Hi All
Its been a long time since I have post on my blog. There were many thoughts going on on what I need to publish. Should I go for Java or Tech stuff or something new. After having few thoughts, I decided to stick to Java. However, this time, I am planning to post more on Clearcase, Unix and How big organization plans and executes their program.
Let me start with Java: A simple question which can confuse many experienced persons. It goes like this.
What is Difference between Set, List and Map in Java? [this post will be followed by Comparator or Comparable]
Set, List and Map are three important Interface of Java collection framework. What are the key differences between Set, List and Map in Java is one of the most frequently asked question for Java Collection.
Some time this question is asked as When to use List, Set and Map in Java. Clearly, interviewer is looking to know that whether you are familiar with fundamentals of Java collection framework or not. Please mind that having understanding of any Framework plays a crucial role.
In order to decide when to use List, Set or Map , you need to know what are these interfaces and what functionality they provide. Lets have a quick look.
1 : (Duplicate) List in Java provides ordered and indexed collection which may contain duplicates. Example: LinkedList and ArrayList are two most popular used List implementation
2: (Unique) Set provides an unordered collection of unique objects, i.e. Set doesn’t allow duplicates. Example: LinkedHashSet, TreeSet and HashSet are frequently used Set implementation.
3: (Duplicate Value – Unique Keys) Map provides a data structure based on key value pair and hashing.
As I said Set, List and Map are interfaces, which defines core contract e.g. a Set contract says that it can not contain duplicates. Based upon our knowledge of List, Set and Map let’s compare them on different metrics.
Duplicate Objects
Main
difference between List and Set interface in Java is that List
allows duplicates while Set doesn’t allow duplicates. All implementation of Set honor this contract. Map holds two object per Entry e.g. key and value and It may contain duplicate values but keys are always unique.
Order
Another key difference between List and Set is that List is an
ordered collection, List’s contract maintains insertion order orelement. Set is an
unordered collection, you get no guarantee on which order element will be stored. Though some of the Set implementation e.g. LinkedHashSet maintains order. Also SortedSet and SortedMap e.g.
TreeSet and TreeMap maintains a sorting order, imposed by using
Comparator or Comparable.
Null elements
List allows null elements and you can have many null objects in a List, because it also allowed duplicates. Set just allow one null element as there is no duplicate permitted while in Map you can have null values and at most one null key. worth noting is that
Hashtable doesn’t allow null key or values but HashMap allows null values and one null keys. This is also the main difference between these two popular implementation of Map interface, aka
HashMap vs Hashtable
Popular implementation
List – ArrayList, LinkedList and Vector
Set – HashSet, TreeSet and LinkedHashSet
Map – HashMap, Hashtable and TreeMap
When to use List, Set and Map in Java
Based upon our understanding of difference between Set, List and Map we can now decide when to use List, Set or Map in Java.
1) If you need to access elements frequently by using
index, than List is a way to go. Its implementation e.g.
ArrayList provides faster access if you know index.
2) If you want to store elements and want them to maintain an
order on which they are inserted into collection then go for List again, as
List is an ordered collection and maintain insertion order.
3) If you want to create collection of unique elements and
don’t want any duplicate than choose any Set implementation e.g.
HashSet, LinkedHashSet or TreeSet. All Set implementation follow there general contract e.g. uniqueness but also add addition feature e.g. TreeSet is a SortedSet and elements stored on TreeSet can be sorted by using
Comparator or Comparable in Java. LinkedHashSet also maintains insertion order.
4) If you store data in form of key and value than Map is the way to go. You can choose from Hashtable, HashMap, TreeMap based upon your subsequent need. In order to choose between first two see
difference between HashSet and HashMap in Java.
That’s all on
difference between Set, List and Map in Java. All three are most fundamental interface of Java Collection framework and any Java developer should know there distinguish feature and given a situation should be able to pick right Collection class to use. It’s also good to remember difference between there implementation e.g.
When to use ArrayList and LinkedList ,
HashMap vs Hashtable or
When to use Vector or ArrayList etc. Collection API is huge and it’s difficult to know every bits and piece but at same time there is no excuse for not knowing fundamentals like Difference between Set, List and Map in Java
Filed under: Java | Tagged: Collection API, Hashtable and TreeMap, LinkedList and Vector Set - HashSet, List, List - ArrayList, Map, Set, TreeSet and LinkedHashSet Map - HashMap | Leave a comment »