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);    }