---
title: Collections
---
# Collections

A Collection in Java is a group of objects which can be ordered (List) 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 the `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 elements from a collection 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 objects.

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:

```java
[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**

You can use a `LinkedList` to implement the `Stack` data structure:
```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
- [Java Collection](https://docs.oracle.com/javase/8/docs/api/java/util/Collection.html)