Hello and welcome! Today, we're exploring practical data manipulation techniques in Java. We'll use Java lists to represent our data stream and perform projection, filtering, and aggregation. And here's the star of the show: our operations will be neatly packaged within a Java class! No mess, all clean code.

Data manipulation is akin to being a sculptor but for data. We chisel and shape our data to get the desired structure. Java lists are perfect for this, and our operations will be conveniently bundled inside a Java class. So, let's get our toolbox ready! Here's a simple Java class, DataStream, that will serve as our toolbox:

Our first stop is data projection. Think of it like capturing a photo of our desired features. Suppose we have data about people. If we're only interested in names and ages, we project our data to include just these details. We'll extend our DataStream class with a projectData method for this:

As you can see, we now have a new list with just the names and ages!

Next, we have data filtering, which is like cherry-picking our preferred data entries. We'll extend our DataStream class with a filterData method that uses a "test" function to filter data:

With the filter method, our output is a list with only Bob’s data, as he's the only one who passes the "age over 26" test.

Last is data aggregation, where we condense our data into a summary. We will add an aggregateData method to our DataStream class for this:

With this script, we calculate the average age, which is 27.5.

Now, let's combine projection, filtering, and aggregation to see the collective power of these techniques. We'll extend our example to demonstrate this flow:

1. Data Projection: Choose only the desired fields.
2. Data Filtering: Filter the data based on certain conditions.
3. Data Aggregation: Summarize the filtered data.

We'll modify our DataStream class to include all the methods and then use them together in a workflow. On top of that, projection and filtering methods will now return an instance of DataStream<T>, not a list as before, so that we can chain these methods when calling them:

Here:

• Projection: We choose only the Name, Age, and Salary fields from our data. The projectData method now returns a DataStream<Map<String, Object>> object, allowing us to chain multiple operations.
• Filtering: We filter the projected data to include only those persons whose age is greater than 30. The filterData method also returns a DataStream<T> object for chaining.
• Aggregation: We calculate the average salary of the filtered data. The final output shows the average salary for those aged over 30, which is 70,000.

By combining these methods, our data manipulation becomes both powerful and concise. Try experimenting and see what you can create!

Brilliant job! You've now grasped the basics of data projection, filtering, and aggregation on Java lists. Plus, you've learned to package these operations in a Java class — a neat bundle of reusable code magic!

Now, why not try applying these fresh skills with some practice exercises? They're just around the corner. Ready? Let's dive into more fun with data manipulation!