`HashMap` is not synchronized and allows one `null` key and multiple `null` values, whereas `HashTable` is synchronized (thread-safe) and doesn't allow `null` keys or values.
The `Iterator` interface provides a way to iterate over a collection of objects, allowing removal of elements during iteration. It has methods `hasNext()`, `next()`, and `remove()`. It's part of Java's `java.util` package.
`Set` is a collection that does not allow duplicate elements and is unordered. `List` allows duplicates and maintains the order of elements. Examples of `Set` include `HashSet` and `TreeSet`, while `List` includes `ArrayList` and `LinkedList`.
ArrayList is based on a dynamic array and allows fast random access but slow insertion and deletion (except at the end). LinkedList is based on a doubly linked list, offering fast insertions and deletions, but slower random access.
The `Comparator` interface provides a method to define custom ordering of objects. It can be used to sort collections of objects based on specific attributes by overriding the `compare()` method.
