Understanding java.util.ConcurrentModificationException and Safe Ways to Remove Elements from a List

This article discusses a common interview question about encountering java.util.ConcurrentModificationException when removing elements from a List using a foreach loop.

It shows the problematic code, explains that foreach internally uses an Iterator whose hasNext() and next() methods check a modification count ( modCount) against an expected count ( expectedModCount), and demonstrates how removing an element inside the loop changes modCount, causing the exception.

Problematic code example:

public static void main(String[] args) {
    List<String> platformList = new ArrayList<>();
    platformList.add("博客园");
    platformList.add("CSDN");
    platformList.add("掘金");

    for (String platform : platformList) {
        if (platform.equals("博客园")) {
            platformList.remove(platform);
        }
    }
    System.out.println(platformList);
}

The article then presents three correct ways to remove elements safely:

Use Iterator 's remove() method.

Iterate with a forward for loop using an index and adjust the index after removal.

Iterate with a reverse for loop, which avoids index shifting.

Example using Iterator.remove():

public static void main(String[] args) {
    List<String> platformList = new ArrayList<>();
    platformList.add("博客园");
    platformList.add("CSDN");
    platformList.add("掘金");

    Iterator<String> iterator = platformList.iterator();
    while (iterator.hasNext()) {
        String platform = iterator.next();
        if (platform.equals("博客园")) {
            iterator.remove();
        }
    }
    System.out.println(platformList);
}

Output: [CSDN, 掘金] Example using a forward index loop (with index correction):

public static void main(String[] args) {
    List<String> platformList = new ArrayList<>();
    platformList.add("博客园");
    platformList.add("CSDN");
    platformList.add("掘金");

    for (int i = 0; i < platformList.size(); i++) {
        String item = platformList.get(i);
        if (item.equals("博客园")) {
            platformList.remove(i);
            i = i - 1; // adjust index after removal
        }
    }
    System.out.println(platformList);
}

Example using a reverse index loop (no index adjustment needed):

public static void main(String[] args) {
    List<String> platformList = new ArrayList<>();
    platformList.add("博客园");
    platformList.add("CSDN");
    platformList.add("掘金");

    for (int i = platformList.size() - 1; i >= 0; i--) {
        String item = platformList.get(i);
        if (item.equals("掘金")) {
            platformList.remove(i);
        }
    }
    System.out.println(platformList);
}

Each method is illustrated with complete Java code examples and the resulting output, showing how they avoid the ConcurrentModificationException.

Additional notes include a brief look at the source code of Iterator.remove(), which synchronizes modCount and expectedModCount, and references to related interview resources.