Why java lets me add a Box(Raw type) to a List<Box<? extends Item>>

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This code works, and I dont understand how the list is able to add a raw type box, I thought type erasure will set the type to their indicated bounds. note: Paper class doesn't extends Bakery class.

EDIT: Is my understanding correct? the compiler typecasts the raw box type so that it can be added to the list? does this works only at compile time? so if I tried to get the value at runtime it will throw an exception?

class Violator {

    public static List<Box<? extends Bakery>> defraud() {
        List<Box<? extends Bakery>> list = new ArrayList<>();
        Paper paper = new Paper();
        Box box = new Box<>();
        box.put(paper);
        list.add(box);
        return list;
    }
}
class Box<T> {
    void put(T item) { /* implementation omitted */ }
    T get() { /* implementation omitted */ }
}
type-erasure generics java
1 Answers

This code does generate an unchecked conversion warning, as to why its permitted, from Angelika Langer's blog

Why are raw types permitted?

To facilitate interfacing with non-generic (legacy) code. Raw types are permitted in the language predominantly to facilitate interfacing with non-generic (legacy) code.

If, for instance, you have a non-generic legacy method that takes a List as an argument, you can pass a parameterized type such as List<String> to that method. Conversely, if you have a method that returns a List , you can assign the result to a reference variable of type List<String> , provided you know for some reason that the returned list really is a list of strings.

Your answer lies in Conversely, if you have a method that returns a List , you can assign the result to a reference variable of type List<String>. In your case you might have a legacy method that returns a Box

 // Legacy code
 private Box getBox(){
     // returns a Box which satisfy the Box<? extends Bakery>
 }

even though the returned result might satisfy the constraint Box<? extends Bakery>, there is no way to be completely sure, so you are allowed to add that box to your list

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