? super Type Java Generics

Question

I have Java 8 generics question .

I have following class hierarchy:

Employee is base class
Manager extends Employee
Executive extends Manager

The following line (1) does not compile:

(1) List<? super Manager> foos1 = new ArrayList<Executive>(); // error

The below lines 2 and 3 compile ok:

(2) List<? super Manager> foos2 = new ArrayList<Manager>();   
(3) List<? super Manager> foos3 = new ArrayList<Employee>();

however, even though I can still add Executive like so:

(4) foos2.add(new Manager("Manager!",123));
(5) foos3.add(new Executive("Executive!",1.0,2));

Please explain the logic why I can not assign variable

<? super Manager> = ArrayList<Executive>

where Executive is a super of Manager but I can still add Executive object to the array list?

many thanks!

the so called duplicate (i.e. PECS) does not answer my question. — Acid Rider, Feb 18 '16 at 06:50
I think it does.. but thats my opinion.. Well, Ive reopened your question :) — TheLostMind, Feb 18 '16 at 06:54
http://stackoverflow.com/questions/2723397/java-generics-what-is-pecs — fps, Feb 18 '16 at 07:01
@FedericoPeraltaSchaffner - Apparently that post doesn't answer the OPs question — TheLostMind, Feb 18 '16 at 07:02
@TheLostMind Sure, I understand you closed the question and reopened it. The link is useful to readers that reach here, though. — fps, Feb 18 '16 at 07:33

score 4 · Answer 1 · answered Feb 18 '16 at 07:02

4

Executive is a sub class of Manager, not a superclass.

The other two work because Employee is a superclass of Manager and type bounds are inclusive (i.e. the class itself or any of its superclasses), so Manager fits in the bounds.

answered Feb 18 '16 at 07:02

Jim Garrison

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I'm betting the OP got confused because this is the opposite of the way the hierarchy works in business. Although of course the hierarchy in his code is the way things _should_ be ... (http://dilbert.com/strip/1994-01-04) – ajb Feb 18 '16 at 07:08

score 4 · Answer 2 · answered Feb 18 '16 at 07:06

List<? super Manager> foos1 = new ArrayList<Executive>(); // error

? super SomeClass
What this means is that you can create a new List of a class which is a Super-class of SomeClass. Since Executive is NOT a super-class of Manager, you can't assign List<? super Manager> foos1 = new ArrayList<Executive>();

Next, why foos3.add(new Executive("Executive!",1.0,2)); works?
Now, List<? super Manager> foos3 = new ArrayList<Employee>(); can accept any Manager instance. Turns out Executive is a Manager, so you can add an Executive to the List

? super Type Java Generics

2 Answers2