Nested wildcards with lower bounds

问题

So I read through the main Java Generic FAQ and the single thing which is holding me up are nested wildcards with lower bounds. I want to give you an example of what I do understand, something specifically which works and how I view it. Maybe you could tell me the way I am thinking about this is wrong even though the compiler isn't complaining in the "good" case.

Example 1 (makes sense):

static void WildcardsMethod(List<? extends Pair<? extends Number>> list)
{
    System.out.println("It worked");
}

static void TestWildcardsMethod()
{
    List<Pair<Integer>> list = null;
    WildcardsMethod(list);
}

I first look at the deepest wildcard and bound in WildcardMethod's signature. It is looking for Pair<? extends Number>. Therefore, I could use Pair<Integer>, Pair<Double> and so on. Now I have something which looks like the below code in my mind if I decided to substitute Pair<Integer> for Pair<? extends Number>:

List<? extends Pair<Integer>>

Now, the wildcard represents a type/subtype of the parametrized type Pair<Integer>. Therefore, I can either pass a Pair<Integer> or SubPair<Integer> to WildcardsMethod.

Example 2 (makes sense):

static void WildcardsMethod(List<? extends Pair<? super Number>> list)
{
    System.out.println("It worked");
}

static void TestWildcardsMethod()
{
    List<Pair<Number>> list = null;
    WildcardsMethod(list);
}

I look and see that I first need a Pair<? super Number> so I decide to pass in Pair<Number> resulting in the following code:

? extends Pair<Number>

I then look at the leftmost wildcard and see that I can use either Pair<Number> or SubPair<Number>. I end up passing List<Pair<Number>>.

So in other words, I see the deepest wildcard as asking for a subtype or supertype of the innermost bound (Number). I then go to the top level wildcard and look for a subtype/supertype of the generic type (Pair).

Example 3 (doesn't make sense):

static void WildcardsMethod(List<? super Pair<? super Number>> list)
{
    System.out.println("It worked");
}

static void TestWildcardsMethod()
{
    List<Pair<Object>> list = null;
    WildcardsMethod(list);
}

Well, in terms of Pair<? super Number>, Object is definitely a supertype of Number so Pair<Object> should work just as it did for the previous examples. The following is what I think of when trying to understand this:

? super Pair<Object>

So I am limited to either Pair<Object> or SuperPair<Object>. However, none of this works.

Example 4 (doesn't make sense):

static void WildcardsMethod(List<? super Pair<? extends Number>> list)
{
    System.out.println("It worked");
}

static void TestWildcardsMethod()
{
    List<Pair<Integer>> list = null;
    WildcardsMethod(list);
}

It's the same thing here. Pair<Integer> belongs to the family of Pair<? extends Number> resulting in the following:

? super Pair<Integer>

I can then pass in either Pair<Integer> or SuperPair<Integer> However, this too does not work.

So I am either thinking of this wrong and somehow that model works for extends but not for super or I am simply missing something about lowerbounds and nested wildcards.

回答1:

Example 1:

Is List<Pair<Integer>> a subtype of List<? extends Pair<? extends Number>>?
It would be if Pair<Integer> is a subtype of Pair<? extends Number>. Is it?
Yes, because Integer is a subtype of Number.

Example 2:

Is List<Pair<Number>> a subtype of List<? extends Pair<? super Number>>?
It would be if Pair<Number> is a subtype of Pair<? super Number>. Is it?
Yes, because Number is a supertype of Number.

Example 3:

Is List<Pair<Object>> a subtype of List<? super Pair<? super Number>>?
It would be if Pair<Object> is a supertype of Pair<? super Number>. Is it?
No, it is not. A parameterized type with a specific parameter can never be a supertype of a parameterized type with a wildcard.

Example 4:

Is List<Pair<Integer>> a subtype of List<? super Pair<? extends Number>>?
It would be if Pair<Integer> is a supertype of Pair<? extends Number>. Is it?
No, it is not. A parameterized type with a specific parameter can never be a supertype of a parameterized type with a wildcard.

来源：https://stackoverflow.com/questions/31889837/nested-wildcards-with-lower-bounds

标签

java

generics

wildcard