Java : ConcurrentLinkedQueue with Examples

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

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

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

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

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

test(new LinkedList<>());

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

test(new ConcurrentLinkedQueue<>());

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

ConcurrentLinkedQueue ()

Creates a ConcurrentLinkedQueue that is initially empty.

final var queue = new ConcurrentLinkedQueue<String>();

queue.add("aaa");
queue.add("bbb");
queue.add("ccc");

System.out.println(queue); // [aaa, bbb, ccc]
System.out.println(queue.size()); // 3

System.out.println(queue.remove()); // aaa
System.out.println(queue.remove()); // bbb
System.out.println(queue.remove()); // ccc

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

ConcurrentLinkedQueue (Collection<? extends E> c)

Creates a ConcurrentLinkedQueue initially containing the elements of the given collection, added in traversal order of the collection's iterator.

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

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

boolean add (E e)

Inserts the specified element at the tail of this queue.

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

System.out.println(queue.add("aaa")); // true
System.out.println(queue); // [aaa]

System.out.println(queue.add("bbb")); // true
System.out.println(queue); // [aaa, bbb]

System.out.println(queue.add("ccc")); // true
System.out.println(queue); // [aaa, bbb, ccc]

boolean addAll (Collection<? extends E> c)

Appends all of the elements in the specified collection to the end of this queue, in the order that they are returned by the specified collection's iterator.

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

System.out.println(queue.addAll(List.of("a1"))); // true
System.out.println(queue); // [a1]

System.out.println(queue.addAll(List.of("b1", "b2"))); // true
System.out.println(queue); // [a1, b1, b2]

System.out.println(queue.addAll(List.of("c1", "c2", "c3"))); // true
System.out.println(queue); // [a1, b1, b2, c1, c2, c3]

void clear ()

Removes all of the elements from this queue.

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

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

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

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

boolean contains (Object o)

Returns true if this queue contains the specified element.

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

System.out.println(queue.contains("aaa")); // true
System.out.println(queue.contains("bbb")); // true
System.out.println(queue.contains("XXX")); // 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 queue = new ConcurrentLinkedQueue<String>();
Collections.addAll(queue, "aaa", "bbb", "ccc");
System.out.println(queue); // [aaa, bbb, ccc]

System.out.println("-- forEach --");
queue.forEach(value -> {
    System.out.println("value = " + value);
});

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

boolean isEmpty ()

Returns true if this queue contains no elements.

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

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

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

Iterator<E> iterator ()

Returns an iterator over the elements in this queue in proper sequence.

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

final var iterator = queue.iterator();

System.out.println("-- forEachRemaining --");
iterator.forEachRemaining(value -> {
    System.out.println("value = " + value);
});

// Result
// ↓
//-- forEachRemaining --
//value = aaa
//value = bbb
//value = ccc

boolean offer (E e)

Inserts the specified element at the tail of this queue.

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

System.out.println(queue.offer("aaa")); // true
System.out.println(queue); // [aaa]

System.out.println(queue.offer("bbb")); // true
System.out.println(queue); // [aaa, bbb]

System.out.println(queue.offer("ccc")); // true
System.out.println(queue); // [aaa, bbb, ccc]

E peek ()

Retrieves, but does not remove, the head of this queue, or returns null if this queue is empty.

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

queue.add("aaa");
queue.add("bbb");
queue.add("ccc");

System.out.println(queue.peek()); // aaa
System.out.println(queue); // [aaa, bbb, ccc]

System.out.println(queue.poll()); // aaa
System.out.println(queue); // [bbb, ccc]

System.out.println(queue.peek()); // bbb
System.out.println(queue); // [bbb, ccc]

System.out.println(queue.poll()); // bbb
System.out.println(queue); // [ccc]

System.out.println(queue.peek()); // ccc
System.out.println(queue); // [ccc]

System.out.println(queue.poll()); // ccc
System.out.println(queue); // []

System.out.println(queue.peek()); // null
System.out.println(queue); // []

E poll ()

Retrieves and removes the head of this queue, or returns null if this queue is empty.

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

queue.add("aaa");
queue.add("bbb");
queue.add("ccc");

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

System.out.println(queue.poll()); // aaa
System.out.println(queue); // [bbb, ccc]

System.out.println(queue.poll()); // bbb
System.out.println(queue); // [ccc]

System.out.println(queue.poll()); // ccc
System.out.println(queue); // []

System.out.println(queue.poll()); // null
System.out.println(queue); // []

boolean remove (Object o)

Removes a single instance of the specified element from this queue, if it is present.

final var queue = new ConcurrentLinkedQueue<String>();
Collections.addAll(queue, "aaa", "BBB", "aaa", "BBB", "ccc");
System.out.println(queue); // [aaa, BBB, aaa, BBB, ccc]

System.out.println(queue.remove("aaa")); // true
System.out.println(queue); // [BBB, aaa, BBB, ccc]

System.out.println(queue.remove("BBB")); // true
System.out.println(queue); // [aaa, BBB, ccc]

System.out.println(queue.remove("BBB")); // true
System.out.println(queue); // [aaa, ccc]

System.out.println(queue.remove("XXX")); // false
System.out.println(queue); // [aaa, ccc]

boolean removeAll (Collection<?> c)

Removes all of this collection's elements that are also contained in the specified collection (optional operation).

final var src = List.of("a", "b", "a", "b", "A", "B");
System.out.println(src); // [a, b, a, b, A, B]

{
    final var queue = new ConcurrentLinkedQueue<>(src);
    System.out.println(queue); // [a, b, a, b, A, B]

    System.out.println(queue.removeAll(List.of())); // false
    System.out.println(queue); // [a, b, a, b, A, B]
}
{
    final var queue = new ConcurrentLinkedQueue<>(src);
    System.out.println(queue); // [a, b, a, b, A, B]

    System.out.println(queue.removeAll(List.of("a"))); // true
    System.out.println(queue); // [b, b, A, B]
}
{
    final var queue = new ConcurrentLinkedQueue<>(src);
    System.out.println(queue); // [a, b, a, b, A, B]

    System.out.println(queue.removeAll(List.of("a", "b"))); // true
    System.out.println(queue); // [A, B]
}
{
    final var queue = new ConcurrentLinkedQueue<>(src);
    System.out.println(queue); // [a, b, a, b, A, B]

    System.out.println(queue.removeAll(List.of("b", "a"))); // true
    System.out.println(queue); // [A, B]
}
{
    final var queue = new ConcurrentLinkedQueue<>(src);
    System.out.println(queue); // [a, b, a, b, A, B]

    System.out.println(queue.removeAll(List.of("A"))); // true
    System.out.println(queue); // [a, b, a, b, B]
}
{
    final var queue = new ConcurrentLinkedQueue<>(src);
    System.out.println(queue); // [a, b, a, b, A, B]

    System.out.println(queue.removeAll(List.of("X", "Y", "Z"))); // false
    System.out.println(queue); // [a, b, a, b, A, B]
}
{
    final var queue = new ConcurrentLinkedQueue<>(src);
    System.out.println(queue); // [a, b, a, b, A, B]

    System.out.println(queue.removeAll(List.of("A", "X", "Y", "Z"))); // true
    System.out.println(queue); // [a, b, a, b, B]
}

boolean removeIf (Predicate<? super E> filter)

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

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

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

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

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

final var queue = new ConcurrentLinkedQueue<String>();
queue.add("aaa");
queue.add("bbb");

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

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

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

boolean retainAll (Collection<?> c)

Retains only the elements in this collection that are contained in the specified collection (optional operation).

final var src = List.of("a", "b", "a", "b", "A", "B");
System.out.println(src); // [a, b, a, b, A, B]

{
    final var queue = new ConcurrentLinkedQueue<>(src);
    System.out.println(queue); // [a, b, a, b, A, B]

    System.out.println(queue.retainAll(List.of())); // true
    System.out.println(queue); // []
}
{
    final var queue = new ConcurrentLinkedQueue<>(src);
    System.out.println(queue); // [a, b, a, b, A, B]

    System.out.println(queue.retainAll(List.of("a", "b"))); // true
    System.out.println(queue); // [a, b, a, b]
}
{
    final var queue = new ConcurrentLinkedQueue<>(src);
    System.out.println(queue); // [a, b, a, b, A, B]

    System.out.println(queue.retainAll(List.of("b", "a"))); // true
    System.out.println(queue); // [a, b, a, b]
}
{
    final var queue = new ConcurrentLinkedQueue<>(src);
    System.out.println(queue); // [a, b, a, b, A, B]

    System.out.println(queue.retainAll(List.of("A"))); // true
    System.out.println(queue); // [A]
}
{
    final var queue = new ConcurrentLinkedQueue<>(src);
    System.out.println(queue); // [a, b, a, b, A, B]

    System.out.println(queue.retainAll(List.of("X", "Y", "Z"))); // true
    System.out.println(queue); // []
}
{
    final var queue = new ConcurrentLinkedQueue<>(src);
    System.out.println(queue); // [a, b, a, b, A, B]

    System.out.println(queue.retainAll(List.of("A", "X", "Y", "Z"))); // true
    System.out.println(queue); // [A]
}
{
    final var queue = new ConcurrentLinkedQueue<>(src);
    System.out.println(queue); // [a, b, a, b, A, B]

    System.out.println(queue.retainAll(List.of("a", "b", "A", "B"))); // false
    System.out.println(queue); // [a, b, a, b, A, B]
}

int size ()

Returns the number of elements in this queue.

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

queue.add("aaa");
System.out.println(queue); // [aaa]
System.out.println(queue.size()); // 1

queue.add("bbb");
System.out.println(queue); // [aaa, bbb]
System.out.println(queue.size()); // 2

queue.add("ccc");
System.out.println(queue); // [aaa, bbb, ccc]
System.out.println(queue.size()); // 3

Spliterator<E> spliterator ()

Returns a Spliterator over the elements in this queue.

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

final var spliterator = queue.spliterator();

System.out.println("-- forEachRemaining --");
spliterator.forEachRemaining(value -> {
    System.out.println("value = " + value);
});

// Result
// ↓
//-- forEachRemaining --
//value = aaa
//value = bbb
//value = ccc

Object[] toArray ()

Returns an array containing all of the elements in this queue, in proper sequence.

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

final Object[] array = queue.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 queue, in proper sequence; the runtime type of the returned array is that of the specified array.

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

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

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

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

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

clear, element, remove

Please see the link below.

• Java API Examples : AbstractQueue

containsAll, toString

Please see the link below.

• Java API Examples : AbstractCollection

containsAll, equals, hashCode, parallelStream, stream, toArray

Please see the link below.

• Java API Examples : Collection

element, remove

Please see the link below.

• Java API Examples : Queue