java - What is the practical application of an assignment involving a wildcard? -

what reason of assignment this?

list<? extends fruit> flist = new arraylist<apple>(); // flist.add(new apple());  // flist.add(new fruit());  // flist.add(new object()); 

once "upcast" apple container in fruit container, not able add in it.

i know like:

list<apple> basket = new arraylist<apple>(); //fill basket tons of juicy apples list<? extends fruit> fruitcontainer = basket; 

and able use fruit interface use elements held fruitcontainer. can practical reason this, if cannot add later?

list<? extends fruit> flist = new arraylist<apple>(); 

that line taken whole has no practical use, you've noted in question, each of the parts of (list<? extends fruit> flist , new arraylist<apple>()), separately, has practical use. they're both valid, , useful, not in combination (unless don't need put in list). that's why compiles, each of parts valid; it's us, programmers, combine them in reasonable , useful way.