Question

目标：我正在尝试从Firebase提取数据并将值从嵌套节点插入字符串（在Android中）。

注意 ：第一个变量/键值对（mNum）很精细但其他人获得NullPointer

研究：关于此特定问题的Firebase文档，Youtube，Stackoverflow，包括：How to get child of child value from firebase in android?

这如何适用于其他人：无法在DB具有多个嵌套时查找有关获取Firebase数据库信息的信息

错误：

java.lang.NullPointerException: println needs a message
                                                                                at android.util.Log.println_native(Native Method)
                                                                                at android.util.Log.i(Log.java:211)
                                                                                at com.netgalaxystudios.timeclock.RegisterSubscriptionActivity$1.onDataChange(RegisterSubscriptionActivity.java:80)

错误指向此行：Log.i（＆＃34; micro name＆＃34;，micro）;

数据库： 正在使用的Firebase DB的图片

RegisterSubscriptionActivity.java ：

public class RegisterSubscriptionActivity extends Activity {

    private DatabaseReference mDatabase;
    private DatabaseReference mDatabaseMicro;

    ArrayList subscriptionInfo;

    //Subscription (String) values
    String micro, small, medium, large, enterprise;
    String mNum, sNum, medNum, lNum, eNum;
    String mPrice, sPrice, medPrice, lPrice, ePrice;


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

        subscriptionInfo = new ArrayList<>();


        //GET INSTANCE OF FIREBASE DB & GRAB SUBSCRIPTION DATA
        mDatabase = FirebaseDatabase.getInstance().getReference("Subscription");
        mDatabaseMicro = mDatabase.child("Micro");
        mDatabaseMicro.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) { //literally a "snapshot" of the data
                //dataSnapshot.

                for(DataSnapshot subscriptionDataSnapshot : dataSnapshot.getChildren()) {

                    if (subscriptionDataSnapshot.getKey().equals("mNum")) {
                        mNum= subscriptionDataSnapshot.getValue().toString();
                    }
                    if(subscriptionDataSnapshot.getKey().equals("mPrice")) {
                        mPrice = subscriptionDataSnapshot.getValue().toString();
                    }
                    if(subscriptionDataSnapshot.getKey().equals("micro")) {
                        micro = subscriptionDataSnapshot.getValue().toString();
                    }

                    Log.i("micro number", mNum);
                    //Log.i("micro price", mPrice);
                    Log.i("micro name", micro);

                    String subscriptions = subscriptionDataSnapshot.getValue(String.class); //Each time it gets the key/value pair per node
                    subscriptionInfo.add(subscriptions);


                }

Answer 1

要获取Micro节点下的数据，请使用以下代码：

DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference microRef = rootRef.child("Subscription").child("Micro");
ValueEventListener eventListener = new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        for(DataSnapshot ds : dataSnapshot.getChildren()) {
            String mNum = ds.child("mNum").getValue(String.class);
            String mPrice = ds.child("mPrice").getValue(String.class);
            String micro = ds.child("micro").getValue(String.class);
            Log.d("TAG", mNum + " / " + mPrice + " / " + micro);
        }
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {}
};
microRef.addListenerForSingleValueEvent(eventListener);

你的出局将是：

10 Employees / 25 / Micro

Answer 2

您的意思是将这些日志语句放在for循环之外吗？第一次通过循环时它将匹配“mNum”，但micro仍将为空。

Firebase - （具有多个子节点的DataSnapshot） - NullPointerException

2 个答案: