Arraylist vs HashMap

Difference between Arraylist and HashMap?

• ArrayList implements List Interface while HashMap is an implementation of Map interface.
• Memory consumption is high in HashMap compared to the ArrayList.
• ArrayList maintains the insertion order while HashMap doesn't maintain any order. ArrayList stores the elements only as value and maintain internally the indexing for every element. While HashMap stores elements with key and value pair, i.e. two objects. So HashMap takes more memory comparatively.
• ArrayList allows duplicate elements but HashMap doesn't allow duplicate keys (It does allow duplicate values)
• In ArrayList, an element can be fetched easily by specifying the index of it. But in HashMap, the elements is fetched by its corresponding key. It means that the key must be remembered always.
•  In ArrayList, any number of null elements can be stored. While in HashMap, only one null key is allowed, but the values can be of any number.

ARRAYLIST	HASHMAP
The java ArrayList implements List Interface	The java HashMap is implements Map interface
ArrayList always maintain insertion order of the elements	HashMap does not maintain the insertion order
ArrayList only stores value or element	HashMap stores key and value pairs
ArrayList can contain duplicate elements	HashMap does not contain duplicate keys but contain duplicate values.
We can have any numbers of null elements in ArrayList	We can have only one null key and any number of null values in HashMap
ArrayList get() method always gives an O(1) performance	HashMap get()method can be O(1) in the best case and O(n) in the worst case