我无法在我的类中描述firebase数据库引用mRef

Question

错误行：

  

尝试在空对象引用上调用虚拟方法'com.google.firebase.database.ValueEventListener com.google.firebase.database.DatabaseReference.addValueEventListener（com.google.firebase.database.ValueEventListener）'

Database.java

package com.juliusj.gym.easygym;
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;
import java.util.List;

public class Database{
    DatabaseReference mRef;
    List<Kontaktas> mList = new ArrayList<>();
    mRef = FirebaseDatabase.getInstance().getReferenceFromUrl("https://easygym-jj173.firebaseio.com/Users");

    public List<Kontaktas> sarasas(){
        mRef.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                mList.clear();
                //shake hands
                for (DataSnapshot unit : dataSnapshot.getChildren()){
                    Kontaktas value = unit.getValue(Kontaktas.class);
                    mList.add(value);
                }
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {

            }
        });

        return mList;
    }


 }

Answer 1

改变这个：

 mRef = FirebaseDatabase.getInstance().getReferenceFromUrl("https://easygym-jj173.firebaseio.com/Users");

到此：

 mRef = FirebaseDatabase.getInstance().getReference().child("Users");

您也无需添加网址来引用数据库。