java cast LinkedHashMap to class extending LinkedHashMap

Question

I have a class (EntireFile) that extends the LinkedHashMap. I try to cast:

EntireFile old = (EntireFile) functionReturningLinkedHashMap();

It throws exception with message: "java.util.LinkedHashMap cannot be cast to com.gmail.maximsmol.YAML.GroupMap".

public class EntireFile extends LinkedHashMap<String, GroupMap>

public class GroupMap extends LinkedHashMap<String, CategoryMap>

public class CategoryMap extends LinkedHashMap<String, LinkedHashMap<String, Integer>>

Please help me to solve the error!

Answer 1

The problem is that the reference returned simply isn't a reference to an instance of EntireFile . If functionReturningLinkedHashMap just returns a LinkedHashMap , it can't be cast to an EntireFile , because it isn't one - how would it get any extra information about it?

(Judging by your exception, you're actually talking about GroupMap rather than EntireFile , but the same thing applies.)

There's nothing special about LinkedHashMap here - the same is always true in Java:

Object foo = new Object();
String bar = (String) foo; // Bang! Exception

Answer 2

Instead of casting you can copy the data to the type of map you need.

EntireFile old = new EntireFile(functionReturningLinkedHashMap());

or

EntireFile old = new EntireFile();
old.putAll(functionReturningLinkedHashMap());