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Java Collection framework: On concrete collections and their

implementation

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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

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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

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LinkedList: doubly-linked list (cont.)

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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)

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Removing an element from a linked list

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Adding an element to a linked list

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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

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Removing an element from an array

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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

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Relationships between framework classes

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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 ();

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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>

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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

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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

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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

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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)

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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

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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

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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

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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