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

// 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();

DequePushPop.java

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

class DequePushPop {
    public static void main(String[] args) {
        // Used as a stack (last-in, first-out).
        Deque<String> stack = new ArrayDeque<>();
        stack.push("Ayanami Rei");
        stack.push("Ikari Shinji");
        stack.push("Soryu Asuka");

        System.out.println(stack.peek()); // Prints "Soryu Asuka" (without removing).
        System.out.println(stack.pop()); // Prints "Soryu Asuka" (removes it).
        System.out.println(stack.pop()); // Prints "Ikari Shinji".

        // 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).
    }
}

The command looks like this:

javac DequePushPop.java
java DequePushPop
Soryu Asuka
Soryu Asuka
Ikari Shinji
0
2
0
2

Common Mistake 1: Calling pop() on an empty Deque causes NoSuchElementException

pop() throws a NoSuchElementException when called on an empty Deque. It's like trying to launch an Eva unit with no pilot available — nothing is there to return.

PopEmptyNg.java

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

class PopEmptyNg {
    public static void main(String[] args) {
        Deque<String> stack = new ArrayDeque<>();
        stack.push("Ayanami Rei");
        stack.pop();
        stack.pop();
    }
}

The command looks like this:

javac PopEmptyNg.java
java PopEmptyNg
Exception in thread "main" java.util.NoSuchElementException

Like Ayanami Rei calmly assessing the situation, use poll() instead — it returns null on an empty Deque without throwing an exception. Alternatively, check with isEmpty() beforehand.

PopEmptyOk.java

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

class PopEmptyOk {
    public static void main(String[] args) {
        Deque<String> stack = new ArrayDeque<>();
        stack.push("Ayanami Rei");
        System.out.println(stack.poll());
        System.out.println(stack.poll());
    }
}

The command looks like this:

javac PopEmptyOk.java
java PopEmptyOk
Ayanami Rei
null

Common Mistake 2: Adding a null element to ArrayDeque causes NullPointerException

ArrayDeque does not allow null elements. Attempting to push null throws a NullPointerException. Like Soryu Asuka having strict conditions for piloting, ArrayDeque only accepts non-null values.

DequeNullNg.java

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

class DequeNullNg {
    public static void main(String[] args) {
        Deque<String> stack = new ArrayDeque<>();
        stack.push("Soryu Asuka");
        stack.push(null);
    }
}

The command looks like this:

javac DequeNullNg.java
java DequeNullNg
Exception in thread "main" java.lang.NullPointerException

Like Katsuragi Misato planning ahead, check for null before pushing, or use a LinkedList if null values are required.

DequeNullOk.java

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

class DequeNullOk {
    public static void main(String[] args) {
        Deque<String> stack = new ArrayDeque<>();
        String pilot = "Soryu Asuka";
        if (pilot != null) {
            stack.push(pilot);
        }
        System.out.println(stack.peek());
    }
}

The command looks like this:

javac DequeNullOk.java
java DequeNullOk
Soryu Asuka

When used as a stack, ArrayDeque is faster than the legacy Stack class. The Stack class extends Vector and carries synchronization overhead, so ArrayDeque is commonly used 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()'.