java.util.HashMap 不能投（android studio, firebase,java）

Question

I've been stuck for two days on this previous segment (Java and Firebase).

I kept a class-type list in the database but when I try to get it to Firebase I get an error.

This is the code:

ArrayList<PostDetails_class> cPostList=new ArrayList<PostDetails_class>();
FirebaseDatabase.getInstance().getReference().addValueEventListener( new ValueEventListener() {
          @Override
          public void onDataChange(@NonNull DataSnapshot snapshot) {
              cPostList= (ArrayList<PostDetails_class>) snapshot.child( "application_details" ).child( "community_postsList" ).getValue();
          }
          @Override
          public void onCancelled(@NonNull DatabaseError error) {

          }
      } );

This is an attempt to read data from the "cPost_List" list which is a list built from the "PostDetails_class" class. ("community_postsList" is a child that is a list of the "PostDetails_class" class):

PostDetails_class postDetails_class=cPostList.get( 0 );  //line 123 in main activity

the error:

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.myapp, PID: 1938
    java.lang.ClassCastException: java.util.HashMap cannot be cast to com.example.myapp.PostDetails_class
        at com.example.myapp.MainActivity.makeList(MainActivity.java:123)

Answer 1

When you call getValue() without passing any argument, Firebase returns the native type for the data, which in this case is a Map<String, Object> .

You need to tell Firebase what class to return by specifying it in the call to getValue , for example with: getValue(PostDetails_class.class) .

---

Firebase can't return a generic list in the way you're asking it to, because the type of the list is returned. I typically prefer looping of the child nodes, and extracting them one by one:

ArrayList<PostDetails_class> cPostList=new ArrayList<PostDetails_class>();
FirebaseDatabase.getInstance().getReference("application_details/community_postsList").addValueEventListener( new ValueEventListener() {
    @Override
    public void onDataChange(@NonNull DataSnapshot snapshot) {
        for (DataSnapshot child: snapshot.getChildren()) {
            PostDetails_class value = child.getValue(PostDetails_class.class);
            cPostList.add(value);
        }
    }
    @Override
    public void onCancelled(@NonNull DatabaseError error) {
        throw error.toException();
    }
});

You'll note that I also changed the code to load data from the deeper node, instead of loading the entire database.

---

If you want to get the entire list in one go, that is possible but you will need a generic type indicator for the list. I recommend reading more on that in these questions about [ firebase-realtime-database][android] generictypeindicator .

Answer 2

ArrayList<PostDetails_class> cPostList=new ArrayList<PostDetails_class>();

Firebase returns HashMap , so you can't receive it into ArrayList

So you need to change the type of the cPostList to HashMap<Integer, PostDetails_class>

HashMap<Integer, PostDetails_class> cPostList = new HashMap<>();

FirebaseDatabase.getInstance().getReference().addValueEventListener( new ValueEventListener() {
          @Override
          public void onDataChange(@NonNull DataSnapshot snapshot) {
              cPostList= (HashMap<Integer, PostDetails_class>) snapshot.child( "application_details" ).child( "community_postsList" ).getValue();
          }
          @Override
          public void onCancelled(@NonNull DatabaseError error) {

          }
      } );