Java增强型循环：引用集合未更改

Question

Something strange is going on, suppose I have this code with 'Attachment' objects, and I want to set all the objects to NULL.

List<Attachment> attachments = getAttachments();

for (Attachment attachment: attachments)
{
   attachment = null;
}

The immediate object attachment is successfully set to NULL. But the backing collection is untouched. It still has the old data. I thought we could always rely on references in Java?

Answer 1

Yes, you can rely on references in Java, if you understand them correctly. You have a reference variable attachment , but it is one of 2 references referring to the object you want to set to null .

attachments -> { Attachment, Attachment, Attachment }
                     |           |           |
                     v           v           v
  attachment -->  (object)    (object)    (object)

When you assign null to attachment , the list reference isn't changed.

attachments -> { Attachment, Attachment, Attachment }
                     |           |           |
                     v           v           v
  attachment      (object)    (object)    (object)
      |
      v
    (null)

An enhanced for loop won't let you change the list contents this way.

You can use a traditional for loop, and call set :

for (int i = 0; i < attachments.size; i++)
{
    attachments.set(i, null);
}

BTW, it's unclear why you want to set all elements to null . This leaves n copies of null in the list. An alternative is to remove all references to the list with attachments.clear() , which will leave no references at all in the list, not even null , in case that's what you really want to do.

Answer 2

You modify the contents of a Collection when traversing it in an enhanced for loop.

The attachment object that you get on every iteration is like a copy (it's actually just another reference to that object) of the object in the collection, so basically you are assigning null to that 'temporary' object, not to the actual element inside the collection.

Answer 3

The assignment of null to your attachment variable can not alter the collection because attachment is a reference to an Attachment object, not the collection itself.

Your attachments collection contains references to Attachment objects. Each iteration of the loop, Java assigns the next object reference in the collection to your attachment variable. If you subsequently change your attachment variable, that can not alter the collection itself.

Answer 4

The identifier in the header of the enhanced for statement declares a local variable , as stated in the Java Language Specification . In your case, attachment is a local variable. When you set it to null , it won't impact the object it is pointing to.