Practice: Which Generic Assignments the Compiler Refuses

Six lines with plain types and wildcards: decide which compile, and why.

A wildcard type, one written with ? for an unnamed type, lets you read but restricts adding, and a list of a subtype is not a list of its supertype. Apply those rules to the six lines below and decide which compile, all six before opening the answer.

java

List<String> strings = new ArrayList<>();

// 1
List<Object> objects = strings;
// 2
List<?> anything = strings;
// 3
anything.add("more");
// 4
Object first = anything.get(0);
// 5
List<? extends Number> numbers = new ArrayList<Integer>();
// 6
numbers.add(1);

Which of the six compile?

Each line, whether it compiles, and why
	Compiles?	Why
1	No	A list of a subtype is not a list of the supertype, because adding anything would break it
2	Yes	A wildcard accepts a list of anything
3	No	Nothing is known about the element type, so nothing is safe to add
4	Yes	Every element is at least an `Object`, so reading one is safe
5	Yes	`Integer` is a subtype of `Number`, which is what `extends` accepts
6	No	The list might hold some other subtype of `Number`, so adding is refused

Lines three and four together are the whole meaning of a wildcard. You may take things out as Object and you may not put anything in, because the compiler knows the list has some element type and not which one.

Line six is the one people argue with. numbers might point at a List<Double>, and adding an Integer to it would be exactly the corruption generics exist to prevent.

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