Data transformation with Streams

Input/Output and Streams in Java

Alex Liu

Software Development Engineer

Data transformation with streams

Data transformation with streams
- Convert data formats, aggregate and filter information
- Store results in new data collection type(eg. from List to Set)
- Supported Collections:
  - Lists (e.g. ArrayList, LinkedList)
  - Sets (e.g. HashSet, TreeSet)
  - Maps (Keys/Values as Streams) (e.g., HashMap, TreeMap)
  - Arrays (via Arrays.stream())
Example ArrayList named namesto use for the rest of the video
```
names: ["Alice", "Bob", "Charlie", "David"]
```

Using Stream to transform elements

Preparation: import required class Set, Collectors and Stream

import java.util.Set;
import java.util.stream.Collectors;
import java.util.stream.Stream;


// Convert the `ArrayList` we created earlier `names` to a `Stream`
Stream<String> namesStream = names.stream();

Current data: [Alice, Bob, Charlie, David]

Using Stream to transform elements (Continued)

// Use `.map()` to convert each element to upper case
Stream<String> upperCaseNameStream = namesStream.map(
        name -> name.toUpperCase())

Current data: [ALICE, BOB, CHARLIE, DAVID]

// use `.collect()` and `Collectors.toSet()` to store the transformed stream data in a `set`
upperCaseNameStream.collect(Collectors.toSet())

Final data:
```
{CHARLIE, ALICE, BOB, DAVID}
```

Using Stream for aggregation

The .reduce() method
- Aggregates elements into a single result
- Useful for summation, concatenation, or finding min/max
Use .reduce() takes two inputs
- Initial Value: the starting value for the accumulation, e.g., 0
- Binary Operator: A function that takes two arguments
  - (x, y) -> x + y means:
    - x represents the accumulated total
    - y represents the current element being processed
    - The operation adds y to x, continuing until all elements are processed

Using Stream for aggregation: Example

Convert the ArrayList names to a Stream object.
```
Stream<String> stream = names.stream();
```

Use .map() to convert name to its length and use .reduce() to add them up together

stream
  .map(name -> name.length())
  .reduce(0, (sum, length) -> sum + length);

Result: 20

Let's practice!

Input/Output and Streams in Java