How to Update Values in a HashMap While Iterating in Java

What is the correct approach to update values in a HashMap while iterating over it in Java?

public class CoolDownTimer implements Runnable {
    private Map<String, Long> playerCooldowns;

    @Override
    public void run() {
        for (Map.Entry<String, Long> entry : playerCooldowns.entrySet()) {
            long updatedValue = entry.getValue() - 20;
            playerCooldowns.put(entry.getKey(), updatedValue);
        }
    }
}

Updating values in a HashMap while iterating over it can be tricky in Java due to the potential for ConcurrentModificationExceptions. This explanation provides a structured approach to safely update the values in your HashMap during iteration.

public class CoolDownTimer implements Runnable {
    private Map<String, Long> playerCooldowns;

    @Override
    public void run() {
        for (Map.Entry<String, Long> entry : playerCooldowns.entrySet()) {
            long updatedValue = entry.getValue() - 20;
            playerCooldowns.put(entry.getKey(), updatedValue);
        }
    }
}

Causes

• Using a simple for-each loop on the values directly is not safe for modification.
• Modifying the map structure while iterating leads to ConcurrentModificationException.

Solutions

• Iterate over the entry set of the map to safely access both keys and values.
• Use the `put` method to update the value directly during iteration.

Mistake: Trying to modify the map directly while looping through its values.

Solution: Always loop through the entry set instead of just the values or keys.

Mistake: Using a traditional for loop or iterator without considering concurrent modifications.

Solution: Use the entry set to avoid modification issues.