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
HashSet is a set, e.g. {1,2,3,4,5} HashMap is a key -> value (key to value) map, e.g. {a -> 1, b -> 2, c -> 2, d -> 1} Notice in my example above that in the HashMap there must not be duplicate keys, but it may have duplicate values. In the HashSet, there must … Read more
Use the retainAll() method of Set: Set<String> s1; Set<String> s2; s1.retainAll(s2); // s1 now contains only elements in both sets If you want to preserve the sets, create a new set to hold the intersection: Set<String> intersection = new HashSet<String>(s1); // use the copy constructor intersection.retainAll(s2); The javadoc of retainAll() says it’s exactly what you … Read more
You can use an enhanced for loop: Set<String> set = new HashSet<String>(); //populate set for (String s : set) { System.out.println(s); } Or with Java 8: set.forEach(System.out::println);
For HashSet<T>, the name is UnionWith. This is to indicate the distinct way the HashSet works. You cannot safely Add a set of random elements to it like in Collections, some elements may naturally evaporate. I think that UnionWith takes its name after “merging with another HashSet“, however, there’s an overload for IEnumerable<T> too.
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
HashSet is much faster than TreeSet (constant-time versus log-time for most operations like add, remove and contains) but offers no ordering guarantees like TreeSet. HashSet the class offers constant time performance for the basic operations (add, remove, contains and size). it does not guarantee that the order of elements will remain constant over time iteration … Read more
There’s no built in type for ConcurrentHashSet because you can always derive a set from a map. Since there are many types of maps, you use a method to produce a set from a given map (or map class). Prior to Java 8, you produce a concurrent hash set backed by a concurrent hash map, … Read more
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