Welcome to today's lesson! Our topic for the day is data aggregation, a crucial aspect of data analysis. Like summarizing a massive book into key points, data aggregation summarizes large amounts of data into important highlights.

By the end of this lesson, you'll be equipped with several aggregation methods to summarize data streams in Java. Let's get started!

Let's say we have a list of integers denoting the ages of a group of people:

Common questions we might ask are: How many people are in the group? What's their total age? Who's the youngest and the oldest? Java's handy Stream API methods like count(), mapToInt().sum(), min(), and max() have our answers:

These functions provide essential aggregation operations and are widely used with data streams.

For deeper analyses, such as calculating the average age or range of ages manually, we can use for loops.

For example, using for loops, we can also find the mode or most frequent age:

While loops can also be used similarly for more complex tasks.

Finally, let's explore the reduce() method, a powerful tool for performing complex aggregations. It applies a binary function to all elements in an iterative and cumulative way. For example, let's calculate the product of all elements in a list using the reduce() method.

By using the multiplication function as the binary function, reduce() has computed the product of all elements in our list.

We can also use reduce() to accomplish more complex tasks. Consider, for instance, the problem of separately computing the sums of even numbers and odd numbers in the ages list.

In this example, we filtered the elements of the list into even and odd categories and then summed them separately.

Fantastic! You've just learned how to use basic and advanced data aggregation methods in Java, including using the reduce() method! These techniques are pivotal in data analysis and understanding. Now, get ready for the practical tasks lined up next. They'll reinforce the skills you've just gained. Remember, the more you practice, the better you become. Good luck with your practice!