What can LinkedList/LinkedHashSet/LinkedHashMap do?

Question

LinkedList/LinkedHashSet/LinkedHashMap, these three do not have ArrayList/HashSet/HashMap popular, what can they do?

List

ArrayList

An ordered list based on an array. Insertion order, that is, getting order.

LinkedList

An ordered list based on a linked list. Insertion order, that is, getting order.

Set

HashSet

A collection with unique elements.

TreeSet

A collection with unique and sorted elements. The default sorter or customized sorter can be used.

LinkedHashSet

A collection with unique and ordered elements. Insertion order, that is, getting order.

Map

HashMap

Mapping table.

TreeMap

A sorted mapping table. The default sorter or customized sorter can be used.

LinkedHashMap

A ordered mapping table. Insertion order, that is, getting order.

Code

@Test
public void testLinkedHashSet1() {
    // Which set can keep the order of elements in array1?

    final String[] array1 = { "c", "b", "a" };

    final Set<String> hashSet = new HashSet<>();
    final Set<String> linkedHashSet = new LinkedHashSet<>();

    for (final String s : array1) {
        hashSet.add(s);
        linkedHashSet.add(s);
    }

    System.out.println(hashSet); // [a, b, c]
    System.out.println(linkedHashSet); // [c, b, a]
}

@Test
public void testLinkedHashSet2() {
    // Keep the order of the elements in array2 and make them distinct

    final String[] array2 = { "a", "a", "b", "b", "c", "c" };

    final Collection<String> arrayList = new ArrayList<String>();
    final Collection<String> linkedHashSet = new LinkedHashSet<String>();

    for (final String s : array2) {
        // Bad smell, without LinkedHashSet
        if (!arrayList.contains(s)) {
            arrayList.add(s);
        }

        // Good smell, with LinkedHashSet
        linkedHashSet.add(s);
    }

    System.out.println(arrayList); // [a, b, c]
    System.out.println(linkedHashSet); // [a, b, c]
}