java - Generics Collections PECS -

java - Generics Collections PECS -

i have question method java.util.collections:

public class collections { public static <t> void copy(list<? super t> dest, list<? extends t> src) { (int i=0; i<src.size();i++) dest.set(i,src.get(i)); } }

i understand how <? super t> works, however, don't understand why first parameter list<? super t> instead of list<t>. think it's useless in situation. using list<t> should work well, shouldn't it? give me examples understand if possible, please?

thanks.

no, makes sense. example, consider situation:

t inputstream dest list<object> src list<fileinputstream>

that works absolutely fine. of course, could create t either object or fileinputstream in situation - imagine calling from method signature of:

public void dosomething(list<? super inputstream> streams) { // want utilize re-create in here reason }

you don't know it's list<inputstream> - it's list<? super inputstream>. if dest parameter in copy just list<t>, we'd stuck... way is written, we're fine.

it makes sense in terms of require destination list - just need able set values of t within it. likewise require of source list can values of t it. <? super t> , <? extends t> express requirements well.

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