Collecting in Java 8

Java 8, released in 2014, was a game-changer for the Java programming language. Among its many new features and improvements, one of the most significant additions was the introduction of the Stream API and the versatile Stream Collectors. These enhancements made it much easier to work with collections and perform complex operations on data in a more concise and expressive way.

In this section, we'll explore the concept of collecting in Java 8 and dive into the world of Stream Collectors, a powerful tool for transforming and aggregating data within streams.

The Need for Collecting

Before Java 8, working with collections in Java often required writing boilerplate code for iterating over elements and accumulating results manually. It was error-prone, verbose, and made code harder to read and maintain. Java 8 addressed these issues by introducing streams and collectors.

Streams provide a higher-level abstraction for processing sequences of data. They allow us to express operations on data in a more functional and declarative manner. However, once we've performed your transformations and operations on a stream, we often need to collect the results into a data structure or perform aggregation operations. This is where collectors come into play.

Stream Collectors

Stream Collectors in Java 8 are a set of utility methods in the java.util.stream.Collectors class that make it easy to accumulate elements from a stream into a collection or perform aggregation operations like summing, averaging, and grouping. Collectors are flexible and can be customized to suit your specific needs.

Here are some common operations that Stream Collectors enable:

1. Collecting to a List or Set:

We can easily collect stream elements into a List or a Set using the Collectors.toList() and Collectors.toSet() methods, respectively.

2. Grouping elements:

We can group elements of a stream based on a certain criterion using Collectors.groupingBy(). For example, grouping a list of persons by their age:

3. Summing and Averaging:

We can easily calculate the sum or average of numeric attributes using Collectors.summingInt(), Collectors.summingLong(), or Collectors.averagingDouble().

4. Joining Strings:

We can concatenate elements of a stream into a single string using Collectors.joining().

Custom Collectors

While the built-in collectors cover many common use cases, we can create custom collectors using the Collectors.collectingAndThen() method. The method allows us to perform additional processing on the result of another collector.

Here's an example of using collectingAndThen() to create a custom collector that collects elements into a Set and then converts it to an immutable Set:

With this custom collector, we can collect elements into an immutable set like this:

Conclusion

In Java 8, the introduction of the Stream API and Stream Collectors revolutionized the way we work with collections and aggregate data. Collectors provide a powerful and flexible mechanism for collecting, transforming, and aggregating data from streams. Whether we need to group data, calculate sums and averages, or create custom collectors, Java 8's Stream Collectors have you covered.

These features have greatly simplified and improved the readability of Java code, making it more expressive and concise. So, if we haven't already embraced Java 8's Stream Collectors, it's time to explore their potential and harness their power in Java projects.