Concepts of changing an item in a collection when iterating?

Question

I found this statement, if the thread modifies the collection directly, when iterates through the collection using a fail-safe iterator, the iterator will throw this exception. at http://download.oracle.com/javase/6/docs/api/java/util/ConcurrentModificationException.html .

I found that simultaneous modification is even selected below the code

List<Employee> lista= new ArrayList(); Employee emp1=new Employee(); Employee emp2=new Employee(); Employee emp3=new Employee(); lista.add(emp1); lista.add(emp2); lista.add(emp3); for(Employee emp:lista) { emp2.setEmpId(2); lista.remove(emp2); } 

Question1: - Can I say that loop gain also uses a failover iterator inside? Although, when I execute the code below, it works great

 for(int i=0;i<lista.size();i++) { Employee empTemp=lista.get(i); lista.remove(emp2); } 

Question2: - Another question when the expression says

if the thread modifies the collection

My guess is that modifying here means deleting or adding not to update an item inside the collection for the list interface, while it also includes modifying the element for the given interface. Correctly? At least the programs I tried are with them.

Edit

As for the set, I doubt my statement above, that is, it throws a parallel modification exception when we try to modify set while iterating.I tried to execute the code below, but it did not throw any exception.

 HashSet<Employee> set1= new HashSet(); Employee emp4=new Employee(); Employee emp5=new Employee(); Employee emp6=new Employee(); set1.add(emp4); set1.add(emp5); set1.add(emp6); Iterator iter1=set1.iterator(); while(iter1.hasNext()) { Employee emp12=(Employee)iter1.next(); System.out.println(""); emp5.setEmpId(2); } 

Ideally according to the instructions, if the set changes at any time after creating the iterator at http://download.oracle.com/javase/6/docs/api/java/util/HashSet.html it should throw a parallel modification exception, but this not this way. Do not know why?

Answer 1

Question1: - Can I say that loop gain also uses a failover iterator inside? Although, when I execute the code below, it works great

Yes, that's right. Take a look at the compiled code with the javap command to check it if you want.

My guess is that modifying here means deleting or adding not to update an item inside the collection for the list interface, while it also includes modifying the element for the given interface. Correctly? At least the programs I've tried deal with them.

That's right, if you execute emp1.setEmpId(2) or something similar, the iteration will not work.

... it should throw a parallel modification exception, but it is not. Do not know why?

It only throws an exception if you modify the list. Keep in mind that the list contains links to objects. If you modify objects, the links do not change, so the list does not change.

Answer 2

If this is not access to two different threads, there is no danger of changing the items in the list. your for does not throw a concurrent modification exception , because you are changing the elements of the set, not the set itself.