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!

You are trying to downcast your LinkedHashMap to an EntireFile. If the cast does not work, that's because the runtime type of the object returned by your method is not an EntireFile. — Alexandre Dupriez
– Alexandre Dupriez, Commented Sep 17, 2012 at 14:46

Jon Skeet · Accepted Answer · 2012-09-17 14:45:34Z

5

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

answered Sep 17, 2012 at 14:45

Jon Skeet

1.5m893 gold badges9.3k silver badges9.3k bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Peter Lawrey · Accepted Answer · 2012-09-17 14:55:45Z

0

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());

answered Sep 17, 2012 at 14:55

Peter Lawrey

535k83 gold badges770 silver badges1.2k bronze badges

Collectives™ on Stack Overflow

java cast LinkedHashMap to class extending LinkedHashMap

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related