Question

Possible Duplicate:
Why is List<Number> not a sub-type of List<Object>?

Isn’t String a subtype of Object in Java?

Then, how come I cannot pass an object of type List<String> into a function that accepts List<Object> as a parameter? I can, though, pass such an object into a function that accepts List as a parameter.

Answer 1

Isn’t String a subtype of Object in Java?

Yes it is, but that doesn’t make List<String> a subtype of List<Object>.

Consider this example:

    List<String> l1 = new ArrayList<String>();
    List<Object> l2 = l1;  // This is a compilation error in real Java
    l2.add(new Integer(42));
    String oops = l1.get(0);

If (hypothetically) List<String> was subtype of List<Object>, then you’d end up assigning a Integer to a String variable in the last statement. In short, we would have broken static type safety.

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I can, though, pass such an object into a function that accepts List as a parameter.

Yes, that is true. But then you are dealing with raw types and you have to use explicit type casts when pulling objects out of the list. For example, the above would need to be rewritten as follows:

    List<String> l1 = new ArrayList<String>();
    List l2 = l1;
    l2.add(new Integer(42));
    String oops = (String) l1.get(0);

Note that the type-cast means we’ve had to replace static type-safety with runtime type-safety. (And, of course, in this case the typecast will fail at runtime because the instance has the wrong type.)