java - Making a function returning the same output-type as input-type -

so let's have following generic function sorts elements in collection (e.g. arraylist<t> or hashset<t>):

public static <t extends comparable> collection<t> sort(collection<t> a) {     list<t> l = a.stream().sorted().collect(collectors.tolist());     return l; } 

the problem when call function following code , have existing variable of type arraylist<integer> has values:

counts = (arraylist<integer>) sort(counts); 

when calling function have cast returned collection arraylist.

is there way can let sort function conversion me? if input arraylist, output arraylist; if input hashset, ouput hashset etc...

you need use generics @6ton suggests in answer, need specify supplier sort method, can create right collector:

public static <t extends comparable<t>, e extends collection<t>> e sort(     collection<t> a,      supplier<e> factory) {      return a.stream().sorted().collect(collectors.tocollection(factory)); } 

then, use sort method way:

list<float> ordered = sort(unordered, arraylist::new); 

this return arraylist, note first argument (i've called unordered), can collection, i.e. hashset, treeset, etc. similarly, can return set, while unordered list or whatever collection:

set<float> ordered = sort(unordered, linkedhashset::new); 

note: others have stated, doesn't make sense sort hashset, , same happens other collections. while specifying supplier factory returned collection won't produce runtime errors, doesn't mean returned collection sorted. example:

set<float> stillunordered = sort(unordered, hashset::new); 

in case, stream collected hashset, doesn't maintain order.