Standard .NET HashSet do not preserve the insertion order. For simple tests the insertion order may be preserved due to an accident, but it’s not guaranteed and would not always work that way. To prove that it is enough to do some removals in between. See this question for more information on that: Does HashSet … Read more
There’s no need to call equals if hashCode differs. There’s no need to call hashCode if (obj1 == obj2). There’s no need for hashCode and/or equals just to iterate – you’re not comparing objects When needed to distinguish in between objects.
Unlike a List<> … A HashSet is a List with no duplicate members. Because a HashSet is constrained to contain only unique entries, the internal structure is optimised for searching (compared with a list) – it is considerably faster Adding to a HashSet returns a boolean – false if addition fails due to already existing … Read more
If the hash codes differ, there is no need to call equals() since it is guaranteed to return false. This follows from the general contract on equals() and hashCode(): If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer … Read more
HashSet vs List vs Dictionary performance test, taken from here. Add 1000000 objects (without checking duplicates) Contains check for half the objects of a collection of 10000 Remove half the objects of a collection of 10000
A HashSet holds a set of objects, but in a way that allows you to easily and quickly determine whether an object is already in the set or not. It does so by internally managing an array and storing the object using an index which is calculated from the hashcode of the object. Take a … Read more
You can use a LinkedHashMap if you want a HashMap whose iteration order does not change. Moreover you should always use it if you iterate through the collection. Iterating over HashMap’s entrySet or keySet is much slower than over LinkedHashMap’s.
There is a shorthand that I use that is not very time efficient, but fits on a single line: Set<String> h = new HashSet<>(Arrays.asList(“a”, “b”)); Again, this is not time efficient since you are constructing an array, converting to a list and using that list to create a set. When initializing static final sets I … Read more
This HashSet MSDN page specifically says: A set is a collection that contains no duplicate elements, and whose elements are in no particular order.
It uses an IEqualityComparer<T> (EqualityComparer<T>.Default unless you specify a different one on construction). When you add an element to the set, it will find the hash code using IEqualityComparer<T>.GetHashCode, and store both the hash code and the element (after checking whether the element is already in the set, of course). To look an element up, … Read more