Why is HashMap faster than HashSet?
hashmap, hashset, java, performance
Solution
Performance:
If you look at the source code of HashSet (at least JDK 6, 7 and 8), it uses HashMap internally, so it basically does exactly what you are doing with sample code.
So, if you need a Set implementation, you use HashSet, if you need a Map - HashMap. Code using HashMap instead of HashSet will have exactly the same performance as using HashSet directly.
Choosing the right collection
Map - maps keys to values (associative array) - http://en.wikipedia.org/wiki/Associative_array.
Set - a collection that contains no duplicate elements - http://en.wikipedia.org/wiki/Set_(computer_science).
If the only thing you need your collection for is to check if an element is present in there - use Set. Your code will be cleaner and more understandable to others.
If you need to store some data for your elements - use Map.
Problem
I have been reading/researching the reason why `HashMap`is faster than `HashSet`. I am not quite understanding the following statements: `HashMap` is faster than `HashSet` because the values are associated to a unique key. In `HashSet`, member object is used for calculating hashcode value which can be same for two objects so `equals()` method is used to check for equality. If it returns `false`, that means the two objects are different. In `HashMap`, the hashcode value is calculated using the key object. The `HashMap` hashcode value is calculated using the key object. Here, the member object is used to calculate the hashcode, which can be the same for two objects, so `equals()` method is used to check for equality. If it returns `false`, that means the two objects are different. To conclude my question: I thought `HashMap` and `HashSet` calculate the hashcode in the same way. Why are they different? Can you provide a concrete example how `HashSet` and `HashMap` calculating the hashcode differently? I know what a "key object" is, but what does it mean by "member object"? `HashMap` can do the same things as `HashSet`, and faster. Why do we need `HashSet`? Example: ``` HashMap <Object1, Boolean>= new HashMap<Object1, boolean>(); map.put("obj1",true); => exist map.get("obj1"); =>if null = not exist, else exist ```