ConcurrentModificationException: Why does removing the null in List throw this Exception if it´s not the first variable

Question

I got this code Snippet which is working fine.

import java.util.ConcurrentModificationException;
import java.util.*;

ArrayList<Object> s = new ArrayList<>();

s.add(null);
s.add("test");

try {
    System.out.println(s + "\n");
    for (Object t : s) {
        System.out.println(t);

        if (t == null || t.toString().isEmpty()) {
            s.remove(t);
            System.out.println("ObjectRemoved = " + t + "\n");
        }

    }

    System.out.println(s + "\n");
} catch (ConcurrentModificationException e) {
    System.out.println(e);
} catch (Exception e) {
    System.out.println(e);
}

But after changing

s.add(null);
s.add("test");

to

s.add("test");
s.add(null);

The code throws a ConcurrentModificationException

So my question is why can I remove null if it's the first object in the list but not if it's the second one?

Answer 1

The first thing to understand is that the code is invalid, as it structurally modifies the list while iterating over it and this is not permitted (NB: this is a slight simplification, as there are pemissible ways to modify a list, but this is not one of them).

The second thing to understand is that ConcurrentModificationException works on a best-effort basis:

Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.

And so invalid code may or may not raise ConcurrentModificationException -- it's really up to the Java runtime and could depend on the platform, version etc.

For example, when I try [test, null, null] locally, I get no exception but also an invalid result, [test, null]. This is a different behaviour to the two behaviours that you observed.

There are several ways to fix your code, the most common of which is probably to use Iterator.remove().