• Write the object invariant for a singly-linked list with a head pointer, a tail pointer and a count of items.

• Given the following definition for a doubly-linked list collection class without cycles or dummy nodes:

  class StringList extends AbstractCollection { private class Node { public String data; public Node prev, next; public Node(String d, Node p, Node n) { data = d; prev = p; next = n; } } private Node head, tail; private int numItems; private int version; private class MyIterator implements Iterator { private Node nextNode = head; private boolean canRemove = false; private int myVersion = version; public String next() { ... // error checking (not shown) String result = nextNode.data; nextNode = nextNode.next; canRemove = true; return result; } public void remove() { // TODO } } } 

  Write the remove method without using any other methods. Ignore invariants. Don't forget to implement fail-fast semantics.