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Debugging Streams with Peek

April 05, 2024


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    Shai Almog

    Author, DevRel, Blogger, Open Source Hacker, Java Rockstar, Conference Speaker, Instructor and Entrepreneur.

Table of Contents

I blogged about Java Stream debugging in the past but I skipped an important method that's worthy of a post of its own: peek.

This article delves into the practicalities of using peek() to debug Java streams, complete with code samples and common pitfalls.

Understanding Java Streams

Java Streams represent a significant shift in how Java developers work with collections and data processing, introducing a functional approach to handling sequences of elements.

Streams facilitate declarative processing of collections, enabling operations such as filter, map, reduce, and more in a fluent style.

This not only makes the code more readable but also more concise compared to traditional iterative approaches.

A Simple Stream Example

To illustrate, consider the task of filtering a list of names to only include those that start with the letter "J" and then transforming each name into uppercase. Using the traditional approach, this might involve a loop and some if statements. However, with streams, this can be accomplished in a few lines:

List<String> names = Arrays.asList("John", "Jacob", "Edward", "Emily");
// Convert list to stream
List<String> filteredNames =       
                  // Filter names that start with "J"
                  .filter(name -> name.startsWith("J"))  
                  // Convert each name to uppercase
                  // Collect results into a new list



This example demonstrates the power of Java streams: by chaining operations together, we can achieve complex data transformations and filtering with minimal, readable code.

It showcases the declarative nature of streams, where we describe what we want to achieve rather than detailing the steps to get there.

What is the peek() Method?

At its core, peek() is a method provided by the Stream interface, allowing developers a glance into the elements of a stream without disrupting the flow of its operations.

The signature of peek() is as follows:

Stream<T> peek(Consumer<? super T> action)

It accepts a Consumer functional interface, which means it performs an action on each element of the stream without altering them. The most common use case for peek() is logging the elements of a stream to understand the state of data at various points in the stream pipeline.

To understand peek lets look at a sample similar to the previous one:

List<String> collected = Stream.of("apple", "banana", "cherry")
                               .filter(s -> s.startsWith("a"))

This code filters a list of strings, keeping only the ones that start with "a".

While it's straightforward, understanding what happens during the filter operation is not visible.

Debugging with peek()

Now, let's incorporate peek() to gain visibility into the stream:

List<String> collected = Stream.of("apple", "banana", "cherry")
                               .peek(System.out::println) // Logs all elements
                               .filter(s -> s.startsWith("a"))
                               .peek(System.out::println) // Logs filtered elements

By adding peek() both before and after the filter operation, we can see which elements are processed and how the filter impacts the stream. This visibility is invaluable for debugging, especially when the logic within the stream operations becomes complex.

We can't step over stream operations with the debugger, but peek() provides a glance into the code that is normally obscured from us.

Uncovering Common Bugs with peek()

Filtering Issues

Consider a scenario where a filter condition is not working as expected:

List<String> collected = Stream.of("apple", "banana", "cherry", "Avocado")
                               .filter(s -> s.startsWith("a"))

Expected output might be ["apple"], but let's say we also wanted "Avocado" due to a misunderstanding of the startsWith method's behavior. Since "Avocado" is spelled with an upper case "A" this code will return false: Avocado".startsWith("a").

Using peek(), we can observe the elements that pass the filter:

List<String> debugged = Stream.of("apple", "banana", "cherry", "Avocado")
                              .filter(s -> s.startsWith("a"))

Large Data Sets

In scenarios involving large datasets, directly printing every element in the stream to the console for debugging can quickly become impractical.

It can clutter the console and make it hard to spot the relevant information. Instead, we can use peek() in a more sophisticated way to selectively collect and analyze data without causing side effects that could alter the behavior of the stream.

Consider a scenario where we're processing a large dataset of transactions, and we want to debug issues related to transactions exceeding a certain threshold:

class Transaction {
    private String id;
    private double amount;

    // Constructor, getters, and setters omitted for brevity

List<Transaction> transactions = // Imagine a large list of transactions

// A placeholder for debugging information
List<Transaction> highValueTransactions = new ArrayList<>();

List<Transaction> processedTransactions =
    // Filter transactions above a threshold
    .filter(t -> t.getAmount() > 5000) 
    .peek(t -> {
        if (t.getAmount() > 10000) {
            // Collect only high-value transactions for debugging

// Now, we can analyze high-value transactions separately, without overloading the console
System.out.println("High-value transactions count: " + 

In this approach, peek() is used to inspect elements within the stream conditionally. High-value transactions that meet a specific criterion (e.g., amount > 10,000) are collected into a separate list for further analysis.

This technique allows for targeted debugging without printing every element to the console, thereby avoiding performance degradation and clutter.

Addressing Side Effects

While streams shouldn't have side effects. In fact, such side effects would break the stream debugger in IntelliJ which I discussed in the past. It's crucial to note that while collecting data for debugging within peek() avoids cluttering the console, it does introduce a side effect to the stream operation, which goes against the recommended use of streams.

Streams are designed to be side-effect-free to ensure predictability and reliability, especially in parallel operations.

Therefore, while the above example demonstrates a practical use of peek() for debugging, it's important to use such techniques judiciously. Ideally, this debugging strategy should be temporary and removed once the debugging session is completed to maintain the integrity of the stream's functional paradigm.

Limitations and Pitfalls

While peek() is undeniably a useful tool for debugging Java streams, it comes with its own set of limitations and pitfalls that developers should be aware of.

Understanding these can help avoid common traps and ensure that peek() is used effectively and appropriately.

Potential for Misuse in Production Code

One of the primary risks associated with peek() is its potential for misuse in production code. Because peek() is intended for debugging purposes, using it to alter state or perform operations that affect the outcome of the stream can lead to unpredictable behavior.

This is especially true in parallel stream operations, where the order of element processing is not guaranteed. Misusing peek() in such contexts can introduce hard-to-find bugs and undermine the declarative nature of stream processing.

Performance Overhead

Another consideration is the performance impact of using peek(). While it might seem innocuous, peek() can introduce a significant overhead, particularly in large or complex streams. This is because every action within peek() is executed for each element in the stream, potentially slowing down the entire pipeline.

When used excessively or with complex operations, peek() can degrade performance, making it crucial to use this method judiciously and remove any peek() calls from production code after debugging is complete.

Side Effects and Functional Purity

As highlighted in the enhanced debugging example, peek() can be used to collect data for debugging purposes, but this introduces side effects to what should ideally be a side-effect-free operation. The functional programming paradigm, which streams are a part of, emphasizes purity and immutability. Operations should not alter state outside their scope.

By using peek() to modify external state (even for debugging), you're temporarily stepping away from these principles. While this can be acceptable for short-term debugging, it's important to ensure that such uses of peek() do not find their way into production code, as they can compromise the predictability and reliability of your application.

The Right Tool for the Job

Finally, it's essential to recognize that peek() is not always the right tool for every debugging scenario. In some cases, other techniques such as logging within the operations themselves, using breakpoints and inspecting variables in an IDE, or writing unit tests to assert the behavior of stream operations might be more appropriate and effective.

Developers should consider peek() as one tool in a broader debugging toolkit, employing it when it makes sense and opting for other strategies when they offer a clearer or more efficient path to identifying and resolving issues.

Navigating the Pitfalls

To navigate these pitfalls effectively:

  • Reserve peek() strictly for temporary debugging purposes. If you have a linter as part of your CI tools it might make sense to add a rule that block code from invoking peek().
  • Always remove peek() calls from your code before committing it to your codebase, especially for production deployments.
  • Be mindful of performance implications and the potential introduction of side effects.
  • Consider alternative debugging techniques that might be more suited to your specific needs or the particular issue you're investigating.

By understanding and respecting these limitations and pitfalls, developers can leverage peek() to enhance their debugging practices without falling into common traps or inadvertently introducing problems into their codebases.

Final Thoughts

The peek() method offers a simple yet effective way to gain insights into Java stream operations, making it a valuable tool for debugging complex stream pipelines.

By understanding how to use peek() effectively, developers can avoid common pitfalls and ensure their stream operations perform as intended.

As with any powerful tool, the key is to use it wisely and in moderation.

The true value of peek() is in debugging massive data sets, these elements are very hard to analyze even with dedicated tools.

By using peek() we can dig into said data set and understand the source of the issue programmatically.

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  • Avatar photo
    Shai Almog

    Author, DevRel, Blogger, Open Source Hacker, Java Rockstar, Conference Speaker, Instructor and Entrepreneur.

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