Why isn"t Collection.remove(Object o) generic?
Collection<E> could have
boolean remove(E o);
Then, when you accidentally try to remove (for example)
Set<String> instead of each individual String from a
Collection<String>, it would be a compile time error instead of a debugging problem later.
Map as well as in
Collection) is not generic because you should be able to pass in any type of object to
remove(). The object removed does not have to be the same type as the object that you pass in to
remove(); it only requires that they be equal. From the specification of
remove(o) removes the object
e such that
(o==null ? e==null : o.equals(e)) is true. Note that there is nothing requiring o and e to be the same type. This follows from the fact that the
equals()method takes in an Object as parameter, not just the same type as the object.
Although it may be commonly true that many classes have
equals() defined so that its objects can only be equal to objects of its own class, that is certainly not always the case. For example, the specification for List.equals() says that two List objects are equal if they are both Lists and have the same contents, even if they are different implementations of List. So coming back to the example in this question, it is possible to have a
Map<ArrayList, Something> and for me to call
remove() with a
LinkedList as argument, and it should remove the key which is a list with the same contents. This would not be possible if
remove() were generic and restricted its argument type.