Streams

A stream is a sequence of elements from a collection that supports processing data in a declarative, chainable way. Rather than manually writing a loop to filter, transform, and collect data, the Stream API lets you describe what you want done, and Java handles how it gets done. For more information about the collections streams operate on, see the Data Structures section.

Creating a Stream

A stream is created by calling .stream() on a collection. Creating a stream doesn't modify the original collection; it just opens a pipeline for processing its elements.

ArrayList<Integer> canIds = new ArrayList<Integer>();
canIds.add(3);
canIds.add(7);
canIds.add(12);

canIds.stream(); // Creates a stream over the ArrayList's elements

Intermediate Operations

Intermediate operations transform a stream into another stream, and can be chained together. They aren't actually executed until a terminal operation is called.

filter()

Keeps only the elements that match a given condition, provided as a lambda. For more information about lambda expressions, see the Lambda Expressions section.

canIds.stream()
      .filter(id -> id > 5); // Keeps only 7 and 12

map()

Transforms each element into something else, such as converting a list of CAN IDs into a list of their corresponding motor names.

canIds.stream()
      .map(id -> "Motor " + id); // Transforms each int into a String

Terminal Operations

Terminal operations end the stream pipeline and produce a result, such as a value, a collection, or a side effect.

Method	Description
forEach()	Performs an action on each element
count()	Returns the number of elements
collect()	Gathers the results back into a collection

canIds.stream()
      .filter(id -> id > 5)
      .forEach(id -> System.out.println(id));

Output:

7
12

Chaining It All Together

The real value of streams comes from chaining multiple operations into a single readable pipeline, instead of nesting loops and if-statements.

ArrayList<Integer> highCanIds = canIds.stream()
      .filter(id -> id > 5)
      .collect(Collectors.toList());

This reads almost like a sentence: "take the CAN IDs, filter for the ones greater than 5, and collect them into a list." Streams are best suited for straightforward data transformations; for more complex or performance-critical logic, a traditional loop (see the Loops section) is often clearer.