Why Removing Elements from a HashMap During a For‑Each Loop Causes ConcurrentModificationException and How to Fix It

When a HashMap is traversed with a for‑each loop over entrySet() and the map’s remove method is invoked at the same time, a ConcurrentModificationException is thrown even in a single‑threaded context.

Reproduction

public static void main(String[] args) {
    Map
map = new HashMap<>();
    map.put("a", 1);
    map.put("b", "b");
    for (Map.Entry
entry : map.entrySet()) {
        if (entry.getKey().equals("a")){
            map.remove("a");
        }
    }
}

Running the above code on Java 17 produces the following stack trace:

Exception in thread "main" java.util.ConcurrentModificationException
  at java.base/java.util.HashMap$HashIterator.nextNode(HashMap.java:1597)
  at java.base/java.util.HashMap$EntryIterator.next(HashMap.java:1630)
  at java.base/java.util.HashMap$EntryIterator.next(HashMap.java:1628)
  at com.example.springcloudtest.spring.postprocessor.Test.main(Test.java:20)

Root cause

The for‑each loop internally uses the iterator java.util.HashMap.EntryIterator . The iterator checks the map’s modCount against its own expectedModCount . Calling HashMap.remove(Object) updates the map’s modCount but does not update the iterator’s expectedModCount , causing the mismatch and the exception.

Decompiled version of the loop (without syntactic sugar) shows the explicit iterator usage:

public class Test {
    public static void main(String[] args) {
        Map
map = new HashMap();
        map.put("a", 1);
        map.put("b", "b");
        Iterator var2 = map.entrySet().iterator();
        while(var2.hasNext()) {
            Entry
entry = (Entry)var2.next();
            if (((String)entry.getKey()).equals("a")) {
                map.remove("a");
            }
        }
    }
}

Therefore, when iterating with an iterator, removal must be performed through the iterator itself, not through the collection’s remove method.

Corrected examples

1. Use the iterator’s remove() method:

public static void main(String[] args) {
    Map
map = new HashMap<>();
    map.put("a", 1);
    map.put("b", "b");
    System.out.println(map);
    Iterator
> iterator = map.entrySet().iterator();
    while (iterator.hasNext()){
        Map.Entry
next = iterator.next();
        if (next.getKey().equals("a")){
            iterator.remove();
        }
    }
    System.out.println("删除后:" + map);
}

2. Use Collection.removeIf directly on the entry set:

public static void main(String[] args) {
    Map
map = new HashMap<>();
    map.put("a", 1);
    map.put("b", "b");
    map.entrySet().removeIf(entry -> entry.getKey().equals("a"));
    System.out.println(map);
}

Summary

The for‑each loop in Java is syntactic sugar for an iterator; removing elements via the map’s own remove while iterating breaks the iterator’s consistency check and triggers ConcurrentModificationException . Use the iterator’s remove() method or removeIf to safely delete entries during iteration.