HashMap
vs EnumMap in Java
What is difference between EnumMap and HashMap in Java is the latest
Java collection interview question which has been asked to couple of my
friends. This is one of the tricky Java question, specially if you are not very much familiar with EnumMap
in Java, which is not uncommon, given you can use it with only Enum
keys. Main difference between EnumMap
and HashMap is that EnumMap is a specialized Map implementation exclusively
for Enum as key. Using Enum as key, allows to do some implementation level
optimization for high performance which is generally not possible with other
object's as key. We have seen lot of interview
questions on HashMap in our article How
HashMap works in Java but what we missed there is this question which is
recently asked to some of my friend. Unlike HashMap, EnumMap is not applicable
for every case but its best suited when you have Enum as key. We have already
covered basics of EnumMap and some EnumMap example in my last article What
is EnumMap in Java and In this post we will focus on key differences
between HashMap and EnumMap in Java.
EnumMap vs HashMap
Before looking differences between EnumMap and HashMap, few words
about What is common between them. Both of them implements Map interface so they can be used in all
methods which accept Map and data can be accessed using common Map
methods e.g. get() and put(). Internally EnumMap is
represented using Array and provides constant time performance for common
methods e.g. get() or put(). Now let's
see few differences between EnumMap vs HashMap :
1) As said earlier, first and foremost difference between EnumMap and HashMap is that,
EnumMap is optimized for enum keys while HashMap is a
general purpose Map implementation similar to Hashtable.
you can not use any type other than Enum
as key in EnumMap but you can use both Enum and any other Object as key in
HashMap.
2) Another difference between EnumMap and HashMap is performance.
as discussed in previous point, due to specialized optimization done for Enum
keys, EnumMap is likely to perform better than HashMap when using
enum as key object.
3) One more thing which can be considered as difference between HashMap
and EnumMap is probability of Collision. Since Enum is internally
maintain as array and they are stored in there natural order using ordinal(), as shown
in following code which is taken from put() method of
EnumMap
int index = ((Enum)key).ordinal();
Object oldValue = vals[index];
vals[index] = maskNull(value);
Since EnumMap doesn't call hashCode
method on keys, there is no chance of collision.
These were some notable difference between EnumMap and HashMap in Java.
In short EnumMap is best suited for enum keys, for which it has optimized and perform better than HashMap in Java. Use EnumMap whenever you can use enum as
keys.
Other Interview Questions from Javarevisited Blog
Source:http://javarevisited.blogspot.com/2012/09/difference-between-enummap-and-hashmap-in-java-vs.html
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