Stacks in Java

A stack is a linear data structure that follows the Last In First Out(LIFO) principle. This means the last element inserted inside the stack is the first to be removed. Think of a stack as a pile of plates. You would typically grab the container that is on the top of the pile. Another example of a stack is the game Tower of Hanoi.

Uses Cases

• Web Browser Back Button

• Text Editors' Undo Function

• Function Call Stack

• Expression Evaluation

• Backtracking Algorithms

• Reverse Polish Notation

• Browser History

• Syntax Checker

Implementation and Methods of a Stack with An Array

//Space Complexity O(n)
public class Stack {
    //the three fields needed 
    private int maxSize;
    private int top;
    private int[] stackArray;

    //constructor for initializing a new stack
    public Stack(int size) {
        maxSize = size;
        stackArray = new int[maxSize];
        top = -1;
    }

    // O(1) added a new element to the top of the stack
    public void push(int val) {
        if(isFull()) {
            System.out.println("StackOverflowException");
        }
        stackArray[++top] = val;
    }

    // O(1) removes the top element from the stack, making 
    //the element below it the new top element
    public int pop() {
        if(isEmpty()) {
            System.out.println("Stack is empty")
            return -1;
        }

        return stackArray[top--];
    }

    // O(1)
    public int peek() {
        if(isEmpty()) {
            System.out.println("Stack is empty.");
            return -1;
        }
        return stackArray[top];
    }

    // O(1)
    public boolean isEmpty() {
        return (top == -1);
    }

    // O(1)
    public boolean isFull() {
        return (top == maxSize - 1);
    }

    // O(1)
    public int size() {
        return top + 1;
    }
}

The basic methods are:

Push: add an element to the top of a stack

Pop: removes an element from the top of a stack

isEmpty: checks if the stack is empty

isFull: checks if the stack is full

Peek: Get the value of the top element without removing it

The time complexity for all the methods are O(1) when implemented with an array.

Additional Details

• Stacks can be implemented with other data structures like Linked List and Deque(Double-ended Queue)

• Most programming languages provide built-in support for stacks through libraries or stand data structures