java - Why can't we return an ArrayList.iterator() for Iterator? -

java - Why can't we return an ArrayList<Foo>.iterator() for Iterator<IFoo>? -

i have method returns iterator<ifoo>. within method, if seek homecoming somearraylist of type arraylist<foo>, compilation error saying: type mismatch: cannot convert iterator<foo> iterator<ifoo>

here's type hierarchy:

interface:ifoo --class: foo (implements ifoo)

and, here's method:

public iterator<ifoo> iterator() { homecoming somearraylist.iterator(); }

my question is, when method wants me homecoming of type iterator<ifoo>, why can't homecoming somearraylist of type arraylist<foo> since foo implements ifoo?

update @ 8:38 on friday, june 20, 2014 (utc)

the method in question public iterator<ifoo> iterator() cannot modified since i'm implementing interface.

i should have mentioned before. sorry.

going off recent edit, can't modify method signature (which bad, because more flexible signature have avoided this...):

one solution create new list proper type , utilize iterator that. works because various constructors lists far more flexible in types take , take subtypes. main disadvantage produce shallow re-create of original arraylist, takes space (albeit not much unless you're working big lists). posted reply in much more compact form.

public iterator<ifoo> iterator() { homecoming new arraylist<ifoo>(somearraylist).iterator(); }

you have specify type arguments explicitly here because otherwise type inferred foo, leaves were.

alternatively, if space required new arraylist() phone call isn't appealing, can write anonymous subclass of iterator acts wrapper (i think got syntax right; might have made minor error or two, should easy fix). it's more writing, it's leaner on memory usage, might desirable:

public iterator<ifoo> iterator() { homecoming new iterator<ifoo>() { private final iterator<? extends ifoo> iterator = somearraylist.iterator(); @override public boolean hasnext() { homecoming iterator.hasnext(); } @override public number next() { homecoming iterator.next(); } @override public void remove() { iterator.remove(); } }; }

one of import thing note these solutions not equivalent.

your solution , first solution homecoming iterators operate on different list original one. distinct list shallow copy, you'll same results hasnext() , next(). however, if phone call remove(), original list remain unaffected, , re-create changed.

the anonymous subclass perform operations on original list, if original solution had worked in first place.

java iterator