What is the main difference between Hashset, Treeset and LinkedHashset, Hashmap and how does it work in Java?

Question

I just understand that LinkedHashSet does not allows duplicate elements when it is inserting. But, I dont understand how does Hashset works in Hava? I know a bit that Hashta

Answer 1

My doubt is, why "a" is inserting in between "B" and "C".

A TreeSet orders the entries.

A LinkedHashSet preserves the insertion order.

A HashSet does not preserve the order of insertion, and is does not sort/order the entries. That means that when you iterate over the set, the entries are returned in an order that is hard to fathom ... and of no practical significance. There is no particular "reason" that "a" is inserted at that point. That's just how it turned out ... given the set of input keys and the order in which they were inserted.

My only doubt is, how does Hashset works in Java.

It is implemented a hash table. Read the Wikipedia page on hash tables for a general overview, and the source code of java.util.HashMap and java.util.HashSet for the details.

The short answer is that HashSet and HashMap are both a hash table implemented as an array of hash chains.

And I know that, LinkedHashset follows doubly linkedlist. If it uses doubly linked list then how does it stores the elements?

LinkedHashSet is essentially a hash table with an additional linked list that records the insertion order. The elements are stored in the main hash table ... and that is what provides fast lookup. Again, refer to the source code for details.

What does mean by doubly linkedlist and how does it works?

Read the article in Wikipedia on doubly linked lists.

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Then where all these three Hashset, Treeset, Linkedhashset would be used in Java and which one has better performance in java?

There are a number of things to think about when choosing between these three classes (and others):

• Do they provide the required functionality. For example, we've already seen that they have different behaviour with respect to the order of iteration.

• Do they have the required concurrency properties? For example, are they thread-safe? do they deal with contention? do they allow concurrent modification?

• How much space do they require?

• What are the performance (time) characteristics.

On the last two points?

• A TreeSet uses the least space, and a LinkedHashSet uses the most.

• A HashSet tends to be fastest for lookup, insertion and deletion for larger sets, and a TreeSet tends to be slowest.