如何从android中的firebase中检索List对象 [英] How to Retrieve a List object from the firebase in android

问题描述

我无法从Firebase中检索列表。我没有麻烦存储它，但只要我试图将dataSnapshot.getValue（）转换为ArrayList，我的应用程序崩溃，给出一个异常：

lockquote
<但是当我试图把它转换成一个HashMap的时候，它也崩溃，给出了异常：

HashMap不能被转换成ArrayList

/ b>

ArrayList不能被转换成hashmap

<请>请帮助​​！这里是创建问题的代码：

$ p $ Fire $ add
public void onDataChange（DataSnapshot dataSnapshot）{
List< TaskDes> td =（ArrayList< TaskDes>）dataSnapshot.getValue（）
notifyDataSetChanged（）;
}

@Override
public void onCancelled（FirebaseError firebaseError）{


}）;

我想将Firebase中的所有数据都作为一个List来检索。类TaskDes包含三个字段：

pre $ class $ Task $ { b $ b字符串taskTitle

$ / code $

解决方案

需要创建一个GenericTypeIndicator对象作为 DataSnapshot.getValue（）参数传递。

代码：

  GenericTypeIndicator< List< String>> t = new GenericTypeIndicator< List< String>>（）{}; 
 
列表< String> yourStringArray = dataSnapshot.getValue（t）; 
  

I am having trouble retrieving a List from the Firebase. I have no trouble storing it, but as soon as I try to cast dataSnapshot.getValue() to ArrayList my app crashes, giving an exception:

HashMap cannot be casted to ArrayList

But when I tried to cast it to a HashMap, it also crashes, giving exception:

ArrayList can't be casted to hashmap

Need help please! Here is the code that is creating the problem:

Fire.addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        List<TaskDes> td = (ArrayList<TaskDes>) dataSnapshot.getValue()
        notifyDataSetChanged();
    }

    @Override
    public void onCancelled(FirebaseError firebaseError) {

    }
});

I want to retrieve all the data in the Firebase as one List. The class TaskDes contains three fields:

class TaskDes { // definition
    boolean done
    String taskDescription
    String taskTitle
}

解决方案

You need to create a GenericTypeIndicator object to pass as DataSnapshot.getValue() parameter.

Code:

GenericTypeIndicator<List<String>> t = new GenericTypeIndicator<List<String>>() {};

List<String> yourStringArray = dataSnapshot.getValue(t);

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