ArrayDeque

This is an optimized Java stack and queue. We often use ArrayDeque in parsers and caches. Its performance is excellent, and it is versatile.

In a parser, state changes based on the tag last encountered. A stack, like ArrayDeque, represents this logic—we deal with the "deepest" elements first.

First example

We add 3 Integers (10, 500 and 1000) to the ArrayDeque. We specify the type in the ArrayDeque—on the right side, the type Integer is inferred.

import java.util.ArrayDeque;

public class Program {
    public static void main(String[] args) {

        // Create ArrayDeque with three elements.
        ArrayDeque<Integer> deque = new ArrayDeque<>();
        deque.push(10);
        deque.push(500);
        deque.push(1000);

        // Peek to get the top item, but do not remove it.
        int peekResult = deque.peek();
        System.out.println(peekResult);

        // Call pop on the Deque.
        int popResult = deque.pop();
        System.out.println(popResult);

        // Pop again.
        popResult = deque.pop();
        System.out.println(popResult);
    }
}
1000
1000
500

Add, looping

We do not need to use push to add elements: we can add add() to add elements onto the end of the ArrayDeque. This type can be used as a stack or a queue.

import java.util.ArrayDeque;

public class Program {
    public static void main(String[] args) {

        ArrayDeque<String> deque = new ArrayDeque<>();

        // Use add on ArrayDeque.
        deque.add("caterpillar");
        deque.add("dinosaur");
        deque.add("bird");

        // Loop over all the elements in the ArrayDeque.
        for (String element : deque) {
            System.out.println(element);
        }
    }
}
caterpillar
dinosaur
bird

IsEmpty, size, clear

An ArrayDeque that has no elements must be specially handled. We can check isEmpty before calling pop() to avoid exceptions.

import java.util.ArrayDeque;

public class Program {
    public static void main(String[] args) {

        ArrayDeque<Double> doubles = new ArrayDeque<>();

        // Add four elements to the ArrayDeque.
        doubles.push(50.25);
        doubles.push(100.40);
        doubles.push(120.25);
        doubles.push(150.50);

        System.out.println(doubles.isEmpty());
        System.out.println(doubles.size());

        // Clear all elements from the ArrayDeque.
        doubles.clear();

        System.out.println(doubles.isEmpty());
        System.out.println(doubles.size());
    }
}
false
4
true
0

Wiki parser

This example uses ArrayDeque as a stack to parse a simple text language. The example "wiki" language uses stars and underscores to indicate italics and bold.

import java.util.ArrayDeque;
import java.lang.StringBuilder;

public class Program {
    enum Tag {
        None, Italics, Bold
    }

    public static void main(String[] args) {

        String wiki = "How are *you my _friend_*?";
        ArrayDeque<Tag> codes = new ArrayDeque<>();
        StringBuilder builder = new StringBuilder();

        // Loop over the string.
        for (int i = 0; i < wiki.length(); i++) {
            char value = wiki.charAt(i);
            // Handle star or underscore characters.
            if (value == '*') {
                // Close italics if on stack, otherwise open it.
                if (codes.peek() == Tag.Italics) {
                    codes.pop();
                    builder.append("</i>");
                } else {
                    codes.push(Tag.Italics);
                    builder.append("<i>");
                }
            } else if (value == '_') {
                // Close bold if on stack, otherwise open it.
                if (codes.peek() == Tag.Bold) {
                    codes.pop();
                    builder.append("</b>");
                } else {
                    codes.push(Tag.Bold);
                    builder.append("<b>");
                }
            } else {
                // Append other characters.
                builder.append(value);
            }
        }

        // Display our results.
        System.out.println(wiki);
        System.out.println(builder);
    }
}
How are *you my _friend_*?
How are <i>you my <b>friend</b></i>?

Benchmark, ArrayDeque

What is the performance difference between ArrayDeque and Stack? The Stack is an older version of ArrayDeque.

import java.util.ArrayDeque;
import java.util.Stack;

public class Program {
    public static void main(String[] args) {

        long t1 = System.currentTimeMillis();

        // Version 1: push 10 elements to ArrayDeque.
        for (int i = 0; i < 10000000; i++) {

            ArrayDeque<Integer> deque = new ArrayDeque<>();
            for (int v = 0; v < 10; v++) {
                deque.push(v);
            }
        }

        long t2 = System.currentTimeMillis();

        // Version 2: push 10 elements to Stack.
        for (int i = 0; i < 10000000; i++) {

            Stack<Integer> stack = new Stack<>();
            for (int v = 0; v < 10; v++) {
                stack.push(v);
            }
        }

        long t3 = System.currentTimeMillis();

        // ... Print benchmark results.
        System.out.println(t2 - t1);
        System.out.println(t3 - t2);
    }
}
 589 ms:   ArrayDeque push
2664 ms:   Stack push

Stack

This is an older collection. As the benchmark revealed, it has serious performance deficits. It exists for the benefit legacy programs. Avoid it.

ArrayDeque is important

It has clear optimizations over older collections like Stack. And it is flexible—it provides both a stack and a queue.

With abstract data types, we raise the conceptual level of our programs. With a stack, like ArrayDeque, we logically push and pop elements, as in a parser.

In a queue, we can implement a help system with tickets. The older entries may be handled first, in FIFO order (first-in, first-out). No items are left too long with no action.