ArrayStack.java

import java.util.Arrays;

public class ArrayStack {

    private int arr[];
    private int top = -1;

    //require an initial value constructor to know how large to make the stack
    public ArrayStack(int capacity){
        arr = new int[capacity];
    }

    //O(1) since the memory location of the element is known/computable
    public void push(int data){
        arr[++top] = data;
    }

    //O(1) since decrementing top
    public int pop(){
        return arr[top--];
    }

    //O(1) constant look up time for the top element in the stack
    public int peek(){
        return arr[top];
    }

    public Boolean isEmpty(){
        return top == -1;
    }

    public String print(){
        return Arrays.toString(arr);
    }

}