CODEWITHSUNDEEP
javagenerics
Andrejs
Andrejs

Reputation: 11981

Creating a Generics collection to which one cannot add the specified type

I understand the PECS concept and why the below add() compile and don't compile:

List<? super ClassCastException> list = new ArrayList<>();
list.add(new ClassCastException()); // OK
list.add(new Exception()); // Does not compile

But shouldn't the below be illegal then? Why not? (specifying the super type that we can't add when instantiating)

List<? super ClassCastException> list2 = new ArrayList<Exception>(); // Compiles
                                                          ^

Upvotes: 0

Views: 43

Answers (1)

Sweeper
Sweeper

Reputation: 274690

Let's examine the reason why the first piece of code is illegal:

List<? super ClassCastException> list = new ArrayList<>();
list.add(new Exception()); // Does not compile

list is of type List<? super ClassCastException> i.e. a list of unknown type that is a parent of ClassCastException. You can't put an Exception in there because what if the list is actually a list of RuntimeExcept? RuntimeException is also a parent class of ClassCastException isn't it? And you certainly can't put an Exception in a list of RuntimeException!

However, this is different:

List<? super ClassCastException> list2 = new ArrayList<Exception>();

What can you assign to list2? list2 is declared as a list of unknown type that is a parent of ClassCastException. Any kind of list that fits the criteria can be assigned.

"But there are other parent classes of ClassCastException though!" you say. Well, yes there are, but in an assignment statement, do we care what the original value of list2 actually is? No. If list2 was originally an ArrayList<RuntimeException>, fine. We can still assign an ArrayList<Exception>. There is no contradiction here. See the italicised in the first paragraph? That's the contradiction that occurs if you try to put an Exception in a list of ? super ClassCastException. You can't point out anything wrong about this assignment statement though.

Upvotes: 1

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