Transcript Implementation of Java Collections (cont.)

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
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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
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