Multiple wildcards on a generic methods makes Java compiler (and me!) very confused

Question

Let's first consider a simple scenario (see complete source on ideone.com):

import java.util.*;

public class TwoListsOfUnknowns {
    static void doNothing(List<?> list1, List<?> list2) { }

    public static void main(String[] args) {
        List<String> list1 = null;
        List<Integer> list2 = null;
        doNothing(list1, list2); // compiles fine!
    }
}

The two wildcards are unrelated, which is why you can call doNothing with a List<String> and a List<Integer>. In other words, the two ? can refer to entirely different types. Hence the following does not compile, which is to be expected (also on ideone.com):

import java.util.*;

public class TwoListsOfUnknowns2 {
    static void doSomethingIllegal(List<?> list1, List<?> list2) {
        list1.addAll(list2); // DOES NOT COMPILE!!!
            // The method addAll(Collection<? extends capture#1-of ?>)
            // in the type List<capture#1-of ?> is not applicable for
            // the arguments (List<capture#2-of ?>)
    }
}

So far so good, but here's where things start to get very confusing (as seen on ideone.com):

import java.util.*;

public class LOLUnknowns1 {
    static void probablyIllegal(List<List<?>> lol, List<?> list) {
        lol.add(list); // this compiles!! how come???
    }
}

The above code compiles for me in Eclipse and on sun-jdk-1.6.0.17 in ideone.com, but should it? Is it not possible that we have a List<List<Integer>> lol and a List<String> list, the analogous two unrelated wildcards situations from TwoListsOfUnknowns?

In fact the following slight modification towards that direction does not compile, which is to be expected (as seen on ideone.com):

import java.util.*;

public class LOLUnknowns2 {
    static void rightfullyIllegal(
            List<List<? extends Number>> lol, List<?> list) {

        lol.add(list); // DOES NOT COMPILE! As expected!!!
            // The method add(List<? extends Number>) in the type
            // List<List<? extends Number>> is not applicable for
            // the arguments (List<capture#1-of ?>)
    }
}

So it looks like the compiler is doing its job, but then we get this (as seen on ideone.com):

import java.util.*;

public class LOLUnknowns3 {
    static void probablyIllegalAgain(
            List<List<? extends Number>> lol, List<? extends Number> list) {

        lol.add(list); // compiles fine!!! how come???
    }
}

Again, we may have e.g. a List<List<Integer>> lol and a List<Float> list, so this shouldn't compile, right?

In fact, let's go back to the simpler LOLUnknowns1 (two unbounded wildcards) and try to see if we can in fact invoke probablyIllegal in any way. Let's try the "easy" case first and choose the same type for the two wildcards (as seen on ideone.com):

import java.util.*;

public class LOLUnknowns1a {
    static void probablyIllegal(List<List<?>> lol, List<?> list) {
        lol.add(list); // this compiles!! how come???
    }

    public static void main(String[] args) {
        List<List<String>> lol = null;
        List<String> list = null;
        probablyIllegal(lol, list); // DOES NOT COMPILE!!
            // The method probablyIllegal(List<List<?>>, List<?>)
            // in the type LOLUnknowns1a is not applicable for the
            // arguments (List<List<String>>, List<String>)
    }
}

This makes no sense! Here we aren't even trying to use two different types, and it doesn't compile! Making it a List<List<Integer>> lol and List<String> list also gives a similar compilation error! In fact, from my experimentation, the only way that the code compiles is if the first argument is an explicit null type (as seen on ideone.com):

import java.util.*;

public class LOLUnknowns1b {
    static void probablyIllegal(List<List<?>> lol, List<?> list) {
        lol.add(list); // this compiles!! how come???
    }

    public static void main(String[] args) {
        List<String> list = null;
        probablyIllegal(null, list); // compiles fine!
            // throws NullPointerException at run-time
    }
}

So the questions are, with regards to LOLUnknowns1, LOLUnknowns1a and LOLUnknowns1b:

• What types of arguments does probablyIllegal accept?
• Should lol.add(list); compile at all? Is it typesafe?
• Is this a compiler bug or am I misunderstanding the capture conversion rules for wildcards?

---

Appendix A: Double LOL?

In case anyone is curious, this compiles fine (as seen on ideone.com):

import java.util.*;

public class DoubleLOL {
    static void omg2xLOL(List<List<?>> lol1, List<List<?>> lol2) {
        // compiles just fine!!!
        lol1.addAll(lol2);
        lol2.addAll(lol1);
    }
}

---

Appendix B: Nested wildcards -- what do they really mean???

Further investigation indicates that perhaps multiple wildcards has nothing to do with the problem, but rather a nested wildcard is the source of the confusion.

import java.util.*;

public class IntoTheWild {

    public static void main(String[] args) {
        List<?> list = new ArrayList<String>(); // compiles fine!

        List<List<?>> lol = new ArrayList<List<String>>(); // DOES NOT COMPILE!!!
            // Type mismatch: cannot convert from
            // ArrayList<List<String>> to List<List<?>>
    }
}

So it looks perhaps a List<List<String>> is not a List<List<?>>. In fact, while any List<E> is a List<?>, it doesn't look like any List<List<E>> is a List<List<?>> (as seen on ideone.com):

import java.util.*;

public class IntoTheWild2 {
    static <E> List<?> makeItWild(List<E> list) {
        return list; // compiles fine!
    }
    static <E> List<List<?>> makeItWildLOL(List<List<E>> lol) {
        return lol;  // DOES NOT COMPILE!!!
            // Type mismatch: cannot convert from
            // List<List<E>> to List<List<?>>
    }
}

A new question arises, then: just what is a List<List<?>>?

Answer 1

• No argument with generics should be accepted. In the case of LOLUnknowns1b the null is accepted as if the first argument was typed as List. For example this does compile :

  List lol = null;
  List<String> list = null;
  probablyIllegal(lol, list);
  

• IMHO lol.add(list); shouldn't even compile but as lol.add() needs an argument of type List<?> and as list fits in List<?> it works.
  A strange example which make me think of this theory is :

  static void probablyIllegalAgain(List<List<? extends Number>> lol, List<? extends Integer> list) {
      lol.add(list); // compiles fine!!! how come???
  }
  

  lol.add() needs an argument of type List<? extends Number> and list is typed as List<? extends Integer>, it fits in. It won't work if it doesn't match. Same thing for the double LOL, and other nested wildcards, as long as the first capture matches the second one, everything is okay (and souldn't be).

• Again, I'm not sure but it does really seem like a bug.

• I'm glad to not be the only one to use lol variables all the time.

Resources :
http://www.angelikalanger.com, a FAQ about generics

EDITs :

1. Added comment about the Double Lol
2. And nested wildcards.

Answer 2

not an expert, but I think I can understand it.

let's change your example to something equivalent, but with more distinguishing types:

static void probablyIllegal(List<Class<?>> x, Class<?> y) {
    x.add(y); // this compiles!! how come???
}

let's change List to [] to be more illuminating:

static void probablyIllegal(Class<?>[] x, Class<?> y) {
    x.add(y); // this compiles!! how come???
}

now, x is not an array of some type of class. it is an array of any type of class. it can contain a Class<String> and a Class<Int>. this cannot be expressed with ordinary type parameter:

static<T> void probablyIllegal(Class<T>[] x  //homogeneous! not the same!

Class<?> is a super type of Class<T> for any T. If we think a type is a set of objects, set Class<?> is the union of all sets of Class<T> for all T. (does it include itselft? I dont know...)