# Java Collection and Generics

## Arrays
Define a fixed length **Array**

    int a[] = new int[4];

If you know the exact size, go for array. It's faster

## Collection

 * Size is not fixed
 * Items can have duplicate values
 * The type here will be int Object type. You can add string, integer, etc...
 * Doesn't have index, use **List** instead of Collection if you need indexing.
 * You can traverse elements in sequence

 *Class Hierarchy:* Collection (Interface) -> List (Interface) -> ArrayList (Class)

    Collection values = new ArrayList();
     values.add(5);
     values.add(6);

Use **Generics** to restrict types on a collection
\<Integer> is a sample Generic, it can be string, integer or custom object.

    Collection<Integer> values = new ArrayList<>();

## Set
* Can only have unique values
* **HashSet** return elements in random order

Sample HashSet

    Set<Integer> numbers = new HashSet<>();

A **TreeSet** returns the element in sorted order.

## Map
* Map have key value pair
* Map is an interface

Sample HashMap

    Map<Integer, String> m = new HashMap<>();






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