Java generics super keyword

I didn't get it for a while. Many of the answers here, and the other questions show specifically when and where certain usages are errors, but not so much why.

This is how I finally got it. If I have a function that adds Numbers to a List, I might want to add them of type MySuperEfficientNumber which is my own custom class that implements Number (but is not a subclass of Integer). Now the caller might not know anything about MySuperEfficientNumber, but as long as they know to treat the elements added to the list as nothing more specific than Number, they'll be fine.

If I declared my method as:

public static void addNumbersToList(List<? extends Number> numbers)

Then the caller could pass in a List<Integer>. If my method added a MySuperEfficientNumber to the end of numbers, then the caller would no longer have a List of Integers and the following code wouldn't work:

List<Integer> numbers = new ArrayList<Integer>();
addNumbersToList(numbers);

// The following would return a MySuperEfficientNumber not an Integer
Integer i = numbers.get(numbers.size()-1)

Obviously this can't work. And the error would be inside the addNumbersToList method. You'd get something like:

The method add... is not applicable for the arguments (MySuperEfficientNumber)

Because numbers could be any specific kind of Number, not necessarily something that MySuperEfficientNumber is compatible with. If I flipped the declaration around to use super, the method would compile without error, but the caller's code would fail with:

The method addNumbersToList(List<? super Number>)... is not applicable for the arguments (List<Integer>)

Because my method is saying, "Don't think that your List can be of anything more specific than Number. I might add all sorts of weird Numbers to the list, you'll just have to deal with it. If you want to think of them as something even more general than Number -- like Object -- that's fine, I guarantee they'll be at least Numbers, but you can treat them more generally if you want."

Whereas extends is saying, "I don't really care what kind of List you give me, as long as each element is at least a Number. It can be any kind of Number, even your own weird, custom, made-up Numbers. As long as they implement that interface, we're good. I'm not going to be adding anything to your list since I don't know what actual concrete type you're using there."


For the first part List<Number> fits in List<? super Number> but you can't add an Object to a List<Number>. That's why you can't add an Object to List<? super Number>.

On the other hand you can add every subclass of Number (Number included) to your list.

For the second part, String is an Object, but String isn't a superclass of Number.

If it worked like this, as every class is a subclass of Object, super would have no meaning.

Let's see every possible cases with List<? super Number> :

  • The passed list is a List<Object>
    • List<Object> will work
    • Object fits in <? super Number>
    • You can add any subtype of Number to a List<Object>
    • Even if you could also add String in it the only thing you're sure of is that you can add any subclass of Number.
  • The passed list is a List<Number> :
    • List<Number> will work
    • Number fits in <? super Number>
    • You can add any subtype of Number to a List<Number>
  • The passed list is a List<Integer> (or any subclass of Number):
    • List<Integer> won't work
    • Integer is a subclass of Number so it is exactly what we want to avoid
    • Even if an Integer fits in a Number you wouldn't be abble to add any subclass of Number in a List<Integer> (for example a Float)
    • super doesn't mean a subclass.
  • The passed list is a List<String> (or any class not extending Number nor in the "super hierarchy" of Number (ie. Number and Object) :
    • List<String> won't work
    • String doesn't fit in Number "super hierarchy"
    • Even if String fits in Object (which is a super class of Number) you woudln't be sure to be able to add a Number to a List that contain any subclass from one of the super classes of Number)
    • super doesn't mean any subclass of one of the super classes, it only means one of the super classes.

How does it work ?

You could say that as long as you can add any subclass of Number with your typed List, it respects the super keyword.


The bounded wildcard in List<? super Number> can capture Number and any of its supertypes. Since Number extends Object implements Serializable, this means that the only types that are currently capture-convertible by List<? super Number> are:

  • List<Number>
  • List<Object>
  • List<Serializable>

Note that you can add(Integer.valueOf(0)) to any of the above types. however, you CAN'T add(new Object()) to a List<Number> or a List<Serializable>, since that violates the generic type safety rule.

Hence it is NOT true that you can add any supertype of Number to a List<? super Number>; that's simply not how bounded wildcard and capture conversion work. You don't declare a List<? super Number> because you may want to add an Object to it (you can't!); you do because you want to add Number objects to it (i.e. it's a "consumer" of Number), and simply a List<Number> is too restrictive.

References

  • Angelika Langer's Generics FAQs
    • What is a bounded wildcard?
    • When would I use a wildcard parameterized type with a lower bound? ("When a concrete parameterized type would be too restrictive.")
    • Why is there no lower bound for type parameters? ("Because it does not make sense.")
  • JLS 5.1.10 Capture Conversion

See also

  • Effective Java 2nd Edition, Item 28: Use bounded wildcards to increase API flexibility
    • "PECS stands for producer-extends, consumer-super

Related questions

  • Too many to list, PECS, new Integer(0) vs valueOf, etc

Tags:

Java

Generics

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