DCP43 - Implementing a stack

import java.util.ArrayList;

public class Stack {

    // Variables
    public ArrayList <Double> stack;

    // Constructor
    Stack(){
        stack = new ArrayList <Double> ();
    }
    // Methods
    //  Push
    public void push(double val) {
        stack.add(val);
    }
    // Pop
    public double pop() {
        if(stack.size() == 0) {
            System.out.println("Error. It is not permitted to pop element from a stack with size = 0");
            return -1000;
        }
        double value = stack.remove(stack.size()-1);
        return value;
    }
    // Max
    public double max() {
        if(stack.size() == 0) {
            System.out.println("Error. I cannot find man element in a stack with size = 0");
            return -1000;
        }
        double MAX = stack.get(0);
        for(int i=1; i<stack.size(); i++) {
            if(stack.get(i) > MAX) {
                MAX = stack.get(i);
            }
        }
        return MAX;
    }
    // Show
    public void show() {
        System.out.println("******** STACK ********");
        for(int i=0; i<stack.size(); i++) {
            System.out.println(stack.get(i));
        }
        System.out.println();
    }

    // MAIN FUNCTION
    public static void main(String args[]) {
        Stack STACK = new Stack();
        STACK.push(1);
        STACK.push(2);
        STACK.push(3);
        STACK.push(4);
        double popped = STACK.pop();
        System.out.println("Popped = " + popped);
        STACK.show();
        STACK.push(5);
        STACK.push(8);
        STACK.push(7);
        STACK.push(6);
        double MAX = STACK.max();
        System.out.println("Max = " + MAX);
        System.out.println();
        STACK.show();
    }

}