Java : PriorityQueue with Examples
Published : July 24, 2023
PriorityQueue (Java SE 20 & JDK 20) API Examples.
You will find code examples on most PriorityQueue methods.
Summary
An unbounded priority queue based on a priority heap. The elements of the priority queue are ordered according to their natural ordering, or by a Comparator provided at queue construction time, depending on which constructor is used.
record Item(String value, int priority) {
@Override
public String toString() {
return "%s(%d)".formatted(value, priority);
}
}
final var comparator = Comparator.comparingInt(Item::priority);
final var queue = new PriorityQueue<Item>(comparator);
System.out.println(queue); // []
System.out.println(queue.offer(new Item("aaa", 1))); // true
System.out.println(queue.offer(new Item("bbb", 2))); // true
System.out.println(queue.offer(new Item("ccc", 3))); // true
System.out.println(queue.offer(new Item("XXX", 100))); // true
System.out.println(queue.offer(new Item("YYY", 99))); // true
System.out.println(queue.offer(new Item("ZZZ", 98))); // true
System.out.println(queue); // [aaa(1), bbb(2), ccc(3), XXX(100), YYY(99), ZZZ(98)]
System.out.println(queue.poll()); // aaa(1)
System.out.println(queue.poll()); // bbb(2)
System.out.println(queue.poll()); // ccc(3)
System.out.println(queue.poll()); // ZZZ(98)
System.out.println(queue.poll()); // YYY(99)
System.out.println(queue.poll()); // XXX(100)
Constructors
PriorityQueue ()
Creates a PriorityQueue with the default initial capacity (11) that orders its elements according to their natural ordering.
final var queue = new PriorityQueue<String>();
System.out.println(queue); // []
System.out.println(queue.offer("aaa")); // true
System.out.println(queue.offer("bbb")); // true
System.out.println(queue.offer("ccc")); // true
System.out.println(queue); // [aaa, bbb, ccc]
PriorityQueue (int initialCapacity)
Creates a PriorityQueue with the specified initial capacity that orders its elements according to their natural ordering.
final var queue = new PriorityQueue<Integer>(10000000);
System.out.println(queue); // []
final var startTime = System.nanoTime();
for (int i = 0; i < 4000000; i++) {
queue.add(i);
}
final var endTime = System.nanoTime();
// 0.115743 sec.
System.out.printf("%f sec.%n", (endTime - startTime) / 1000000000.0);
final var queue = new PriorityQueue<Integer>(1);
System.out.println(queue); // []
final var startTime = System.nanoTime();
for (int i = 0; i < 4000000; i++) {
queue.add(i);
}
final var endTime = System.nanoTime();
// 0.188015 sec.
System.out.printf("%f sec.%n", (endTime - startTime) / 1000000000.0);
PriorityQueue (int initialCapacity, Comparator<? super E> comparator)
Creates a PriorityQueue with the specified initial capacity that orders its elements according to the specified comparator.
Please see :
- PriorityQueue(int initialCapacity) for initialCapacity parameter.
- PriorityQueue(Comparator<? super E> comparator) for comparator parameter.
PriorityQueue (Collection<? extends E> c)
Creates a PriorityQueue containing the elements in the specified collection.
final var c = List.of("a", "b", "c");
final var queue = new PriorityQueue<>(c);
System.out.println(queue); // [a, b, c]
System.out.println(queue.size()); // 3
PriorityQueue (Comparator<? super E> comparator)
Creates a PriorityQueue with the default initial capacity and whose elements are ordered according to the specified comparator.
record Item(String value, int priority) {
@Override
public String toString() {
return "%s(%d)".formatted(value, priority);
}
}
final var comparator = Comparator.comparingInt(Item::priority);
final var queue = new PriorityQueue<Item>(comparator);
System.out.println(queue); // []
System.out.println(queue.offer(new Item("aaa", 1))); // true
System.out.println(queue.offer(new Item("bbb", 2))); // true
System.out.println(queue.offer(new Item("ccc", 3))); // true
System.out.println(queue.offer(new Item("XXX", 100))); // true
System.out.println(queue.offer(new Item("YYY", 99))); // true
System.out.println(queue.offer(new Item("ZZZ", 98))); // true
System.out.println(queue); // [aaa(1), bbb(2), ccc(3), XXX(100), YYY(99), ZZZ(98)]
System.out.println(queue.poll()); // aaa(1)
System.out.println(queue.poll()); // bbb(2)
System.out.println(queue.poll()); // ccc(3)
System.out.println(queue.poll()); // ZZZ(98)
System.out.println(queue.poll()); // YYY(99)
System.out.println(queue.poll()); // XXX(100)
PriorityQueue (PriorityQueue<? extends E> c)
Creates a PriorityQueue containing the elements in the specified priority queue.
final var c = new PriorityQueue<String>(Comparator.reverseOrder());
System.out.println(c.offer("aaa")); // true
System.out.println(c.offer("bbb")); // true
System.out.println(c.offer("ccc")); // true
final var queue = new PriorityQueue<>(c);
System.out.println(queue.poll()); // ccc
System.out.println(queue.poll()); // bbb
System.out.println(queue.poll()); // aaa
PriorityQueue (SortedSet<? extends E> c)
Creates a PriorityQueue containing the elements in the specified sorted set.
final var c = new TreeSet<String>(Comparator.reverseOrder());
System.out.println(c.add("aaa")); // true
System.out.println(c.add("bbb")); // true
System.out.println(c.add("ccc")); // true
System.out.println(c); // [ccc, bbb, aaa]
final var queue = new PriorityQueue<>(c);
System.out.println(queue.poll()); // ccc
System.out.println(queue.poll()); // bbb
System.out.println(queue.poll()); // aaa
Methods
boolean add (E e)
Inserts the specified element into this priority queue.
final var queue = new PriorityQueue<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]
void clear ()
Removes all of the elements from this priority queue.
final var queue = new PriorityQueue<String>();
System.out.println(queue); // []
System.out.println(queue.offer("a")); // true
System.out.println(queue); // [a]
System.out.println(queue.offer("b")); // true
System.out.println(queue); // [a, b]
queue.clear();
System.out.println(queue); // []
Comparator<? super E> comparator ()
Returns the comparator used to order the elements in this queue, or null if this queue is sorted according to the natural ordering of its elements.
final var queue = new PriorityQueue<String>();
System.out.println(queue); // []
final var comparator = queue.comparator();
System.out.println(comparator); // null
final var queue = new PriorityQueue<String>(Comparator.reverseOrder());
System.out.println(queue); // []
final var comparator = queue.comparator();
System.out.println(Objects.equals(comparator, Comparator.reverseOrder())); // true
final var queue = new PriorityQueue<>(String.CASE_INSENSITIVE_ORDER);
System.out.println(queue); // []
final var comparator = queue.comparator();
System.out.println(Objects.equals(comparator, String.CASE_INSENSITIVE_ORDER)); // true
boolean contains (Object o)
Returns true if this queue contains the specified element.
final var queue = new PriorityQueue<String>();
System.out.println(queue.offer("aaa")); // true
System.out.println(queue.offer("bbb")); // true
System.out.println(queue.offer("ccc")); // true
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 PriorityQueue<String>();
System.out.println(queue.offer("aaa")); // true
System.out.println(queue.offer("bbb")); // true
System.out.println(queue.offer("ccc")); // true
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
Iterator<E> iterator ()
Returns an iterator over the elements in this queue.
final var queue = new PriorityQueue<String>();
System.out.println(queue.offer("aaa")); // true
System.out.println(queue.offer("bbb")); // true
System.out.println(queue.offer("ccc")); // true
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 into this priority queue.
final var queue = new PriorityQueue<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 PriorityQueue<String>();
System.out.println(queue); // []
System.out.println(queue.offer("aaa")); // true
System.out.println(queue.offer("bbb")); // true
System.out.println(queue.offer("ccc")); // true
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 PriorityQueue<String>();
System.out.println(queue); // []
System.out.println(queue.offer("aaa")); // true
System.out.println(queue.offer("bbb")); // true
System.out.println(queue.offer("ccc")); // true
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 PriorityQueue<String>();
System.out.println(queue.offer("aaa")); // true
System.out.println(queue.offer("aaa")); // true
System.out.println(queue.offer("bbb")); // true
System.out.println(queue.offer("bbb")); // true
System.out.println(queue.offer("ccc")); // true
System.out.println(queue); // [aaa, aaa, bbb, bbb, ccc]
System.out.println(queue.remove("aaa")); // true
System.out.println(queue); // [aaa, bbb, bbb, ccc]
System.out.println(queue.remove("bbb")); // true
System.out.println(queue); // [aaa, ccc, bbb]
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 PriorityQueue<>(src);
System.out.println(queue); // [A, a, B, b, b, a]
System.out.println(queue.removeAll(List.of())); // false
System.out.println(queue); // [A, a, B, b, b, a]
}
{
final var queue = new PriorityQueue<>(src);
System.out.println(queue); // [A, a, B, b, b, a]
System.out.println(queue.removeAll(List.of("a"))); // true
System.out.println(queue); // [A, B, b, b]
}
{
final var queue = new PriorityQueue<>(src);
System.out.println(queue); // [A, a, B, b, b, a]
System.out.println(queue.removeAll(List.of("a", "b"))); // true
System.out.println(queue); // [A, B]
}
{
final var queue = new PriorityQueue<>(src);
System.out.println(queue); // [A, a, B, b, b, a]
System.out.println(queue.removeAll(List.of("b", "a"))); // true
System.out.println(queue); // [A, B]
}
{
final var queue = new PriorityQueue<>(src);
System.out.println(queue); // [A, a, B, b, b, a]
System.out.println(queue.removeAll(List.of("A"))); // true
System.out.println(queue); // [B, a, b, b, a]
}
{
final var queue = new PriorityQueue<>(src);
System.out.println(queue); // [A, a, B, b, b, a]
System.out.println(queue.removeAll(List.of("X", "Y", "Z"))); // false
System.out.println(queue); // [A, a, B, b, b, a]
}
{
final var queue = new PriorityQueue<>(src);
System.out.println(queue); // [A, a, B, b, b, a]
System.out.println(queue.removeAll(List.of("A", "X", "Y", "Z"))); // true
System.out.println(queue); // [B, a, b, b, a]
}
boolean removeIf (Predicate<? super E> filter)
Removes all of the elements of this collection that satisfy the given predicate.
final var queue = new PriorityQueue<String>();
System.out.println(queue.offer("aaa")); // true
System.out.println(queue.offer("BBB")); // true
System.out.println(queue.offer("ccc")); // true
System.out.println(queue.offer("DDD")); // true
System.out.println(queue); // [BBB, DDD, ccc, aaa]
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 PriorityQueue<String>();
System.out.println(queue.offer("aaa")); // true
System.out.println(queue.offer("bbb")); // true
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 PriorityQueue<>(src);
System.out.println(queue); // [A, a, B, b, b, a]
System.out.println(queue.retainAll(List.of())); // true
System.out.println(queue); // []
}
{
final var queue = new PriorityQueue<>(src);
System.out.println(queue); // [A, a, B, b, b, a]
System.out.println(queue.retainAll(List.of("a", "b"))); // true
System.out.println(queue); // [a, a, b, b]
}
{
final var queue = new PriorityQueue<>(src);
System.out.println(queue); // [A, a, B, b, b, a]
System.out.println(queue.retainAll(List.of("b", "a"))); // true
System.out.println(queue); // [a, a, b, b]
}
{
final var queue = new PriorityQueue<>(src);
System.out.println(queue); // [A, a, B, b, b, a]
System.out.println(queue.retainAll(List.of("A"))); // true
System.out.println(queue); // [A]
}
{
final var queue = new PriorityQueue<>(src);
System.out.println(queue); // [A, a, B, b, b, a]
System.out.println(queue.retainAll(List.of("X", "Y", "Z"))); // true
System.out.println(queue); // []
}
{
final var queue = new PriorityQueue<>(src);
System.out.println(queue); // [A, a, B, b, b, a]
System.out.println(queue.retainAll(List.of("A", "X", "Y", "Z"))); // true
System.out.println(queue); // [A]
}
{
final var queue = new PriorityQueue<>(src);
System.out.println(queue); // [A, a, B, b, b, a]
System.out.println(queue.retainAll(List.of("a", "b", "A", "B"))); // false
System.out.println(queue); // [A, a, B, b, b, a]
}
int size ()
Returns the number of elements in this collection.
final var queue = new PriorityQueue<String>();
System.out.println(queue); // []
System.out.println(queue.size()); // 0
System.out.println(queue.offer("aaa")); // true
System.out.println(queue); // [aaa]
System.out.println(queue.size()); // 1
System.out.println(queue.offer("bbb")); // true
System.out.println(queue); // [aaa, bbb]
System.out.println(queue.size()); // 2
System.out.println(queue.offer("ccc")); // true
System.out.println(queue); // [aaa, bbb, ccc]
System.out.println(queue.size()); // 3
queue.clear();
System.out.println(queue); // []
System.out.println(queue.size()); // 0
final Spliterator<E> spliterator ()
Creates a late-binding and fail-fast Spliterator over the elements in this queue.
final var queue = new PriorityQueue<String>();
System.out.println(queue.offer("aaa")); // true
System.out.println(queue.offer("bbb")); // true
System.out.println(queue.offer("ccc")); // true
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.
final var queue = new PriorityQueue<String>();
System.out.println(queue.offer("a")); // true
System.out.println(queue.offer("b")); // true
System.out.println(queue.offer("c")); // true
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; the runtime type of the returned array is that of the specified array.
final var queue = new PriorityQueue<String>();
System.out.println(queue.offer("a")); // true
System.out.println(queue.offer("b")); // true
System.out.println(queue.offer("c")); // true
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 PriorityQueue<String>();
System.out.println(queue.offer("a")); // true
System.out.println(queue.offer("b")); // true
System.out.println(queue.offer("c")); // true
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)); // [null]
System.out.println(Arrays.toString(ret)); // [a, b, c]
}
Methods declared in AbstractQueue
Methods declared in AbstractCollection
Methods declared in Collection
containsAll, equals, hashCode, isEmpty, parallelStream, stream, toArray
Please see the link below.
Related posts
- API Examples