Question

public class from_the_database {

    public String last_entry(String object) {

        DatabaseReference database = FirebaseDatabase.getInstance().getReference();
        final String ok = object;
        final ArrayList<String> list = new ArrayList<String>(); // array to save result
        Query query = database.child("records").orderByChild("object").equalTo(object);

        query.addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {

                if (dataSnapshot.exists()) {
                    for (DataSnapshot post : dataSnapshot.getChildren()) {
                        Data_model_milk dmm = 
                        post.getValue(Data_model_milk.class);
                        Log.e("value",dmm.getDate()); // this works though
                        list.add("item was added last at " + dmm.getDate()); // not being added
                    }
                } else {
                    list.add("no insert for " + ok); // not being added
                }
            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {
                Log.e("from the db cancelled", databaseError.toString());
            }

        });

        Log.e(object, list.get(0)); // not available
        return list.get(0); // list.get(0) is empty
    }
}

所以问题是 dmm.getDate（）返回一个值，但未保存在数组列表或需要由主函数last_entry返回的任何其他变量中

我使用了以0,1和10大小初始化的最终数组，但似乎没有任何作用。

Answer 1

您正在采取的方法，将无法奏效。我猜您正在尝试使您的类成为实用程序类。

您应该更改方法，可能下面的代码可以提供帮助

public class from_the_database {

    public void last_entry(String object, OnDataChangeCallback listener) {

        DatabaseReference database = FirebaseDatabase.getInstance().getReference();
        final String ok = object;
        Query query = database.child("records").orderByChild("object").equalTo(object);

        query.addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                if (dataSnapshot.exists()) {
                    for (DataSnapshot post : dataSnapshot.getChildren()) {
                        Data_model_milk dmm = 
                        post.getValue(Data_model_milk.class);
                        Log.e("value",dmm.getDate()); // this works though
                        listener.onDataChange("item was added last at " + dmm.getDate());
                    }
                } else {
                    listener.onDataChange("no insert for " + ok);
                }
            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {
                Log.e("from the db cancelled", databaseError.toString());
            }

        });
    }

    public interface OnDataChangeCallback{
        void onDataChange(String data);
    }
}

现在如下调用last_entry方法

last_entry("queryString", new OnDataChangeCallback(){
    public void onDataChange(String data){
      // do something with data
    }
});

Answer 2

只有在基类和内部类中都存在相同的对象名称时，您的代码中出现的情况才会发生

public class LockTest {
  public static void main(String[] args) {
    final ArrayList<String> strList = new ArrayList<>(); // Same object name variable in base-class and inner-class
    Hello hello = new Hello() {
        @Override
        public void run() {
            strList.add("Hello");
        }
    };
    System.out.println(strList.size()); // Output is 0
  } 
}

interface Hello {
ArrayList<String> strList = new ArrayList<>();
void run();
}

在您的情况下，这似乎不太可能，但请看一下这位天使提出的问题。也许是这样

Answer 3

onDataChange是asynchronous，所以您的

Log.e(object, list.get(0)); // not available
return list.get(0); // list.get(0) is empty

将在调用onDataChange之前执行，因此您的列表始终为空。

因此，与其从函数中返回一个字符串，不如创建一个带有字符串参数的新函数并像这样在onDataChange内部调用该函数，效果更好：

public void last_entry(String object) {

    DatabaseReference database = FirebaseDatabase.getInstance().getReference();
    final String ok = object;
    final ArrayList<String> list = new ArrayList<String>(); // array to save result
    Query query = database.child("records").orderByChild("object").equalTo(object);

    query.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {

            if (dataSnapshot.exists()) {
                for (DataSnapshot post : dataSnapshot.getChildren()) {
                    Data_model_milk dmm =
                            post.getValue(Data_model_milk.class);
                    Log.e("value",dmm.getDate()); // this works though
                    list.add("item was added last at " + dmm.getDate()); // not being added
                }
            } else {
                list.add("no insert for " + ok); // not being added
            }

            performSomeAction(list.get(0));
        }

        @Override
        public void onCancelled(@NonNull DatabaseError databaseError) {
            Log.e("from the db cancelled", databaseError.toString());
        }

    });
}

public void performSomeAction(String msg){
    //Do your action here
}

最终列表或数组将不存储值

3 个答案: