaseReference user = mDatabase.child(UserId);
    final DatabaseReference friendlist = user.child("contact");
    friendlist.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
         UserList=(ArrayList)dataSnapshot.getValue();

        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });
}

    public ArrayList<User> getUserList() {
        return UserList;
    }
}

my database

Answer 1

要通过电子邮件地址获取用户，您需要运行 Firebase 数据库查询。

FirebaseDatabase
        .getInstance()
        .getReference("User")
        .orderByChild("email")
        .equalTo(email)
        .addListenerForSingleValueEvent(mValueEventListener);

这将为您的听众提供正确的结果。

但是您的 onDataChange 也不会工作。您试图从快照中获取 List，但它实际上是一个带有字符串键 (EuEr...,等)和属性作为一个值。所以你需要在你的回调中转换它:

public void onDataChange(DataSnapshot dataSnapshot) {
    UserList = new ArrayList<User>();
    for (DataSnapshot userSnapshot: dataSnapshot.getChildren()) {
        System.out.println(userSnapshot.getKey());
        UserList.add(userSnapshot.getValue(User.class));
    }
}