---
title: Collections
---
# Collections

A Collection in Java is a group of objects which can be ordered(LinkedList) or unordered(Set). The Collection interface is at the top of the hierarchy and all other classes and interfaces extend from this interface. It is located in the java.util package.

The Collection interface also extends Iterable interface, which means that every collection in java must be iterable. This in turn means that a for-each loop can be used to fetch them in a sequence.

```java
public interface Collection<E> extends Iterable<E>
```

Some of the most common methods provided by this interface are:

```java
boolean add(E e) // Adds the specified element to the collection if not present and returns true if this collection changed.

void clear() // Removes all the elements from the collection.

boolean contains(Object o) // Returns true if the specified element is in the collection else false

boolean isEmpty() // Returns true if the collection is empty else false

boolean remove(Object o) // Removes the specifies element and return true on successful removal else false.

int size() // Returns number of items in the collection.
```

These and various other methods have to be implemented by any class implementing Collection interface.


## Interfaces extending Collection interface

Other important interfaces extending the collection interface are:

Set:
	A collection containing only unique elements.

Queue:
	Implement the queue behaviour where elements are added only in the beginning and removed from the end.

List:
	This collection handles a list/sequence of object.

These four interfaces (Collection, Set, Queue, List) along with SortedSet, Deque and NavigableSet form the collective Collection hierarchy.

# The LinkedList class

LinkedList is one the most important Collection classes which provides a doubly-linked list implementation. It implements the List, Deque, Cloneable and Serializable interfaces.

**Create a LinkedList**

```java
LinkedList<Integer> intList = new LinkedList<Integer>(); // Creates a new list of Integer objects.
```

You can also create a list of any other object type. For eg.

```java
LinkedList<String> stringList = new LinkedList();

LinkedList<LinkedList<Integer>> listOfList = new LinkedList();
```

Note: All collections in Java have been converted to generic types since JDK 1.5

**Add elements to the list**

```java
intList.add(new Integer(1)); // Add 1 to the end.

intList.add(2); // This works as Java provides autoboxing and unboxing of primitive datatypes and their respective wrapper classes

intList.addFirst(3); // Add to the beginning of the list

intList.addLast(2); // Add to the end of the list

intList.add(2, 5); // Add element 5 at index 2
```

Let us print the list

```java
System.out.println(intList); // toString() method is automatically called on the list
```

Output:
[3, 1, 5, 2, 2]


**Retrieve elements from the list**
```java
intList.get(3); // Returns element at index 3 i.e. 2

intList.getFirst(); // Get the first element i.e. 3

intList.getLast(); // Returns last element i.e. 2

intList.indexOf(2); // Returns first occured index of 2 i.e. 3

intList.lastIndexOf(2); // Returns last occured index of 2 i.e. 4
```

**LinkedList as a Stack**

Since Java does not provide a separate 
```java
intList.push(5); // Add element to the end of list. Works same as addLast()

intList.pop(); // Removes and returns the last element of the list.
```

**Remove elements from the list**
```java
intList.remove(3); // Removes the element at index 3 of the list

intList.removeFirst(); // Removes first element of the list

intList.removeLast(); // Removes last element of the list
```

Note: All the above mentioned methods for removing and fetching an element return NoSuchElementException on an empty list.

#### More Information:
* Source: <a href='https://docs.oracle.com/javase/9/docs/api/overview-summary.html' target='_blank' rel='nofollow'>Java Documentation</a>