How to Safely Use the map() Operation in Java 8 Streams

Java 8 introduced a new way of manipulating collections with the introduction of streams. Streams allow developers to write declarative code that expresses the intent of the operation being performed on the collection, making it easier to reason about and debug. One of the most commonly used operations in Java 8 streams is the map() operation, which transforms the elements of a collection based on a specified function. However, when using the map() operation, it's important to be aware of the risk of ConcurrentModificationException when modifying the state of the source collection. In this article, we'll explore how to safely use the map() operation in Java 8 streams.

What is the map() operation in Java 8 streams?

The map() operation in Java 8 streams transforms the elements of a collection based on a specified function. It takes a Function<T, R> as input, where T is the type of the input element and R is the type of the output element. The map() operation applies the specified function to each element in the collection and returns a new collection of the transformed elements.

Here's an example of using the map() operation to transform a collection of strings to uppercase:

List<String> words = Arrays.asList("hello", "world", "java")
List<String> upperCaseWords = words.stream()
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? .map(String::toUpperCase)
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? .collect(Collectors.toList());        

In this example, we're using the map() operation to transform a List of strings to uppercase using the String::toUpperCase method reference. The transformed elements are collected into a new List using the collect() operation.

The Risk of ConcurrentModificationException

One of the risks when using the map() operation is the ConcurrentModificationException that can occur when modifying the state of the source collection while iterating over it. Consider the following example:

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5)
numbers.stream()
? ? ? ?.map(number -> {
? ? ? ? ? ?if (number % 2 == 0) {
? ? ? ? ? ? ? ?numbers.remove(number);
? ? ? ? ? ?}
? ? ? ? ? ?return number;
? ? ? ?})
? ? ? ?.collect(Collectors.toList());        

In this example, we're using the map() operation to remove all even numbers from the List of integers. However, when we try to remove elements from the List while iterating over it, we get a ConcurrentModificationException. This exception occurs because the map() operation iterates over the collection using an iterator, and modifying the collection while iterating over it can cause the iterator to become invalid.

How to Safely Use the map() Operation

To safely use the map() operation in Java 8 streams, we need to ensure that we don't modify the state of the source collection while iterating over it. One way to achieve this is to separate the mapping and modifying steps into two separate operations. For example:

List<Integer> numbers = new ArrayList<>(Arrays.asList(1, 2, 3, 4, 5))
List<Integer> filteredNumbers = new ArrayList<>();

numbers.stream()
? ? ? ?.filter(number -> number % 2 != 0)
? ? ? ?.map(number -> number * 2)
? ? ? ?.forEach(filteredNumbers::add);        

In this example, we're first filtering out all even numbers from the List of integers using the filter() operation. Then we're using the map() operation to multiply each remaining number by 2 and adding the transformed elements to a new List using the forEach() operation. By using the forEach() operation instead of the collect() operation, we're able to add the transformed elements to a new List without modifying the state of the source collection.

Another way to safely use the map() operation is to create a new collection with the transformed elements, as shown in the following example:

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5)
List<Integer> filteredNumbers = numbers.stream()
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?.filter(number -> number % 2 != 0)
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?.map(number -> number * 2)
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?.collect(Collectors.toList());        

In this example, we're using the collect() operation to collect the transformed elements into a new List instead of modifying the state of the source collection.

Conclusion

The map() operation in Java 8 streams is a powerful tool for transforming elements in a collection. However, when using the map() operation, it's important to be aware of the risk of ConcurrentModificationException when modifying the state of the source collection. To safely use the map() operation, we should separate the mapping and modifying steps into two separate operations or create a new collection with the transformed elements. By following these best practices, we can write safe and effective code that takes advantage of the power of Java 8 streams.