Question

How to do this in Java – passing a collection of subtype to a method requiring a collection of base type?

The example below gives:

The method foo(Map<String,List>) is not applicable for the arguments (Map<String,MyList>) 

I can implement by creating a class hierarchy for the typed collections – but is it possible otherwise?

public void testStackOverflow() {      class MyList extends AbstractList {         public Object get(int index) {             return null;         }         public int size() {             return 0;         }                };      Map <String, List>   baseColl = null;     Map <String, MyList> subColl  = null;      foo (subColl);               }  private void foo (Map <String, List> in) {       } 

EDIT: Turns out from the answers that this requires ‘bounded wildcard’ so adding this text for searchability

Answer 1

Change foo to:

private void foo(Map <String, ? extends List> in) { } 

That will restrict what you can do within foo, but that’s reasonable. You know that any value you fetch from the map will be a list, but you don’t know what kind of value is valid to put into the map.