Unsound Java

unsound.java

class Test {
  interface Supplier<T> { List<T> get(); }
 
  public static void main(String[] args) {
    Supplier<String> strs = supplier("foo", "bar");
    Supplier<Integer> ints = supplier(1, 2);
    mergeSuppliers(Arrays.asList(strs, ints));
    String s = strs.get().get(2); // boom!
  }
 
  private static void mergeSuppliers(List<Supplier<?>> suppliers) {
    mergeLists(suppliers.stream().map(s -> s.get())); // shouldn't type-check, but does
  }
 
  private static<T> void mergeLists(Stream<List<T>> listStream) {
    List<List<T>> lists = listStream.collect(Collectors.toList());
    if (lists.size() >= 2) {
      lists.get(0).addAll(lists.get(1));
    }
  }
 
  private static<T> Supplier<T> supplier(T ... elements) {
    List<T> model = new ArrayList<>(Arrays.asList(elements));
    return () -> model;
  }
}