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Java Dictionary

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Java Dictionary
new LinkedList<>() / queue.offer() / poll() / peek()

new LinkedList<>() / queue.offer() / poll() / peek()

Since:		Java 5（2004）

A queue is a First In First Out (FIFO) data structure. In Java, LinkedList implements the Queue interface and can be used as a queue. Use offer() to add elements, poll() to retrieve and remove them, and peek() to inspect the front element without removing it.

Syntax

// Create a LinkedList as a queue.
Queue<Type> queueName = new LinkedList<>();

// Add an element to the tail.
queue.offer(element);

// Retrieve and remove the head element (returns null if empty).
queue.poll();

// Inspect the head element without removing it (returns null if empty).
queue.peek();

// Check whether the queue is empty.
queue.isEmpty();

Method List

Method	Description
new LinkedList<>()	Creates a queue (or deque) backed by a doubly linked list.
offer(E e)	Adds an element to the tail. Returns `true` if the element was added successfully.
poll()	Retrieves and removes the head element. Returns `null` if the queue is empty.
peek()	Returns the head element without removing it. Returns `null` if the queue is empty.
isEmpty()	Returns a `boolean` indicating whether the queue is empty.

Sample Code

sample_QueueOfferPoll.java

import java.util.LinkedList;
import java.util.Queue;

class QueueOfferPoll {
    public static void main(String[] args) {
        Queue<String> queue = new LinkedList<>();

        // Add elements to the tail.
        queue.offer("Kogami Shinya");
        queue.offer("Tsunemori Akane");
        queue.offer("Ginoza Nobuchika");

        // Inspect the head element without removing it.
        System.out.println(queue.peek()); // Prints "Kogami Shinya".

        // Retrieve and remove the head element.
        System.out.println(queue.poll()); // Prints "Kogami Shinya".
        System.out.println(queue.poll()); // Prints "Tsunemori Akane".

        // Check the size.
        System.out.println(queue.size()); // Prints "1".

        // Retrieve all elements in order.
        Queue<Integer> numQueue = new LinkedList<>();
        numQueue.offer(10);
        numQueue.offer(20);
        numQueue.offer(30);

        while (!numQueue.isEmpty()) {
            System.out.println(numQueue.poll()); // Prints 10, 20, 30 in order.
        }
    }
}

QueueOfferPoll.java

javac QueueOfferPoll.java
java QueueOfferPoll
Kogami Shinya
Kogami Shinya
Tsunemori Akane
1
10
20
30

Notes

Both offer() and add() add an element to the queue, but when a capacity-restricted queue cannot accept the element, offer() returns false while add() throws an exception. It is generally recommended to use the safer offer().

Similarly, both poll() and remove() retrieve the head element, but when called on an empty queue, remove() throws an exception whereas poll() returns null. Check with isEmpty() before retrieving, or use poll().

For stack and deque operations, see 'new ArrayDeque<>() / push() / pop()'.

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