此異常發生在使用迭代器（Iterator）時，如果迭代器內的物件發生改變，則會發生此異常。

程式碼演示

public class iteratorDemo {
    public static void main(String[] args) {
        Collection c = new HashSet();
        c.add(new Name("ffff1","222"));
        c.add(new Name("f2","adwe"));
        c.add(new Name("fff3","ssha"));
        Iterator i = c.iterator();
        while
 
(i.hasNext()){
            Name name  = (Name) i.next();
            if(name.getFirstName().length() < 3){
                i.remove(name);
                //如果換成c.remove(name);會產生例外；
                //java.util.ConcurrentModificationException
            }
        }
        System.out.println(c);
}

Name類

public class Name {
    private String firstName,lastName;
    public Name(){}
    public Name(String firstName, String lastName) {
        super();
        this.firstName = firstName;
        this.lastName = lastName;
    }
    public String getFirstName() {
        return firstName;
    }
    public
 
 void setFirstName(String firstName) {
        this.firstName = firstName;
    }
    public String getLastName() {
        return lastName;
    }
    public void setLastName(String lastName) {
        this.lastName = lastName;
    }
    public String toString(){
        return firstName+" "+lastName;
    }
}

在while迴圈內部，執行的是Iterator物件，遍歷的過程是封閉的過程，所以當迴圈內部的物件發生變化時，就會發生異常