When to use HashMap over LinkedList or ArrayList and vice-versa

Question

What is the reason why we cannot always use a HashMap  even though it is much more efficient than ArrayList or LinkedList in add remove operations  also irrespective of the number of the elements   I googled it and found some reasons  but there was always a workaround for using HashMap  with advantages still alive

User · Answer

Lists and Maps are different data structures. Maps are used for when you want to associate a key with a value and Lists are an ordered collection.

Map is an interface in the Java Collection Framework and a HashMap is one implementation of the Map interface. HashMap are efficient for locating a value based on a key and inserting and deleting values based on a key. The entries of a HashMap are not ordered.

ArrayList and LinkedList are an implementation of the List interface. LinkedList provides sequential access and is generally more efficient at inserting and deleting elements in the list, however, it is it less efficient at accessing elements in a list. ArrayList provides random access and is more efficient at accessing elements but is generally slower at inserting and deleting elements.

User · Answer

I will put here some real case examples and scenarios when to use one or another  it might be of help for somebody else    HashMap  When you have to use cache in your application  Redis and membase are some type of extended HashMap   Doesn t matter the order of the elements  you need quick   O 1    read access  a value   using a key     LinkedList  When the order is important  they are ordered as they were added to the LinkedList   the number of elements are unknown  don t waste memory allocation  and you require quick insertion time   O 1     A list of to-do items that can be listed sequentially as they are added is a good example

User · Answer

Lists represent a sequential ordering of elements  Maps are used to represent a collection of key   value pairs   While you could use a map as a list  there are some definite downsides of doing so   Maintaining order  - A list by definition is ordered    You add items and then you are able to iterate back through the list in the order that you inserted the items     When you add items to a HashMap  you are not guaranteed to retrieve the items in the same order you put them in    There are subclasses of HashMap like LinkedHashMap that will maintain the order  but in general order is not guaranteed with a Map   Key Value semantics  - The purpose of a map is to store items based on a key that can be used to retrieve the item at a later point     Similar functionality can only be achieved with a list in the limited case where the key happens to be the position in the list    Code readability Consider the following examples             Adding to a List     list add myObject              adds to the end of the list     map put myKey  myObject        sure  you can do this  but what is myKey      map put  1   myObject          you could use the position as a key but why          Iterating through the items     for  Object o   myList               nice and easy     for  Object o   myMap values         more code and the order is not guaranteed   Collection functionality Some great utility functions are available for lists via the Collections class    For example              Randomize the list     Collections shuffle myList           Sort the list     Collections sort myList  myComparator       Hope this helps

User · Answer

The downfall of ArrayList and LinkedList is that when iterating through them  depending on the search algorithm  the time it takes to find an item grows with the size of the list   The beauty of hashing is that although you sacrifice some extra time searching for the element  the time taken does not grow with the size of the map  This is because the HashMap finds information by converting the element you are searching for  directly into the index  so it can make the jump   Long story short    LinkedList  Consumes a little more memory than ArrayList  low cost for insertions add  amp  remove  ArrayList  Consumes low memory  but similar to LinkedList  and takes extra time to search when large  HashMap  Can perform a jump to the value  making the search time constant for large maps  Consumes more memory and takes longer to find the value than small lists

[java] When to use HashMap over LinkedList or ArrayList and vice-versa

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