Question

我正在写一份申请，与家人分享燃料停止。但是，我是firebase的新手。在这个例子中，我创建了类Eintrag（Entry）

public class Eintrag {
    String name;
    double kilometerstand;
    double liter;
    double preis;
    String datum;

    public Eintrag()
    {}

    public Eintrag(String name, double kilometerstand, double liter, double preis, String datum) {
        this.name = name;
        this.kilometerstand = kilometerstand;
        this.liter = liter;
        this.preis = preis;
        this.datum = datum;

    }

    public static void neuerEintrag(int index, String name, double kilometerstand, double liter, double preis, String datum) {

        MainActivity.log.add(index, new Eintrag(name, kilometerstand, liter, preis, datum));
    }

    public static void neuerEintrag(String name, double kilometerstand, double liter, double preis, String datum) {

        int index = MainActivity.log.size();
        MainActivity.log.add(index, new Eintrag(name, kilometerstand, liter, preis, datum));
    }

    // FIREBASE
    public String getName() {
        return name;
    }

    public double getKilometerstand() {
        return kilometerstand;
    }

    public double getLiter() {
        return liter;
    }

    public double getPreis() {
        return preis;
    }

    public String getDatum() {
        return datum;
    }

    public String setName() {
        return name;
    }

    public double setKilometerstand() {
        return kilometerstand;
    }

    public double setLiter() {
        return liter;
    }

    public double setPreis() {
        return preis;
    }

    public String setDatum() {
        return datum;
    }
    //FIREBASE
}

并尝试检索我之前上传的对象列表。现在，当我尝试拉取List，或使用子方法检索单个值时，它总是返回null。我不知道为什么，请帮忙。

public class Firebase {

    static FirebaseDatabase database = FirebaseDatabase.getInstance();
    static DatabaseReference lastEntry = database.getReference("LastEntry");
    static DatabaseReference eintrag = database.getReference("Eintrag");

    static int index;
    protected static List<Eintrag> freshLog;
    static int LastEntry;
    static String test;

    public static void lastEntry() {

        lastEntry.addListenerForSingleValueEvent(new ValueEventListener(){
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                // This method is called once with the initial value and again
                // whenever data at this location is updated.
                LastEntry=toIntExact((Long) dataSnapshot.getValue(Long.class));
            }

            @Override
            public void onCancelled(DatabaseError error) {
                // Failed to read value
            }
        });
    }

    public Firebase(){
    }

    protected static void getData(){

        eintrag.addListenerForSingleValueEvent(new ValueEventListener(){
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                // This method is called once with the initial value and again
                // whenever data at this location is updated.
                GenericTypeIndicator<List<Eintrag>> e = new GenericTypeIndicator<List<Eintrag>>() {};

                // freshLog= dataSnapshot.getValue(e);

                test= dataSnapshot.child("3").child("datum").getValue(String.class);
            }

            @Override
            public void onCancelled(DatabaseError error) {
                // Failed to read value
            }
        });
    }

    public static void setData(){
        Eintrag.neuerEintrag("R", 23000, 4.5, 6.7, "12.08.99");
        Eintrag.neuerEintrag("L", 24000, 3.4, 7.2, "UWE");
        Eintrag.neuerEintrag("C", 25000, 42.8, 2.4, "KAI");
        Eintrag.neuerEintrag("R", 26000, 4, 5.4, "Burkhardt");
        eintrag.setValue(MainActivity.log);
    }
}

这是我的Firebase结构： https://imgur.com/p9sGfl0

这是我的调试信息： https://imgur.com/Qq2Aqkk

PS：我知道我的代码很乱，我从学校记得的基础知识的基础知识开始。

Answer 1

要阅读Eintrag节点子节点，您应该这样做：

    database.getReference().child("Eintrag")

另外，为了使其更通用，请使用ChildEventListener而不是ValueEventListener。这样您就可以监听Eintrag中的更改，无论是添加的元素，更改的值，删除，移动还是取消。您可以在ChildEventListener中的on方法中分别处理其中一个更改的策略。

Firebase读取列表返回null

1 个答案: