Collection Framework in Java

The Collection Framework is a set of classes and interfaces

• Helps in handling groups of objects
• Standardizes the way in which groups of objects are handled

• The important interfaces of the Collection Framework are
  • Collection
  • List
  • Queue
  • Set
  • SortedSet
• All these interfaces are generic interfaces
• Collection interface declares the methods that any Collection should have
• Any class that defines a Collection should implement this interface
• Some of the important methods are
  • add
  • remove
  • contains
  • isEmpty

List

• List interface extends Collection interface
• List interface declares the behavior of a Collection that stores a sequence of elements
• Elements can be inserted and accessed by their position
• Some of the important methods are
  • add (adds to the specified position)
  • get (gets from the specified position)
  • indexOf

Set

• Set interface extends Collection interface
• Set interface declares the behavior of a Collection that does not allow duplicate elements
  • Does not declare any new method on its own
  • The add method returns false if a program attempts to insert a duplicate element
• SortedSet interface extends Set interface
• SortedSet interface declares the behavior of a Set sorted in the ascending order
• Some of the important methods are
  • first
  • last
• The Collection Framework provides some classes that have already implemented the Collection interfaces
• Some important Collection classes are
  • ArrayList
  • LinkedList
  • HashSet
  • LinkedHashSet
• Moreover, programmers can create Collection classes by implementing the collection interfaces or extending the collection classes

 

Iterator

• An Iterator object is used to access the elements of a Collection, one element at a time
• Iterator standardizes the way in which elements are accessed from a Collection
  • Invoke the method iterator() of the collection object that returns an Iterator
  • Invoke the method next() of the iterator object that returns the next element
  • Invoke the method hasNext() of the iterator object to check whether there are more elements in the collection object

public void printAll(Collection<String> collection){
Iterator<String> iterator = collection.iterator();
while(iterator.hasNext()){
out.println(iterator.next());
}
}

 

• Collections that implement List has a method listIterator() that returns a ListIterator object
• ListIterator extends Iterator and has extra methods
  • previous
  • hasPrevious
  • nextIndex
  • previousIndex

 

Enhanced For Loop

The for-each version of the for loop can also be used to access the elements of a collection

for(String element : collection){
                System.out.println(element);
}

Map

• A map stores associations between keys and values
  • Keys and Values are Objects
  • Given a key, its value can be found
  • Keys must be unique, values can be repeated
• A class that defines the behavior of a map should implement the Map interface
• Some of the important methods are
  • get
  • put
• A Map is not a collection
  • An iterator cannot be obtained for a Map
  • The for-each loop cannot be used with a Map
• A collection-view of a Map can be obtained
  • The method values return a Collection view of a Map
• SortedMap interface extends Map interface
• The elements are maintained in the ascending order of the keys
• Java provides some classes that have already implemented the Map interfaces
• Some important Map classes are
  • HashMap
  • TreeMap
  • LinkedHashMap
• The Collection Framework defines several algorithms that can be applied to collections and maps
• These algorithms are defined as static methods in a class called Collections
  • binarySearch
  • reverse
  • sort etc..