1. Java Collection framework: On concrete collections and their implementation

2. Concrete collections in Java library
ArrayList: indexed sequence that grows/shrinks dynamically
LinkedList: ordered sequence that allows efficient insertions and removal at any location
HashSet: unordered collection that rejects duplicates
TreeSet: sorted set
EnumSet: set of enumerated type values (implements Set)
LinkedHashSet: set that remembers the order in which elements were inserted
PriorityQueue: allows efficient removal of the smallest element
HashMap: stores key/value associations
TreeMap: map in which the keys are sorted
EnumMap: keys belong to an enumerated type (impl. Map)
LinkedHashMap: remembers the order in which added
WeakHashMap: keys and entries that can be reclaimed by the garbage collector if the keys are not used elsewhere
IdentityHashMap: keys that are compared by ==, not equals

3. LinkedList: doubly-linked list
LinkedList implements the List interface and extends the AbstractCollection class (indirectly through AbstractSequentialList)
offers inexpensive operations to add into and remove from the middle of a data structure
LinkedList also gives (expensive) implementations for using array indices: get (i), listIterator (i), etc.
LinkedList provides methods to get, remove and insert an element at the beginning and end
these extra (see API) operations support efficient stack, queue, or double-ended queue (deque) handling; implements the Queue interface

4. LinkedList: doubly-linked list (cont.)

5. LinkedList: doubly-linked list (cont.)
List <E> list = new LinkedList <E> (aCollection)
list.addFirst (obj)
list.addLast (obj)
obj = list.getFirst ()
obj = list.getLast ()
obj = list.removeFirst ()
obj = list.removeLast ()
list traversal is done by an implementation of ListIterator
a recursive list having itself as an element is not permitted (but not checked)

6. Removing an element from a linked list

7. Adding an element to a linked list

8. ArrayList: sequentially allocated list
the class ArrayList implements List
internally, contains an Object [ ] array that is automatically reallocated when needed
methods to manipulate the array that stores the elements: ensureCapacity (minCapacity), and trimToSize ()
gives efficient (O(1)) implementations for methods with index positions, such as get (i), set (i, obj), etc.
implements the marker interface RandomAccess
note that ArrayList does not provide operations for stack and queue handling (not a Queue)
element traversals are made by an implementation of ListIterator

9. Removing an element from an array

10. Implementation of Java Collections
uses object-oriented mechanisms and patterns to organize the services and the classes:
encapsulation, inheritance, polymorphism;
Template Method, Factory Method, Iterator, Strategy, Proxy, etc.
however, data structures are not organized into a single-root class library
some libraries handle maps uniformly as collections of pair elements
to support code reuse, the framework provides abstract classes, e.g., AbstractCollection
AbstractCollection implements Collection, leaving operations, such as iterator () and size (), abstract

11. Relationships between framework classes

12. Implementation of Java Collections (cont.)
the skeleton class either gives a method some default definition, say, throwing an exception, or implements it in terms of more basic operations
e.g., utility routines, such as toString () and addAll (aCollection), can be defined in terms of other operations:
public class AbstractCollection <E> implements Collection <E> {
. . .
public abstract Iterator<E> iterator ();

13. Implementation of Java Collections (cont.)
. . .
public boolean add (E obj) { // illegal by default
throw new UnsupportedOperationException (); }
// a hypothetical implementation:
public boolean contains (Object obj) {
for (E element : this) // calls iterator ()
if (element.equals (obj))
return true;
return false;
} // AbstractCollection <E>

14. Implementation of Java Collections (cont.)
the abstract classes are meant to be used by programmers wanting to extend the framework with new implementations of data structures
of course, must implement the missing abstract methods to make default ones to work
additionally, an implementation may override a ready-made skeleton method to make it legitimate, or to give it a more efficient implementation for a specific concrete data structure

15. Error Handling very thorough checks; uses unchecked exceptions
denied ops throw UnsupportedOperationException
unacceptable type throws ClassCastException
element constraint (e.g. range) violation throws IllegalArgumentException
an accessed empty collection or exhausted iterator throws NoSuchElementException
a null parameter (unless null is accepted as an element) throws NullPointerException
invalid element index (< 0 or >= size ()) throws IndexOutOfBoundsException
especially, constant views may throw UnsupportedOperationException

16. Iterator error handling
Iterator.remove () removes the last element visited by next ()
removal depends on the state of the iterator
throws UnsupportedOperationException if the remove operation is not supported, and
throws IllegalStateException if the next method has not yet been called
generally, the behavior of an Iterator is unspecified if the underlying collection is modified while the iteration is in progress in any way other than by calling the method remove

17. Iterator error handling (cont.)
however, ListIterator.remove is guaranteed to throw IllegalStateException if the list was structurally modified (i.e., remove or add have been called) since the last next or previous
ListIterator.add (anObject) throws
UnsupportedOperationException if the add method is not supported by this list iterator
ClassCastException (IllegalArgumentException) if the class (or some other aspect) of the specified element prevents it from being added to this list
note that adding does not depend on the state of the iterator (whether next or previous has been called)

18. Iterator error handling (cont.)
the method set (newElement) replaces the last element returned by last next or previous
the update is done "in place", i.e., without structurally modifying the data structure
however, the method set () itself throws IllegalStateException
if neither next nor previous have been called, or
if the list was structurally modified (remove or add have been called) since the last next or previous

19. Fail-Fast Feature
iterators generally support the so-called fail-fast feature
this means that if the collection is modified after an iterator is created, in any way except through the iterator's own remove or add methods, an exception is thrown
only one iterator may be both reading and changing a list, and multiple readers are allowed if they don't do any structural modifications (so, set (obj) is OK)
thus, in case of concurrent modification, the iterator fails quickly, rather than risking arbitrary behaviour at an undetermined time in the future

20. Fail-Fast Feature (cont.)
For example, consider the following code:
List<String> list = . . .;
ListIterator<String> iter1 = list.listIterator ();
ListIterator<String> iter2 = list.listIterator(list.size());
iter1.next ();
iter1.remove ();
iter2.next (); // throws exception
last call throws a ConcurrentModificationException since iter2 detects that the list was modified externally

21. Fail-Fast Feature: Summary
an iterator throws ConcurrentModificationException, if it founds that the data structure has been structurally modified by other iterators or by collection operations
simple rule: attach as many reader iterators to a collection as you like; alternatively, you can attach a single iterator that can both read and write
modification detection is achieved by a mutation count values per each iterator and the collection object
note that here "ConcurrentModificationException" doesn't mean to involve any multithreading