ArrayDequeThis 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.
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.
Push() adds an element to the front (or the top) of the ArrayDeque. The type must be matched that specified in the declaration.ArrayDeque's contents.ArrayDeque and returns it. Pop throws an exception if empty.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
500We 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.
ArrayDeque with a for-each loop. No index is needed.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
birdIsEmpty, size, clearAn ArrayDeque that has no elements must be specially handled. We can check isEmpty before calling pop() to avoid exceptions.
Size always equals 0 when the ArrayDeque is empty. The size() changes as elements are added and removed.ArrayDeque. To restart an algorithm, clear() is a simpler option than calling pop() many times.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
0This 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.
enum called Tag. We push our tags (None, Italics, Bold) to the ArrayDeque to help with parsing.StringBuilder to build up a result from the parser. In the end this holds finished HTML.char is a star or underscore. And we manipulate the ArrayDeque based on its current contents.ArrayDeque to manage a parser's state: we know whether to open or close a tag.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>?ArrayDequeWhat is the performance difference between ArrayDeque and Stack? The Stack is an older version of ArrayDeque.
ArrayDeque.Stack, and push() elements to it. We perform the same number of operations as version 1.ArrayDeque can be populated with 10 integers many times faster than a Stack. ArrayDeque should always be preferred.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 pushStackThis is an older collection. As the benchmark revealed, it has serious performance deficits. It exists for the benefit legacy programs. Avoid it.
ArrayDeque is importantIt 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.