Stack Basics

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Stack;

public class Stack_Basics {
    public static void main(String[] args) {
        Stack<Integer> stack = new Stack<>();
        System.out.println(stack.push(1));  // it returns the number pushed
        System.out.println(stack.add(2));       // it returns true
        stack.push(3);
        stack.push(4);
        stack.push(5);

        System.out.println(stack);          // o/p: [1, 2, 3, 4]
        System.out.println(stack.peek());   // o/p: 4
        for (int x : stack) {
            System.out.print(x + "\t");     // o/p: 1  2  3  4
        }
        System.out.println();
        //Exception in thread "main" java.util.ConcurrentModificationException
        // at java.util.Vector$Itr.checkForComodification(Vector.java:1212)
        // at java.util.Vector$Itr.next(Vector.java:1165)
        // at pep.Day25.Stack_Basics.main(Stack_Basics.java:19)
        for (int x : stack)
            System.out.println(stack.pop());

for each loop calculates the size first and iterates that many number of times, while execution if there is any modification in size then, it gives ConcurrentModificationException. Similarly in ArrayList.

Hence, to remove elements we should use for loop, instead of foreach loop

ArrayList<Integer> arr = new ArrayList(Arrays.asList(1, 2, 3));
for (int x:arr)
    System.out.println(arr.remove(x));