Java Streams peek() API

Azure Container Apps is a fully managed serverless container service that enables you to build and deploy modern, cloud-native Java applications and microservices at scale. It offers a simplified developer experience while providing the flexibility and portability of containers.

Of course, Azure Container Apps has really solid support for our ecosystem, from a number of build options, managed Java components, native metrics, dynamic logger, and quite a bit more.

To learn more about Java features on Azure Container Apps, visit the documentation page.

You can also ask questions and leave feedback on the Azure Container Apps GitHub page.

Of course, Azure Container Apps has really solid support for our ecosystem, from a number of build options, managed Java components, native metrics, dynamic logger, and quite a bit more.

To learn more about Java features on Azure Container Apps, you can get started over on the documentation page.

And, you can also ask questions and leave feedback on the Azure Container Apps GitHub page.

Modern software architecture is often broken. Slow delivery leads to missed opportunities, innovation is stalled due to architectural complexities, and engineering resources are exceedingly expensive.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

With Orkes Conductor managed through Orkes Cloud, developers can focus on building mission critical applications without worrying about infrastructure maintenance to meet goals and, simply put, taking new products live faster and reducing total cost of ownership.

Try a 14-Day Free Trial of Orkes Conductor today.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

Try a 14-Day Free Trial of Orkes Conductor today.

End-to-end testing is a very useful method to make sure that your application works as intended. This highlights issues in the overall functionality of the software, that the unit and integration test stages may miss.

Playwright is an easy-to-use, but powerful tool that automates end-to-end testing, and supports all modern browsers and platforms.

When coupled with LambdaTest (an AI-powered cloud-based test execution platform) it can be further scaled to run the Playwright scripts in parallel across 3000+ browser and device combinations:

>> Automated End-to-End Testing With Playwright

1. Introduction

The Java Stream API introduces us to a powerful alternative for processing data.

In this short tutorial, we’ll focus on peek(), an often misunderstood method.

2. Quick Example

Let’s get our hands dirty and try to use peek(). We have a stream of names, and we want to print them to the console.

Since peek() expects a Consumer<T> as its only argument, it seems like a good fit, so let’s give it a try:

Stream<String> nameStream = Stream.of("Alice", "Bob", "Chuck");
nameStream.peek(System.out::println);

However, the snippet above produces no output. To understand why, let’s do a quick refresher on aspects of the stream lifecycle.

3. Intermediate vs. Terminal Operations

Recall that streams have three parts: a data source, zero or more intermediate operations, and zero or one terminal operation.

The source provides the elements to the pipeline.

Intermediate operations get elements one by one and process them. All intermediate operations are lazy, and, as a result, no operations will have any effect until the pipeline starts to work.

Terminal operations mean the end of the stream lifecycle. Most importantly for our scenario, they initiate the work in the pipeline.

4. peek() Usage

The reason peek() didn’t work in our first example is that it’s an intermediate operation and we didn’t apply a terminal operation to the pipeline. Alternatively, we could have used forEach() with the same argument to get the desired behavior:

Stream<String> nameStream = Stream.of("Alice", "Bob", "Chuck");
nameStream.forEach(System.out::println);

peek()‘s Javadoc page says: “This method exists mainly to support debugging, where you want to see the elements as they flow past a certain point in a pipeline“.

Let’s consider this snippet from the same Javadoc page:

Stream.of("one", "two", "three", "four")
  .filter(e -> e.length() > 3)
  .peek(e -> System.out.println("Filtered value: " + e))
  .map(String::toUpperCase)
  .peek(e -> System.out.println("Mapped value: " + e))
  .collect(Collectors.toList());

It demonstrates, how we observe the elements that passed each operation.

On top of that, peek() can be useful in another scenario: when we want to alter the inner state of an element. For example, let’s say we want to convert all user’s name to lowercase before printing them:

Stream<User> userStream = Stream.of(new User("Alice"), new User("Bob"), new User("Chuck"));
userStream.peek(u -> u.setName(u.getName().toLowerCase()))
  .forEach(System.out::println);

Alternatively, we could have used map(), but peek() is more convenient since we don’t want to replace the element.

5. peek() vs map()

In Java Streams, peek() and map() serve distinct purposes and should be used appropriately. map() is used for transforming the element such as for converting the type of element or changing the element:

List<Integer> integers = Arrays.asList(1, 2, 3, 4)
List<Integer> transformedElements = integers.stream()
  .map(e -> e * 2)
  .collect(Collectors.toList());
List<Integer> expected = Arrays.asList(2, 4, 6,  8);
assertEquals(expected, transformedElements)

peek() is mainly used for debugging or observing elements:

integers.stream()
  .peek(System.out::println)
  .collect(Collectors.toList());

Elements that are processed inside peek() might not be eligible for terminal operation:

List<Integer> peekedList = new ArrayList<>();
List<Integer> result = integers.stream()
  .peek(peekedList::add)
  .filter(e -> e < 3)
  .collect(Collectors.toList());
assertEquals(Arrays.asList(1, 2), result);
assertEquals(Arrays.asList(1, 2, 3, 4), peekedList);

As we can see peekedList contains all the elements while the result only contains filtered results.

It’s tempting to use peek() to transform the elements, but we should always keep in mind that peek() doesn’t transform and pass the element downstream in a stream operation chain, unlike map(). Hence to transform the elements we should always use a map(), while with peek() we should use it for debugging or logging purposes.

6. Conclusion

In this short article, we saw a summary of the stream lifecycle to understand how peek() works. We also saw two everyday use cases when using peek() is the most straightforward option.

The code backing this article is available on GitHub. Once you're logged in as a Baeldung Pro Member, start learning and coding on the project.

Of course, Azure Container Apps has really solid support for our ecosystem, from a number of build options, managed Java components, native metrics, dynamic logger, and quite a bit more.

To learn more about Java features on Azure Container Apps, visit the documentation page.

You can also ask questions and leave feedback on the Azure Container Apps GitHub page.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

Try a 14-Day Free Trial of Orkes Conductor today.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

Try a 14-Day Free Trial of Orkes Conductor today.