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Java Dictionary

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Java Dictionary
new ArrayDeque<>() / deque.push() / pop()

new ArrayDeque<>() / deque.push() / pop()

A Deque (Double Ended Queue) is a data structure that allows adding and removing elements from both ends. ArrayDeque can be used as a stack (last-in, first-out) or as a double-ended queue. Use push() and pop() for stack operations.

Syntax

// Creates an ArrayDeque.
Deque<Type> dequeName = new ArrayDeque<>();

// Adds an element to the front (stack push).
deque.push(element);

// Removes and returns the front element (stack pop).
deque.pop();

// Returns the front element without removing it.
deque.peekFirst();

// Returns the last element without removing it.
deque.peekLast();

Method List

Method	Description
new ArrayDeque<>()	Creates an array-based double-ended queue. Can be used as a stack or queue.
push(E e)	Adds an element to the front. Equivalent to a stack push.
pop()	Removes and returns the front element. Equivalent to a stack pop. Throws an exception if the deque is empty.
peekFirst()	Returns the front element without removing it. Returns `null` if the deque is empty.
peekLast()	Returns the last element without removing it. Returns `null` if the deque is empty.

Sample Code

import java.util.ArrayDeque;
import java.util.Deque;

// Used as a stack (last-in, first-out).
Deque<String> stack = new ArrayDeque<>();
stack.push("first");
stack.push("second");
stack.push("third");

System.out.println(stack.peek());  // Prints "third" (without removing).
System.out.println(stack.pop());   // Prints "third" (removes it).
System.out.println(stack.pop());   // Prints "second".

// Used as a double-ended queue.
Deque<Integer> deque = new ArrayDeque<>();
deque.addFirst(1);   // Adds to the front.
deque.addLast(2);    // Adds to the back.
deque.addFirst(0);   // Adds to the front.

System.out.println(deque.peekFirst()); // Prints "0".
System.out.println(deque.peekLast());  // Prints "2".

System.out.println(deque.pollFirst()); // Prints "0" (removes it).
System.out.println(deque.pollLast());  // Prints "2" (removes it).

Notes

When used as a stack, ArrayDeque is faster than the legacy Stack class. The Stack class extends Vector and carries synchronization overhead, so it is recommended to use ArrayDeque in single-threaded environments.

Internally, push() calls addFirst() and pop() calls removeFirst(). pop() throws a NoSuchElementException on an empty deque, so check with isEmpty() beforehand.

For queue operations (first-in, first-out), see 'new LinkedList<>() / offer() / poll() / peek()'.

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