Collection Interface - Java Collections Framework - DSA

Most people consider the collection as the root interface of Collections Framework and it is true to a great extent but another part of Collections Framework is Map Interface, we will see that later, Most Common methods which are applicable to all collections are defined in this interface for example add() to add an element, size() to get the size and much more, below is a table of most common methods.

Hierarchey of the Collection Interface.

The Parent of Collection Interface is Iterator Interface and the Collection is base class for List Interface, Set Interface and Queue Interface, the respective classes which impliments either of the sub classes will also implement the defined methods from the Collection Interface, below are some of the commonly used methods.

Defined Methods:

Method	Description
add()	This method returns a Boolean value true if it inserts the specified element in this collection.
addAll()	This method returns a Boolean value true if all elements were added sucessfully.
clear()	Removes all the elements of the collection.
contains()	returns a Boolean value if the specified object exists true else false.
containsAll()	returns a Boolean value if the specified Objects from the argument of collection and if all the elements exist inside the collection then it returns true else false.
equals()	returns true if the specified Object is equal to this current Collection.
hashCode()	returns the hash code value of this collection.
isEmpty()	returns true if this collection has no elements otherwise returns false.
iterator()	returns an Iterator over the elements in proper sequence.
remove()	Removes the first element where the specified element is found, but only the 1st element of the specified object will be removed.
removeAll()	same like remove() but all objects matching will be removed from this collection.
removeIf()	It removes all the elements of this collection that satisfy the given predicate filter.
retainAll()	The Objects inside the arguments collection will be kept and all other Objects will be removed from this collection.
size()	This returns the number of objects present inside this collection.
spliterator()	It returns a spliterator over the elements in this collection.
toArray()	It returns an array containing all the elements of this collection which are in proper sequence.

Classes that implements Collection

ArrayList, LinkedList, Vector, Stack, PriorityQueue, ArrayDeque, TreeSet, HashSet, LinkedHashSet

Which means all the above methods in the table can be used from the objects of these classes which implements Collection interface, along with these diffrent collections have their own interfaces they implement depending on the underlaying collections.

Sample Code:


import java.util.*;
class TestJavaCollection1{
public static void main(String args[]){
ArrayList < String> list=new ArrayList < String>(); //Creating arraylist
list.add("Ravi");//Adding object in arraylist
list.add("Vijay");
list.add("Ravi");
list.add("Ajay");
//Traversing list through Iterator
Iterator itr=list.iterator();
while(itr.hasNext()){
System.out.println(itr.next());
}
}
}

//output
Ravi
Vijay
Ravi
Ajay