import java.util.Iterator;
import java.util.NoSuchElementException;

public class Stack<E> implements Iterable<E>{
    private E[] arr;
    private int top = -1; // highest element id

    public Stack(int cap) {
        // Create array
        this.arr = (E[]) new Object[cap];
    }

    public E pop() {
        // If no elements yet - throw exception
        if (top < 0) {
            throw new NoSuchElementException();
        }

        E result = arr[top];
        arr[top] = null; // Allow garbage collector clearing
        top--; // decrease size

        // return element
        return result;
    }

    public void push(E e) {
        // If stack is full - throw exception
        if (top == arr.length) {
            throw new ArrayIndexOutOfBoundsException();
        }
        top += 1; // increase size
        arr[top] = e; // set element
    }

    public String toString() {
        if (top < 0) {
            return "[]";
        }

        StringBuilder sb = new StringBuilder();
        sb.append("[");

        for (int i = 0; i < top; i++) {
            sb.append(this.arr[i]).append(", ");
        }

        return sb.append(this.arr[top]).append("]").toString();
    }

    public static void main(String[] args) {

        Stack<String> stack = new Stack<>(11);
        stack.push("hello");
        stack.push("world");

        System.out.println(stack);

        stack.pop();
        System.out.println(stack);

        stack.pop();
        System.out.println(stack);

        Stack<Integer> intStack = new Stack<>(11);
        intStack.push(1);
        intStack.push(2);
        System.out.println(intStack);

        for (Integer i: intStack) {
            System.out.println(i);
        }

        intStack.pop();
        System.out.println(intStack);

        intStack.pop();
        System.out.println(intStack);

        for (Integer i: intStack) {
            System.out.println(i);
        }
    }

    @Override
    public Iterator<E> iterator() {
        return new StackIterator(this);
    }

    private class StackIterator implements Iterator<E> {
        private Stack<E> thisStack;
        private int position;

        private StackIterator(Stack<E> thisStack) {
            this.thisStack = thisStack;
            position = 0;
        }

        @Override
        public boolean hasNext() {
            return position <= thisStack.top;
        }

        @Override
        public E next() {
            position++;
            return thisStack.arr[position - 1];
        }
    }
}