Java Language - 30 - Stacks

Collections and Data Structures – Stacks

Introduction

Stacks are a fundamental data structure in Java used to manage a collection of elements with a Last-In-First-Out (LIFO) order. In this guide, we will explore the concept of stacks, understand how they work, and learn how to implement and use them in Java for various applications.

What Is a Stack?

A stack is a linear data structure that follows the LIFO principle, meaning the last element added to the stack is the first one to be removed. Think of it as a collection of items stacked on top of each other, similar to a stack of plates. You can only add or remove items from the top.

Implementing Stacks in Java

In Java, you can implement a stack using the ‘Stack’ class or the ‘Deque’ interface, where ‘ArrayDeque’ is a common choice. Here’s an example using ‘ArrayDeque’ to create and manipulate a stack:


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

public class StackExample {
    public static void main(String[] args) {
        Deque<String> stack = new ArrayDeque<>();

        // Pushing elements onto the stack
        stack.push("Item 1");
        stack.push("Item 2");
        stack.push("Item 3");

        // Popping elements from the stack
        while (!stack.isEmpty()) {
            String item = stack.pop();
            System.out.println("Popped: " + item);
        }
    }
}

In this example, we use ‘ArrayDeque’ to create a stack and push items onto it using the ‘push’ method. Then, we pop items from the stack using the ‘pop’ method, demonstrating the LIFO order.

Common Stack Operations

Stacks support basic operations, including:

Push: Adding an element to the top of the stack using the ‘push’ method.
Pop: Removing and returning the top element using the ‘pop’ method. It throws an exception if the stack is empty.
Peek: Viewing the top element without removing it using the ‘peek’ method.
Check for Empty: Using ‘isEmpty’ to check if the stack is empty.

Use Cases of Stacks

Stacks have various practical applications, including:

Expression Evaluation: Stacks are used to evaluate mathematical expressions, such as infix to postfix conversion and postfix expression evaluation.
Undo Mechanisms: Stacks are employed to implement undo functionality in applications, allowing users to revert actions in a step-wise manner.
Backtracking Algorithms: In algorithms like depth-first search, stacks are used to keep track of the traversal path.
Function Calls: The call stack is used to manage function calls and returns in programming languages.

Conclusion

Stacks are a valuable data structure in Java that follows the Last-In-First-Out order. They are used in a wide range of applications, from expression evaluation to undo mechanisms. By understanding how to implement and use stacks in Java, you can enhance the efficiency and functionality of your programs.

Java Language – 30 – Stacks