Visualize Java Stream Debugging: Turn Complex Streams into Clear Views

Preface

Since Java 8, developers rely heavily on Stream APIs, but the results of stream operations sometimes do not match expectations, requiring step‑by‑step debugging to locate issues.

Conventional Debugging

Consider the following code:

public static void main(String[] args) {
  Object[] res = Stream.of(1,2,3,4,5,6,7,8)
    .filter(i -> i%2 == 0)
    .filter(i -> i>3)
    .toArray();
  System.out.println(Arrays.toString(res));
}

Setting breakpoints on the Stream operations lets you inspect intermediate results. However, this approach is not very intuitive, and a visual overview is often needed.

Visual Debugging

Enter Debug mode and run the program. The tool opens a Stream Trace view that displays the entire Stream pipeline visually.

You can also click the Flat Mode button to flatten the view.

More Complex Example

In real projects, streams often operate on collections of objects. The following example demonstrates a more involved pipeline:

List<Optional<Customer>> customers = Arrays.asList(
    Optional.of(new Customer("日拱一兵", 18)),
    Optional.of(new Customer("卑微的小开发", 22)),
    Optional.empty(),
    Optional.of(new Customer("OOT", 21)),
    Optional.empty(),
    Optional.of(new Customer("温柔一刀", 23)),
    Optional.empty()
);

long numberOf65PlusCustomers = customers
    .stream()
    .flatMap(c -> c
      .map(Stream::of)
      .orElseGet(Stream::empty))
    .filter(c -> c.getAge() > 18)
    .count();

System.out.println(numberOf65PlusCustomers);

Running this code in the visual debugger produces a Stream Trace view that clearly shows the flow of data and the evaluation of each operation.

Conclusion

This simple yet powerful feature can greatly improve daily debugging of Stream pipelines. Stay tuned for more advanced debugging techniques you may not have noticed before.

Have you used this visual debugging feature? What other advanced tricks do you know?