Collections Framework

  • The java collections Framework defines several classes and interfaces which can be used to represent a group of individual object as a single entity.
  • These classes and interface implement the collection data structures.
  • For Example : list, stack, queue or maps.
  • The collection framework was designed and developed primarily by Joshua Bloch
  • It was introduced in JDK 1.2
  • Why to use these collections?
    • We don’t have to implement every algorithm and data structure from the scratch, its already written into java.
    • It is already been tested.
  • Almost all Collections are derived from the java.utils.Collection interface
  • The main advantage of Collections over array is that collections are grow-able in size, i.e we need not know the size of elements in advance.
  • toArray() method can transform any collection into one dimensional array.
  • The Collection interface extends the java.lang.Iterable interface, this is why we can use for-each loop.

We write for each loop like below example :

for(String fruits : listOfFruits){

After compilation of .java file, .class file contains below code :

for (Iterator<String> i = listOfFruits.iterator(); i.hasNext();) {
   String fruits =;
Fixed in SizeGrow-able in size
Can hold primitive, non primitive and object data type.Can hold only non primitive and object data type.
No underlying Data structure, hence no ready-made method supportImplemented base on standard data structure hence for every collection ready-made collection support is available.

Collection Interface

  • Collection interface defines the most common methods which are applicable for any Collection object.
  • In general Collection interface is considered as root interface of collection framework.
  • Collection Interface extends Iterable Interface.
  • There is no concrete class which implements collection interface directly.
Collection InterfaceCollections Class
If you want to represent a group of individual objects we should use collection InterfaceCollections is a utility class (java.utils.package) which defines several utility methods like sorting,searching for collection object.

Methods of Collection Interface :

boolean add(E e);
boolean addAll(Collection<? extends E> c);
boolean remove(Object o);
boolean removeAll(Collection<?> c);
void clear();
boolean retainAll(Collection<?> c);
boolean contains(Object o);
boolean containsAll(Collection<?> c);

int size();
boolean isEmpty();
Iterator<E> iterator();
Object[] toArray();

boolean equals(Object o);
int hashCode();

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