java set 删除元素 java.util.ConcurrentModificationException

public static void main(String[] args) {
Set<String> set = new HashSet<String>();

set.add("a");
set.add("b");
set.add("c");
set.add("d");

Iterator<String> iter = set.iterator();

while (iter.hasNext()) {
String s = iter.next();

if ("a".equals(s)) {
iter.remove();
}
}

System.out.println(set);

Set<Integer> set1 = new HashSet<Integer>();

set1.add(1);
set1.add(2);
set1.add(3);
set1.add(4);

Iterator<Integer> iter1 = set1.iterator();

while (iter1.hasNext()) {
int s = iter1.next();

if (3 == s) {
iter1.remove();//big error
}
}

System.out.println(set1);
}