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Java : Queue with Examples

Last Modified : February 10, 2024 Published : June 8, 2022

Queue (Java SE 21 & JDK 21) with Examples.
You will find code examples on most Queue<E> methods.


Summary

A collection designed for holding elements prior to processing. Besides basic Collection operations, queues provide additional insertion, extraction, and inspection operations.

Queue (Java SE 21 & JDK 21)

Class diagram

final Queue<String> queue = new ArrayDeque<>();

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

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

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

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

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

Methods

boolean add (E e)

Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returning true upon success and throwing an IllegalStateException if no space is currently available.

Queue (Java SE 21 & JDK 21) 
// capacity = 3
final Queue<String> queue = new ArrayBlockingQueue<>(3);
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]

try {
    queue.add("ddd");
} catch (IllegalStateException e) {
    System.out.println("IllegalStateException! : " + e.getMessage());
}

// Result
// ↓
//IllegalStateException! : Queue full

E element ()

Retrieves, but does not remove, the head of this queue.

Queue (Java SE 21 & JDK 21) 
final Queue<String> queue = new ArrayDeque<>();
System.out.println(queue); // []

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

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

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

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

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

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

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

try {
    queue.element();
} catch (NoSuchElementException e) {
    System.out.println("NoSuchElementException!");
}

// Result
// ↓
//NoSuchElementException!

boolean offer (E e)

Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions.

Queue (Java SE 21 & JDK 21) 
// capacity = 3
final Queue<String> queue = new ArrayBlockingQueue<>(3);
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]

System.out.println(queue.offer("ddd")); // false
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.

Queue (Java SE 21 & JDK 21) 
final Queue<String> queue = new ArrayDeque<>();
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.

Queue (Java SE 21 & JDK 21) 
final Queue<String> queue = new ArrayDeque<>();
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); // []

E remove ()

Retrieves and removes the head of this queue.

Queue (Java SE 21 & JDK 21) 
final Queue<String> queue = new ArrayDeque<>();
System.out.println(queue); // []

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

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

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

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

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

try {
    queue.remove();
} catch (NoSuchElementException e) {
    System.out.println("NoSuchElementException!");
}

// Result
// ↓
//NoSuchElementException!

Methods declared in Collection

addAll, clear, contains, containsAll, equals, hashCode, isEmpty, iterator, parallelStream, remove, removeAll, removeIf, retainAll, size, spliterator, stream, toArray, toArray, toArray

Please see the link below.

Methods declared in Iterable

forEach

Please see the link below.


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