Objects in hashsets should either be immutable, or you need to exercise discipline in not changing them after they’ve been used in a hashset (or hashmap). In practice I’ve rarely found this to be a problem – I rarely find myself needing to use complex objects as keys or set elements, and when I do … Read more
My experiment shows that HashSet is faster than an ArrayList starting at collections of 3 elements inclusively. A complete results table | Boost | Collection Size | | 2x | 3 elements | | 3x | 10 elements | | 6x | 50 elements | | 12x | 200 elements | <= proportion 532-12 vs … Read more
You need to override the Object#hashCode() method in the Move class to let it return the same hashCode() value for the state of the Move instance. Don’t forget to override Object#equals() as well. See also: Overriding equals and hashCode in Java Hint: if you’re using an IDE like Eclipse, you can also just autogenerate them. … Read more
The difference between the two are, as you’ve stated: A LinkedHashSet is an ordered version of HashSet that maintains a doubly-linked List across all elements. Use this class instead of HashSet when you care about the iteration order. When you iterate through a HashSet the order is unpredictable, while a LinkedHashSet lets you iterate through … Read more
The second one (just using HashSet) is only a coincidence. From the JavaDocs: This class implements the Set interface, backed by a hash table (actually a HashMap instance). It makes no guarantees as to the iteration order of the set; in particular, it does not guarantee that the order will remain constant over time. This … Read more
EDIT: As of Java 8 and later, the following is no longer applicable. This proves that you shouldn’t rely on undocumented Java behaviours. This behaviour is caused by several separate reasons: Integers hash to themselves in Java, HashMaps and HashSets are backed up by an array they also modify hashes using the higher bits to … Read more
Here is a possible comparer that compares an IEnumerable<T> by its elements. You still need to sort manually before adding. One could build the sorting into the comparer, but I don’t think that’s a wise choice. Adding a canonical form of the list seems wiser. This code will only work in .net 4 since it … Read more
You can manually iterate over the elements of the set: Iterator<Integer> iterator = set.iterator(); while (iterator.hasNext()) { Integer element = iterator.next(); if (element % 2 == 0) { iterator.remove(); } } You will often see this pattern using a for loop rather than a while loop: for (Iterator<Integer> i = set.iterator(); i.hasNext();) { Integer element … Read more
The important thing about HashSet<T> is right there in the name: it’s a set. The only things you can do with a single set is to establish what its members are, and to check whether an item is a member. Asking if you can retrieve a single element (e.g. set[45]) is misunderstanding the concept of … Read more
Absolutely not. The insertion order directly influences the iteration order whenever you have a bucket collision: When two elements end up in the same bucket, the first one that was inserted will also be the first one returned during iteration, at least if the implementation of collision handling and iteration is straightforward (and the one … Read more