Tags: Collection , Interview

One of the popular interview question asked to java developer. To know how well you understand the iterator.

Fail Fast
When you perform add, update or remove on collection while iterating on it. It throws java.util.ConcurrentModificationException.

Source code (FailFastExample.java)

import java.util.HashMap;
import java.util.Map;

/**
 * Example of Fail Fast using Map.
 * @author javaQuery
 * @date 4th August, 2016
 * @Github: https://github.com/javaquery/Examples 
 */
public class FailFastExample {
    public static void main(String[] args) {
        Map<String, Double> carPrice = new HashMap<String, Double>();
        carPrice.put("Porsche", 100000D);
        carPrice.put("Ferrari", 200000D);

        /* Iterate over Map */
        for(Map.Entry<String, Double> car : carPrice.entrySet()){
            System.out.println("Car: " + car.getKey());
            System.out.println("Price: " + car.getValue());
            
            /* Will cause java.util.ConcurrentModificationException */
            carPrice.put("Lamborghini", 300000D);
        }
    }
}

Output

Car: Ferrari
Price: 200000.0
Exception in thread "main" java.util.ConcurrentModificationException
 at java.util.HashMap$HashIterator.nextNode(HashMap.java:1429)
 at java.util.HashMap$EntryIterator.next(HashMap.java:1463)
 at java.util.HashMap$EntryIterator.next(HashMap.java:1461)
 at com.javaquery.collections.map.FailFastExample.main(FailFastExample.java:23)

You can also consider Collection (List, Set, Map) shared between two or more Threads. One is iterating on collection and other tries to add element in it then the thread iterating over it, will throw java.util.ConcurrentModificationException

In interview if you are able to answer this question. They might me interested in checking your knowledge little bit deep. You may face the question How Fail Fast Iterator identifies that the collection is modified?

---

Fail Safe
It iterate over the clone(copy) of original collection. So It won't throw java.util.ConcurrentModificationException.

Source code (FailSafeExample.java)

/**
 * Example of Fail Safe using ConcurrentHashMap.
 * @author javaQuery
 * @date 4th August, 2016 
 * @Github: https://github.com/javaquery/Examples 
 */
public class FailSafeExample {

    public static void main(String[] args) {
        Map<String, Double> carPrice = new ConcurrentHashMap<String, Double>();
        carPrice.put("Porsche", 100000D);
        carPrice.put("Ferrari", 200000D);

        /* Iterate over Map */
        for (Map.Entry<String, Double> car : carPrice.entrySet()) {
            System.out.println("Car: " + car.getKey());
            System.out.println("Price: " + car.getValue());

            /* Will be added in original copy of Map but won't available in Iterator */
            carPrice.put("Lamborghini", 300000D);
        }
        System.out.println("=== loop finished ===");
        System.out.println("Car: Lamborghini" + ", Price: " + carPrice.get("Lamborghini"));
    }
}

Output

Car: Ferrari
Price: 200000.0
Car: Porsche
Price: 100000.0
=== loop finished ===
Car: Lamborghini, Price: 300000.0

Tags: Collection , Interview