Set types

Definition

A Set holds no duplicates. HashSet has no defined order and is the fastest. LinkedHashSet keeps insertion order. TreeSet keeps items sorted. Pick by the order you need.

Example code

Three sets, three orders

import java.util.HashSet;
import java.util.LinkedHashSet;
import java.util.Set;
import java.util.TreeSet;

class Demo {
    public static void main(String[] args) {
        String[] fruits = {"pear", "apple", "fig", "apple", "banana"};

        Set<String> hash = new HashSet<>();               // no order guarantee
        Set<String> linked = new LinkedHashSet<>();       // insertion order
        Set<String> sorted = new TreeSet<>();             // sorted order

        for (String fruit : fruits) {
            hash.add(fruit);
            linked.add(fruit);
            sorted.add(fruit);
        }

        System.out.println("linked: " + linked);          // the duplicate "apple" was ignored
        System.out.println("sorted: " + sorted);
        System.out.println("size of hash: " + hash.size());
    }
}

// Output:
// linked: [pear, apple, fig, banana]
// sorted: [apple, banana, fig, pear]
// size of hash: 4