How to Remove Elements from a List Without Causing java.util.ConcurrentModificationException

When beginners try to remove elements from a List using a foreach loop, a java.util.ConcurrentModificationException is thrown because the loop internally uses an Iterator that checks structural modifications via modCount and expectedModCount.

The article first shows the problematic code:

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);
}

Running this code results in a ConcurrentModificationException. The article then explains that the foreach construct uses an Iterator whose next() method calls checkForComodification(), comparing the two counters.

To avoid the exception, three safe removal methods are presented.

1. 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]

2. Using a forward indexed for loop

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);
}

The index is decremented after a removal to avoid skipping the next element.

3. Using a reverse indexed for loop

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);
}

Because the loop iterates from the end, no index correction is needed.

References: https://blog.csdn.net/zjwcdd/article/details/51513879 https://blog.csdn.net/wangjun5159/article/details/61415358