Question

Unsure as to how I would be able to have an array behave as a stack in java by removing the last (top) element of an array as well as pushing a new element into the top to the top of the stack. All without using any packages.

Answer 1

/* ARRAY IMPLEMENTATION OF STACK */

import java.util.*;

class Stack
{
   private int arr[];
   private int top;
   private int capacity;

   // Constructor to initialize stack
   Stack(int size)
   {
       arr = new int[size];
       capacity = size;
       top = -1;
   }

   // Utility function to add an element x in the stack
   public void push(int x)
   {
       if (isFull())
       {
           System.out.println("OverFlow\nProgram Terminated\n");
           System.exit(1);
       }

       System.out.println("Inserting " + x);
       arr[++top] = x;
   }

   // Utility function to pop top element from the stack
   public int pop()
   {
       // check for stack underflow
       if (isEmpty())
       {
           System.out.println("UnderFlow\nProgram Terminated");
           System.exit(1);
       }

       System.out.println("Removing " + peek());

       // decrease stack size by 1 and (optionally) return the popped element
       return arr[top--];
   }

   // Utility function to return top element in a stack
   public int peek()
   {
       if (!isEmpty())
           return arr[top];
       else
           System.exit(1);

       return -1;
   }

   // Utility function to return the size of the stack
   public int size()
   {
       return top + 1;
   }

   // Utility function to check if the stack is empty or not
   public Boolean isEmpty()
   {
       return top == -1;   // or return size() == 0;
   }

   // Utility function to check if the stack is full or not
   public Boolean isFull()
   {
       return top == capacity - 1;   // or return size() == capacity;
   }

   public static void main (String[] args)
   {
       Stack stack = new Stack(3);

       stack.push(1);       // Inserting 1 in the stack
       stack.push(2);       // Inserting 2 in the stack

       stack.pop();       // removing the top 2
       stack.pop();       // removing the top 1

       stack.push(3);       // Inserting 3 in the stack

       System.out.println("Top element is: " + stack.peek());
       System.out.println("Stack size is " + stack.size());

       stack.pop();       // removing the top 3

       // check if stack is empty
       if (stack.isEmpty())
           System.out.println("Stack Is Empty");
       else
           System.out.println("Stack Is Not Empty");
   }
}