How to Safely Remove Elements from a List While Iterating in Java

What is the best way to remove elements from a list while iterating over it in Java?

for (String fruit : list) {
    if("banana".equals(fruit))
        list.remove(fruit);
    System.out.println(fruit);
}

When iterating over a list in Java and attempting to modify it by removing elements, you may encounter a ConcurrentModificationException. This issue arises because the iterator detects that the list structure has changed during iteration, making it unsafe to continue. Here, we'll discuss safe methods to remove items from a list while iterating over it.

Iterator<String> iterator = list.iterator();
while (iterator.hasNext()) {
    String fruit = iterator.next();
    if ("banana".equals(fruit)) {
        iterator.remove(); // Safely remove using the iterator
    }
}

Causes

• Using an enhanced for-loop (for-each) directly on a list while modifying the list structure.
• Not using an explicit Iterator when performing modifications.

Solutions

• Use an Iterator explicitly to remove elements safely while iterating.
• Use the Java 8 Stream API to filter items and create a new collection which excludes the undesired elements.
• Collect elements to be removed in a separate list then remove them after the iteration.

Mistake: Using a for-each loop for removal instead of an iterator.

Solution: Always use an iterator when you need to remove elements during iteration.

Mistake: Removing items from the list within the for-each block.

Solution: Switch to using an explicit Iterator to prevent ConcurrentModificationException.