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Java Fundamentals - Collections - Defining

Collection (Interface)


   --> List
Have an order, index


  -->Set
Have only one element without be duplicated

-->SortedSet
Have unique elements in an order.
You can´t add element in any position


  -->Queue
Has an order: FIFO Fisrt in First Out


  -->Deque
Has a doble ending queue: FIFO & LIFO


Key Collections

Both are collections in pairs: Keys/Values

  -->Map

  -->SortedMap
Can iterate over the Map and see the values in certain order


Implementation

 -->List
     -->ArrayList

getElement - ok
add - Fail
contains - fail
next - ok
remove - fail


     -->LinkedList

getElement - fail
add - ok
contains - fail
next - ok
remove - ok

  -->Set
      -->HashSet

  -->SortedSet
     -->TreeSet

  -->Queue
     -->PriorityQueue


In this case you need to create a Comparator in order to create the Priority.

Before JAVA 8

private static final Comparator<MY_CLASS> COMPARATOR = new Comparator<MY_CLASS>(){
     public int compare (final MY_CLASS o1, final MY_CLASS o2){
                     return o1.getAtribute.compareTo( o2.getAtribute() );
     }
}

In JAVA 8

private static final Comparator<MY_CLASS> COMPARATOR = comparing( MYCLASS::getAtribute );


 PriorityQueue
private Queue<MY_CLASS> enquiries = new PriorityQueue<>(COMPARATOR);


  -->Deque
     -->LinkedList
    -->ArrayDeque

Return false if the queue is full 
boolean offerFirst(E e)
boolean offerLast(E e)

 Throws an exception if the queue is full
void addFirst(E e)
void addLast(E e)

Returns null
E removeFirst()
E removeLast()

Throws exception when empty
E removeFirst()
E removeLast()

Returns null
E peekFirst()
E peekLast()

Throws exception when empty
E getFirst()
E getLast()

 Commands to work as LIFO
void push(E e)
E pop()



  -->Map
    -->HashMap

keySet()
Return all the keys from the Map

final Map<Integer, CLASS> map = new HashMap<>();
final Set<Integer> keys = map.keySet();


values()
Return all the values from the map
final Collection<CLASS> values = map .values();


entrySet()
To iterate over a Map

 final Set<Map.Entry<Integer, CLASS>> entries = map.entrySet();
        for(Map.Entry<Integer, CLASS> entry : entries)
        {
            System.out.println(entry);
            System.out.println(entry.getKey() );
            System.out.println(entry.getValue() );
        }

JAVA 8
map.forEach((key, value) -> System.out.println(key + " -> " + value));



  -->SortedMap (Interface)
     -->TreeMap

K firstKey()
K lastKey()

SortedMap<K,V> tailMap( E fromKey)

SortedMap<K,V> headMap( E toKey)

SortedMap<K,V> subMap( E fromKey, E toKey)


  -->NavigableMap (Java 6)

Map.Entry<K,V> firstEntry()
Map.Entry<K,V> lastEntry()

Return and get element

Map.Entry<K,V> pollFirstEntry()
Map.Entry<K,V> pollLastEntry()

Map.Entry<K,V> floorEntry(K key)
Map.Entry<K,V> ceilingEntry(K key)

Get the next element (floorKey, ceilingKey)

K floorKey(K key)
K ceilingKey(K key)

By default the order is ascending
NavigableMap<K, V> descendingMap()
NavigableSet<K> descendingKeySet()

To iterate a through the keys
NavigableSet<K> navigableKeySet()

NavigableMap<K, V> tailMap( E fromKey, boolean includeElement)
NavigableMap<K, V> headMap( E toKey, boolean includeElement)

NavigableMap<K, V> subMap( E fromKey, boolean includeElement, E toKey, boolean includeElement)

   -->EnumSet



NEW METHODS - JAVA 8


getOrDefault
putIfAbsent
compute
computeIfPresent -  Review if an element exist otherwise it added

hashMap.computeIfPresent( KEY_TO_SEARCH,  (id) -> new MY_CLASS(id, ATTRIBUTE)

computeIfAbsent
merge - combine the key value, the value and the age

forEach

replaceAll -> Use lambda expressions
hashMap.replaceAll(  
    (key, value)  -> new Object( key,  object.getAttribute1(),  object.getAttribute2() + 10 )
);


Other collections

  • TreeMap
  • LinkedHashMap
  • WeakHashMap
    • Used as a cache
  • EnumMap
    • Store less than 64 elements


Collection Algorithms


Collections.rotate( LIST ,  #elementsToRotate )

Collections.shuffle( LIST ) 

Collections sort( My_Comparable )
In java 8 List contains sort method included


Collection Factories

Singleton
Set< Integer> set = Collections.singleton(1);
List<String> list = Collections.singletonList("one");
Map<Integer, String> amp  = Collections.singletonMap(1 , "a");

Empty Collections
Set< Integer> set = Collections.emptySet();
List<String> list = Collections.emptyList();
Map<Integer, String> amp  = Collections.emptyMap();


Collections.unmodifiableList( MY_LIST)
You can't change the list or a exception will be throw.

Collection Utilities

Collections.addAll( MY_LIST, element1, element2, ...);
Collections.min( MY_LIST, MY_COMPARATOR );
Collections.max( MY_LIST, MY_COMPARATOR );



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