Java generic method call

Question

Consider the following code:

public class MainClass { public static void main(String[] args) { ArrayList<HashMap<String, Integer>> collection = new ArrayList<>(); ArrayList<HashMap<String, Integer>> outCollecttion = <String, Integer, HashMap<String, Integer>, ArrayList<HashMap<String, Integer>>>doSomeWork(collection); } public static <V extends Object, U extends Object, K extends Map<V, U>, J extends Collection<K>> Collection<K> doSomeWork(J collection) { Collection<K> result = new ArrayList<>(); for (K element : collection) { result.add(element); //here is supposed other code, this is just example } return result; } }

I want to do some work on a generic collection that contains map of some generic types. I know that Java has hard time figuring complex generic expressions, so I put explicit types before method call:

<String, Integer, HashMap<String, Integer>, ArrayList<HashMap<Integer, String>>>doSomeWork(collection)

But compiler will not compile that. I do understand that it might have something to do with the fact that I'm trying to use generic type in generic type, but I don't know how to write this method without using casts and than deprecating warnings (I usually compile with -Xlint:unchecked flag).

Maybe you accidentally switched Integer and String in ArrayList<Hashmap<...>>? — Cephalopod
– Cephalopod, Commented Jun 3, 2014 at 9:23
Thank you for pointing that out, but it still no use. Edited code in question. — ProgramPower
– ProgramPower, Commented Jun 3, 2014 at 9:26

Cephalopod · Accepted Answer · 2014-06-03 09:30:54Z

First, when you want to explicitly provide the type arguments, you have to call the method from its class:

... = MainClass.<String, Integer, HashMap<String, Integer>, ArrayList<HashMap<String, Integer>>>doSomeWork(collection);

Second, your method returns Collection<K>, but the variable is declared ArrayList<...>. The following works for me:

Collection<HashMap<String, Integer>> outCollecttion = ...

TulaGingerbread · Accepted Answer · 2014-06-03 09:37:57Z

0

You don't need to use generics in assignment.

Collection<HashMap<String, Integer>> outCollecttion = doSomeWork(collection);

edited Jun 3, 2014 at 9:37

answered Jun 3, 2014 at 9:26

TulaGingerbread

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ProgramPower Over a year ago

Have you tried to compile it? Because it will not.

Error:(12, 72) java: incompatible types; no instance(s) of type variable(s) V,U,K exist so that java.util.Collection<K> conforms to java.util.ArrayList<java.util.HashMap<java.lang.String,java.lang.Integer>> required: java.util.ArrayList<java.util.HashMap<java.lang.String,java.lang.Integer>> found: <V,U,K>java.util.Collection<K>

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