Sort a Map / EntrySet using Lambdaj

Question

I'm wondering if its possible to sort an EntrySet< K, V > based on some property of K using lambdaj ?

I dont think Lambdaj will let me I just wanted to confirm.

For example if I have a Map< A, B > where A has the following structure:

public class A{
private double published;

     public double getPublished()
     {
           return double; 
     }
}

Am I able to sort an entryset, using lambdaj, on A.getPublished ?

What does it mean to sort a `Set`? `Set`s don't have order, right? — missingfaktor, Jun 29 '12 at 15:54
I mean sort a returned collection. Thats what lambdaj does right ? It doesnt mutate the input collection. — MikePatel, Jun 29 '12 at 15:58
I think your assumption is correct, but I am not a lambdaj expert. Aside: If this is not just an academic question, you may want to use a LinkedHashMap, where the order is determined by the insertion order. — HellishHeat, Jun 29 '12 at 21:47

score 3 · Accepted Answer · answered Jun 29 '12 at 22:02

3

Unfortunately what you're asking is not possible because of type erasure. Indeed supposing you have map of type Map what you would like to do is something as:

sort(map.entrySet(), on(Map.Entry.class).getKey().getPublished());

That doesn't work because lambdaj doesn't know that Map.Entry is actually a Map.Entry (and of course a syntax like on(Map.Entry.class).getKey()... is not allowed). It means that the on(Map.Entry.class).getKey() invocation just returns an object of Object class and then you're not allowed to invoke the getPublished() method on it.

answered Jun 29 '12 at 22:02

Mario Fusco

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Thanks, this is what I suspected. Some libraries (like jackson) provide ways of passing classes erased by type erasure so I was wondering if there was any such facility for lambdaj – MikePatel Jun 30 '12 at 11:23
Can you elaborate a bit more on that? Do you think there's the possibility to improve this in lambdaj? If so how? What does jackson do in this case? – Mario Fusco Jun 30 '12 at 12:50
@MikePatel, can you please give a link to this Jackson library you are referring to? I am unable to find it. – missingfaktor Jul 01 '12 at 17:27
jackson allows you to provide a TypeReference class which is its own implementation encapsulating the information usually disposed of at type erasure time. e.g. Map result = mapper.readValue(src, new TypeReference>() { }); potentially you could utilize this approach but i'm not sure how much work it is. @missingfaktor http://jackson.codehaus.org/ – MikePatel Jul 01 '12 at 17:31

Sort a Map / EntrySet using Lambdaj

1 Answers1