Java : CopyOnWriteArraySet with Examples

void test(Set<Integer> set) throws InterruptedException {
    for (int i = 0; i < 5; i++) {
        set.add(i);
    }

    try (final var executor = Executors.newSingleThreadScheduledExecutor()) {

        executor.schedule(() -> {
            System.out.println("-- add value! --");
            set.add(9999);
        }, 5, TimeUnit.SECONDS);

        try {
            for (final var value : set) {
                System.out.println("value = " + value);
                TimeUnit.SECONDS.sleep(2);
            }
        } catch (ConcurrentModificationException e) {
            System.out.println("ConcurrentModificationException!");
        }
    }

    System.out.println("-- end --");
    System.out.println("set = " + set);
}

test(new LinkedHashSet<>());

// Result
// ↓
//value = 0
//value = 1
//value = 2
//-- add value! --
//ConcurrentModificationException!
//-- end --
//set = [0, 1, 2, 3, 4, 9999]

test(new CopyOnWriteArraySet<>());

// Result
// ↓
//value = 0
//value = 1
//value = 2
//-- add value! --
//value = 3
//value = 4
//-- end --
//set = [0, 1, 2, 3, 4, 9999]

CopyOnWriteArraySet ()

Creates an empty set.

final var set = new CopyOnWriteArraySet<String>();
System.out.println(set); // []
System.out.println(set.isEmpty()); // true

set.add("a");
set.add("b");
set.add("c");

System.out.println(set); // [a, b, c]
System.out.println(set.size()); // 3

CopyOnWriteArraySet (Collection<? extends E> c)

Creates a set containing all of the elements of the specified collection.

final var c = List.of("a", "b", "c");

final var set = new CopyOnWriteArraySet<>(c);
System.out.println(set); // [a, b, c]
System.out.println(set.size()); // 3

boolean add (E e)

Adds the specified element to this set if it is not already present.

final var set = new CopyOnWriteArraySet<String>();
System.out.println(set); // []

System.out.println(set.add("a")); // true
System.out.println(set); // [a]

System.out.println(set.add("b")); // true
System.out.println(set); // [a, b]

System.out.println(set.add("c")); // true
System.out.println(set); // [a, b, c]

System.out.println(set.add("a")); // false
System.out.println(set); // [a, b, c]

boolean addAll (Collection<? extends E> c)

Adds all of the elements in the specified collection to this set if they're not already present.

final var src = List.of("a", "b", "c");

{
    final var set = new CopyOnWriteArraySet<>(src);
    System.out.println(set); // [a, b, c]

    System.out.println(set.addAll(List.of("d", "e", "f"))); // true
    System.out.println(set);  // [a, b, c, d, e, f]
}
{
    final var set = new CopyOnWriteArraySet<>(src);
    System.out.println(set); // [a, b, c]

    System.out.println(set.addAll(List.of("a", "b", "c"))); // false
    System.out.println(set); // [a, b, c]
}
{
    final var set = new CopyOnWriteArraySet<>(src);
    System.out.println(set); // [a, b, c]

    System.out.println(set.addAll(List.of("a", "c", "e", "f"))); // true
    System.out.println(set); // [a, b, c, e, f]
}

void clear ()

Removes all of the elements from this set.

final var set = new CopyOnWriteArraySet<String>();
System.out.println(set); // []

set.add("a");
System.out.println(set); // [a]

set.add("b");
System.out.println(set); // [a, b]

set.clear();
System.out.println(set); // []

boolean contains (Object o)

Returns true if this set contains the specified element.

final var set = new CopyOnWriteArraySet<String>();
Collections.addAll(set, "a", "b", "c");
System.out.println(set); // [a, b, c]

System.out.println(set.contains("")); // false
System.out.println(set.contains("a")); // true
System.out.println(set.contains("b")); // true
System.out.println(set.contains("c")); // true

System.out.println(set.contains("X")); // false
System.out.println(set.contains("abc")); // false

boolean containsAll (Collection<?> c)

Returns true if this set contains all of the elements of the specified collection.

final var set = new CopyOnWriteArraySet<String>();
Collections.addAll(set, "a", "b", "c");
System.out.println(set); // [a, b, c]

System.out.println(set.containsAll(Set.of())); // true

System.out.println(set.containsAll(Set.of("a"))); // true
System.out.println(set.containsAll(Set.of("a", "b"))); // true
System.out.println(set.containsAll(Set.of("b", "a"))); // true
System.out.println(set.containsAll(Set.of("c", "b", "a"))); // true

System.out.println(set.containsAll(Set.of("X"))); // false
System.out.println(set.containsAll(Set.of("a", "X"))); // false
System.out.println(set.containsAll(Set.of("a", "b", "c", "X"))); // false

boolean equals (Object o)

Compares the specified object with this set for equality.

final var set = new CopyOnWriteArraySet<String>();
Collections.addAll(set, "a", "b", "c");
System.out.println(set); // [a, b, c]

System.out.println(set.equals(Set.of("a"))); // false
System.out.println(set.equals(Set.of("a", "b"))); // false
System.out.println(set.equals(Set.of("a", "b", "c"))); // true
System.out.println(set.equals(Set.of("c", "b", "a"))); // true

final var set = new CopyOnWriteArraySet<String>();
Collections.addAll(set, "a", "b", "c");
System.out.println(set); // [a, b, c]

final var hashSet = new HashSet<>(set);
System.out.println(hashSet); // [a, b, c]
System.out.println(set.equals(hashSet)); // true

final var treeSet = new TreeSet<>(set);
System.out.println(treeSet); // [a, b, c]
System.out.println(set.equals(treeSet)); // true

final var list = new ArrayList<>(set);
System.out.println(list); // [a, b, c]
System.out.println(set.equals(list)); // false

void forEach (Consumer<? super E> action)

Performs the given action for each element of the Iterable until all elements have been processed or the action throws an exception.

final var set = new CopyOnWriteArraySet<String>();
Collections.addAll(set, "aaa", "bbb", "ccc");
System.out.println(set); // [aaa, bbb, ccc]

System.out.println("-- forEach --");
set.forEach(element -> {
    System.out.println(element);
});

// Result
// ↓
//-- forEach --
//aaa
//bbb
//ccc

boolean isEmpty ()

Returns true if this set contains no elements.

final var set = new CopyOnWriteArraySet<String>();
set.add("a");
set.add("b");
set.add("c");

System.out.println(set); // [a, b, c]
System.out.println(set.isEmpty()); // false

set.clear();
System.out.println(set); // []
System.out.println(set.isEmpty()); // true

Iterator<E> iterator ()

Returns an iterator over the elements contained in this set in the order in which these elements were added.

final var set = new CopyOnWriteArraySet<String>();
Collections.addAll(set, "aaa", "bbb", "ccc");
System.out.println(set); // [aaa, bbb, ccc]

System.out.println("-- iterator --");
set.iterator().forEachRemaining(System.out::println);

// Result
// ↓
//-- iterator --
//aaa
//bbb
//ccc

boolean remove (Object o)

Removes the specified element from this set if it is present.

final var src = List.of("a", "b", "c");

{
    final var set = new CopyOnWriteArraySet<>(src);
    System.out.println(set); // [a, b, c]

    System.out.println(set.remove("")); // false
    System.out.println(set); // [a, b, c]
}
{
    final var set = new CopyOnWriteArraySet<>(src);
    System.out.println(set); // [a, b, c]

    System.out.println(set.remove("a")); // true
    System.out.println(set); // [b, c]
}
{
    final var set = new CopyOnWriteArraySet<>(src);
    System.out.println(set); // [a, b, c]

    System.out.println(set.remove("b")); // true
    System.out.println(set); // [a, c]
}
{
    final var set = new CopyOnWriteArraySet<>(src);
    System.out.println(set); // [a, b, c]

    System.out.println(set.remove("X")); // false
    System.out.println(set); // [a, b, c]
}

boolean removeAll (Collection<?> c)

Removes from this set all of its elements that are contained in the specified collection.

final var src = List.of("a", "b", "c");

{
    final var set = new CopyOnWriteArraySet<>(src);
    System.out.println(set); // [a, b, c]

    System.out.println(set.removeAll(Set.of())); // false
    System.out.println(set); // [a, b, c]
}
{
    final var set = new CopyOnWriteArraySet<>(src);
    System.out.println(set); // [a, b, c]

    System.out.println(set.removeAll(Set.of("a"))); // true
    System.out.println(set); // [b, c]
}
{
    final var set = new CopyOnWriteArraySet<>(src);
    System.out.println(set); // [a, b, c]

    System.out.println(set.removeAll(Set.of("a", "b"))); // true
    System.out.println(set); // [c]
}
{
    final var set = new CopyOnWriteArraySet<>(src);
    System.out.println(set); // [a, b, c]

    System.out.println(set.removeAll(Set.of("b", "a"))); // true
    System.out.println(set); // [c]
}
{
    final var set = new CopyOnWriteArraySet<>(src);
    System.out.println(set); // [a, b, c]

    System.out.println(set.removeAll(Set.of("a", "b", "c"))); // true
    System.out.println(set); // []
}
{
    final var set = new CopyOnWriteArraySet<>(src);
    System.out.println(set); // [a, b, c]

    System.out.println(set.removeAll(Set.of("X", "Y"))); // false
    System.out.println(set); // [a, b, c]
}
{
    final var set = new CopyOnWriteArraySet<>(src);
    System.out.println(set); // [a, b, c]

    System.out.println(set.removeAll(Set.of("a", "X"))); // true
    System.out.println(set); // [b, c]
}

boolean removeIf (Predicate<? super E> filter)

Removes all of the elements of this collection that satisfy the given predicate.

final var set = new CopyOnWriteArraySet<String>();
set.add("aaa");
set.add("BBB");
set.add("ccc");
set.add("DDD");

System.out.println(set); // [aaa, BBB, ccc, DDD]

final var ret = set.removeIf(s -> {
    return s.equals(s.toUpperCase());
});

System.out.println(ret); // true
System.out.println(set); // [aaa, ccc]

final var set = new CopyOnWriteArraySet<String>();
set.add("aaa");
set.add("bbb");

System.out.println(set); // [aaa, bbb]

final var ret = set.removeIf(s -> s.equals(s.toUpperCase()));

System.out.println(ret); // false
System.out.println(set); // [aaa, bbb]

boolean retainAll (Collection<?> c)

Retains only the elements in this set that are contained in the specified collection.

final var src = List.of("a", "b", "c");

{
    final var set = new CopyOnWriteArraySet<>(src);
    System.out.println(set); // [a, b, c]

    System.out.println(set.retainAll(Set.of())); // true
    System.out.println(set); // []
}
{
    final var set = new CopyOnWriteArraySet<>(src);
    System.out.println(set); // [a, b, c]

    System.out.println(set.retainAll(Set.of("a"))); // true
    System.out.println(set); // [a]
}
{
    final var set = new CopyOnWriteArraySet<>(src);
    System.out.println(set); // [a, b, c]

    System.out.println(set.retainAll(Set.of("a", "b"))); // true
    System.out.println(set); // [a, b]
}
{
    final var set = new CopyOnWriteArraySet<>(src);
    System.out.println(set); // [a, b, c]

    System.out.println(set.retainAll(Set.of("b", "a"))); // true
    System.out.println(set); // [a, b]
}
{
    final var set = new CopyOnWriteArraySet<>(src);
    System.out.println(set); // [a, b, c]

    System.out.println(set.retainAll(Set.of("a", "b", "c"))); // false
    System.out.println(set); // [a, b, c]
}
{
    final var set = new CopyOnWriteArraySet<>(src);
    System.out.println(set); // [a, b, c]

    System.out.println(set.retainAll(Set.of("a", "b", "c", "X", "Y"))); // false
    System.out.println(set); // [a, b, c]
}
{
    final var set = new CopyOnWriteArraySet<>(src);
    System.out.println(set); // [a, b, c]

    System.out.println(set.retainAll(Set.of("X", "a", "b", "Y"))); // true
    System.out.println(set); // [a, b]
}
{
    final var set = new CopyOnWriteArraySet<>(src);
    System.out.println(set); // [a, b, c]

    System.out.println(set.retainAll(Set.of("X", "Y"))); // true
    System.out.println(set); // []
}

int size ()

Returns the number of elements in this set.

final var set1 = new CopyOnWriteArraySet<String>();
System.out.println(set1); // []
System.out.println(set1.size()); // 0

final var set2 = new CopyOnWriteArraySet<String>();
Collections.addAll(set2, "a", "b", "c");
System.out.println(set2); // [a, b, c]
System.out.println(set2.size()); // 3

Spliterator<E> spliterator ()

Returns a Spliterator over the elements in this set in the order in which these elements were added.

final var set = new CopyOnWriteArraySet<String>();
Collections.addAll(set, "aaa", "bbb", "ccc");
System.out.println(set); // [aaa, bbb, ccc]

final var spliterator = set.spliterator();
spliterator.forEachRemaining(System.out::println);

// Result
// ↓
//aaa
//bbb
//ccc

Object[] toArray ()

Returns an array containing all of the elements in this set.

final var set = new CopyOnWriteArraySet<String>();
Collections.addAll(set, "a", "b", "c");
System.out.println(set); // [a, b, c]

final Object[] array = set.toArray();
System.out.println(Arrays.toString(array)); // [a, b, c]

<T> T[] toArray (T[] a)

Returns an array containing all of the elements in this set; the runtime type of the returned array is that of the specified array.

final var set = new CopyOnWriteArraySet<String>();
Collections.addAll(set, "a", "b", "c");
System.out.println(set); // [a, b, c]

final String[] ret = set.toArray(new String[0]);
System.out.println(Arrays.toString(ret)); // [a, b, c]

final var set = new CopyOnWriteArraySet<String>();
Collections.addAll(set, "a", "b", "c");
System.out.println(set); // [a, b, c]

{
    final String[] array = new String[3];
    System.out.println(Arrays.toString(array)); // [null, null, null]

    final var ret = set.toArray(array);
    System.out.println(Arrays.toString(array)); // [a, b, c]
    System.out.println(Arrays.toString(ret)); // [a, b, c]
}
{
    final String[] array = new String[5];
    System.out.println(Arrays.toString(array)); // [null, null, null, null, null]

    final var ret = set.toArray(array);
    System.out.println(Arrays.toString(array)); // [a, b, c, null, null]
    System.out.println(Arrays.toString(ret)); // [a, b, c, null, null]
}
{
    final String[] array = new String[1];
    System.out.println(Arrays.toString(array)); // [null]

    final var ret = set.toArray(array);
    System.out.println(Arrays.toString(array)); // [null]
    System.out.println(Arrays.toString(ret)); // [a, b, c]
}

hashCode

Please see the link below.

• Java API Examples : AbstractSet

toString

Please see the link below.

• Java API Examples : AbstractCollection

parallelStream, stream, toArray

Please see the link below.

• Java API Examples : Collection