jsp useBean with hashmap

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jsp useBean with hashmap

How do I use jsp:useBean with collections like Lists and Maps correctly?

I can get Lists by using

    type="List<MyObject>"

but I cannot get the List to instantiate if nothing is passed.

    class="ArrayList<MyObject>"

And the above attribute gives an error:

The value for the useBean class attribute ArrayList`<MyObject`> is invalid.

And both the attributes fail for a HashMap.

Currently, I'm reading the objects using getAttribute() but it'll be cleaner to use the tags and set them up.

+1 A:

Have you tried just

class="java.util.ArrayList"

Remember, the actual type name doesn't include the generic type, that's syntactic sugar used within the Java language itself, and <jsp:useBean> isn't part of the Java language.

skaffman 2010-07-16 09:53:28

I guess my phrasing was unclear. I've edited my post a bit. The class attribute is the one throwing the error. The type attribute works for lists, but doesn't instantiate the list if null is passesd.

manu1001 2010-07-16 10:03:32

@manu101: Yes, but `class="ArrayList<MyObject>"` is not valid, because `ArrayList<MyObject>` is not a valid type. The type is `java.util.ArrayList`

skaffman 2010-07-16 10:14:50

@manu1001, skaffman is entirely right. I just wanted to add, the `jsp:useBean` is superfluous if you don't need to set items in the list (for which there's no way using `jsp:` or `c:` tags by the way) and when you are accessing it using pure EL (e.g. `${foo}` and so on). You could leave the `jsp:useBean` line away, unless for pure documentation or IDE-support purposes.

BalusC 2010-07-16 11:43:08

Ok, got it. It works for Lists now. But what about HashMap? The same doesn't work. Basically, I want to know how to read a HashMap<MyObject1, MyObject2> from the request scope using the useBean tag.

manu1001 2010-07-16 12:20:05

@manu1001: if `java.util.ArrayList` worked, then so will `java.util.HashMap`

skaffman 2010-07-16 12:27:43

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