Ensuring a an object type

Question

Everybody knows that before doing any cast we have to check the object type with instanceof, however, with generics the compiler doesn't allow us to check it with generics like this.

if(object instanceof List<Message>)

I was investigating its solution and some people say that we have to check with List only, however, this is not valid for me. I also checked that some developers recommend using isAssignableFrom, but it requires object to have an instance. So how can I check that object is a List of Message (and not a just a list) without creating a trick object?

You don't *have* to use `instanceof` in order to cast. In fact if you're doing a lot of unknown casts, you most likely have a design flaw that you should fix. — Kayaman, Jan 10 '18 at 08:51
You should use [PECS](https://stackoverflow.com/questions/2723397/what-is-pecs-producer-extends-consumer-super) when working with generics. This, of course, only works if all elements in the list satisfy the condition/constraint. — Turing85, Jan 10 '18 at 08:53

score 0 · Answer 1 · answered Jan 10 '18 at 08:52

0

A bit naive approach

 Object deserializedList = fetchList();
 if (deserializedList instanceof List) {
   for (Object obj : (List)deserializedList) {
     if (obj instanceof Message) {
       Message message = (Message)obj;
       System.out.println("message: " + message);
     } else {
       System.out.println("not a message: " + obj);
     }
   }
 }

answered Jan 10 '18 at 08:52

Boris Pavlović

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What if the list is empty? – Hola Soy Edu Feliz Navidad Jan 10 '18 at 08:53
1

@HolaSoyEduFelizNavidad then the `foreach`-loop is not entered. You may want to check that the `List` is not `null`: `if ((deserializedList != null) && (deserializedList instanceof List)) {` – Turing85 Jan 10 '18 at 08:54
That's exactly my problem. I don't want to do it. I might need to add something to the list afterwards. – Hola Soy Edu Feliz Navidad Jan 10 '18 at 08:59
"*That's exactly my problem. I don't want to do it.*" - you do not want to do what? "*I might need to add something to the list afterwards.*" - To the original list? There is nothing stopping you form doing that. Please give us a clear problem statement. – Turing85 Jan 10 '18 at 09:01
IF the list is empty, and I add one message, but it might be a list of something else. – Hola Soy Edu Feliz Navidad Jan 10 '18 at 09:09

Ensuring a an object type

1 Answers1