how to cast hashmap.values().toArray() where such values() are also hashmaps?

Question

I expect the subject is clear, suggestions are appreciated.

I have a hhm=new HashMap<String,HashMap<String,Test>>().

I have a function(HashMap<String,Test>... array).

I need to call function(hhm.values().toArray(new HashMap<String,Test>[0])) but I cant find a way to do that, this code wont compile.

Casting will cause exception in run-time: function((HashMap<String,Test>[])hhm.values().toArray())

what now?

• ernest_k: Better stick with collections to avoid arrays of generic types... Change your function code to function(Collection<HashMap<String, Test>> array) and just call it using function(hhm.values()). You may need to change the implementation of function to read a collection/set rather than an array.
• Aquarius Power: @ErnestKiwele because of this I guess? stackoverflow.com/a/33178372/1422630
• ernest_k: You could say that. It's always a good time to rethink your design or contracts when you catch yourself making arrays of generic types. And, yes, that answer gives the first reason of the challenge.
• Alexey Romanov: No, that answer isn't relevant: it's talking about new E[...] where E is a type parameter, not a generic class. See ibm.com/developerworks/java/library/j-jtp01255/index.html under "More covariance troubles" instead.

Answer 1

Hacky solution in case you can't change function, not recommended otherwise:

function(hhm.values().toArray((HashMap<String,Test>[]) new HashMap<?,?>[0]))

Since it is legal to create a HashMap<?,?>[], and you can then cast it because HashMap<Whatever, Whatever>[] is really just HashMap[] at runtime (if it weren't, there would be no problem with new HashMap<String,Test>[0] in the first place).

Answer 2

I undertand what you want to implement. But your intention is not appropriate on Java.

Collections Using Generic. The purpose of Generic is Type-Saftey on compile-time.

Java array is deficient, so it force type on run-time.

Your intention is possible to implement. But you will be more safe to change varags parameter to List and more convenient to implement your purpose.

List<Map<String, Test1>> list =new ArrayList<>();
for(String key : hhm.keySet()){
 list.add(hhm.get(key));
}
function(list);
public static void function(List<Map<String, Test1>> list){
 for(Map<String, Test1> map : list){
 for(String key : map.keySet()){
 System.out.println(key +" : "+map.get(key));
 }
 }
}