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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, 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.

Partner – Orkes – NPI EA (cat=Spring)
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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.

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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.

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eBook – Java Streams – NPI EA (cat=Java Streams)
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Since its introduction in Java 8, the Stream API has become a staple of Java development. The basic operations like iterating, filtering, mapping sequences of elements are deceptively simple to use.

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Spring Data JPA is a great way to handle the complexity of JPA with the powerful simplicity of Spring Boot.

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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.

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eBook – Java Streams – NPI (cat=Java Streams)
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Since its introduction in Java 8, the Stream API has become a staple of Java development. The basic operations like iterating, filtering, mapping sequences of elements are deceptively simple to use.

But these can also be overused and fall into some common pitfalls.

To get a better understanding on how Streams work and how to combine them with other language features, check out our guide to Java Streams:

>> Join Pro and download the eBook

1. Overview

In this quick article, we explain different ways of merging Java Streams – which is not a very intuitive operation.

2. Using Plain Java

The JDK 8 Stream class has some useful static utility methods. Let’s take a closer look at the concat() method.

2.1. Merging Two Streams

The simplest way to combine 2 Streams is to use the static Stream.concat() method:

@Test
public void whenMergingStreams_thenResultStreamContainsElementsFromBoth() {
    Stream<Integer> stream1 = Stream.of(1, 3, 5);
    Stream<Integer> stream2 = Stream.of(2, 4, 6);

    Stream<Integer> resultingStream = Stream.concat(stream1, stream2);

    assertEquals(
      Arrays.asList(1, 3, 5, 2, 4, 6),
      resultingStream.collect(Collectors.toList()));
}

2.2. Merging Multiple Streams

When we need to merge more than 2 Streams, things become a bit more complex. One possibility is to concatenate the first two streams, then concatenate the result with the next one and so on.

The next code snippet shows this in action:

@Test
public void given3Streams_whenMerged_thenResultStreamContainsAllElements() {
    Stream<Integer> stream1 = Stream.of(1, 3, 5);
    Stream<Integer> stream2 = Stream.of(2, 4, 6);
    Stream<Integer> stream3 = Stream.of(18, 15, 36);

    Stream<Integer> resultingStream = Stream.concat(
      Stream.concat(stream1, stream2), stream3);

    assertEquals(
      Arrays.asList(1, 3, 5, 2, 4, 6, 18, 15, 36),
      resultingStream.collect(Collectors.toList()));
}

As we can see, this approach becomes unfeasible for more streams. Of course, we can create intermediate variables or helper methods to make it more readable, but here is a better option:

@Test
public void given4Streams_whenMerged_thenResultStreamContainsAllElements() {
    Stream<Integer> stream1 = Stream.of(1, 3, 5);
    Stream<Integer> stream2 = Stream.of(2, 4, 6);
    Stream<Integer> stream3 = Stream.of(18, 15, 36);
    Stream<Integer> stream4 = Stream.of(99);

    Stream<Integer> resultingStream = Stream.of(
      stream1, stream2, stream3, stream4)
      .flatMap(i -> i);

    assertEquals(
      Arrays.asList(1, 3, 5, 2, 4, 6, 18, 15, 36, 99),
      resultingStream.collect(Collectors.toList()));
}

What happens here is:

  • We first create a new Stream containing the 4 Streams, which results in a Stream<Stream<Integer>>
  • Then we flatMap() this into a Stream<Integer> using the identity function

3. Using StreamEx

StreamEx is an open-source Java library that extends possibilities of Java 8 Streams. It uses the StreamEx class as an enhancement to the JDK’s Stream interface.

3.1. Merging Streams

The StreamEx library allows us to merge streams using the append() instance method:

@Test
public void given4Streams_whenMerged_thenResultStreamContainsAllElements() {
    Stream<Integer> stream1 = Stream.of(1, 3, 5);
    Stream<Integer> stream2 = Stream.of(2, 4, 6);
    Stream<Integer> stream3 = Stream.of(18, 15, 36);
    Stream<Integer> stream4 = Stream.of(99);

    Stream<Integer> resultingStream = StreamEx.of(stream1)
      .append(stream2)
      .append(stream3)
      .append(stream4);

    assertEquals(
      Arrays.asList(1, 3, 5, 2, 4, 6, 18, 15, 36, 99),
      resultingStream.collect(Collectors.toList()));
}

Since it is an instance method, we can easily chain it and append multiple streams.

Note that we could also create a List out of the stream by using toList() if we type the resultingStream variable to the StreamEx type.

3.2. Merging Streams Using prepend()

StreamEx also contains a method that adds elements before one another called prepend():

@Test
public void given3Streams_whenPrepended_thenResultStreamContainsAllElements() {
    Stream<String> stream1 = Stream.of("foo", "bar");
    Stream<String> openingBracketStream = Stream.of("[");
    Stream<String> closingBracketStream = Stream.of("]");

    Stream<String> resultingStream = StreamEx.of(stream1)
      .append(closingBracketStream)
      .prepend(openingBracketStream);

    assertEquals(
      Arrays.asList("[", "foo", "bar", "]"),
      resultingStream.collect(Collectors.toList()));
}

4. Using Jooλ

jOOλ is a JDK 8 compatible library that provides useful extensions to the JDK. The most important stream abstraction here is called Seq. Note that this is a sequential and ordered stream, so calling parallel() will have no effect.

4.1. Merging Streams

Just like the StreamEx library, jOOλ has an append() method:

@Test
public void given2Streams_whenMerged_thenResultStreamContainsAllElements() {
    Stream<Integer> seq1 = Stream.of(1, 3, 5);
    Stream<Integer> seq2 = Stream.of(2, 4, 6);

    Stream<Integer> resultingSeq = Seq.ofType(seq1, Integer.class)
      .append(seq2);

    assertEquals(
      Arrays.asList(1, 3, 5, 2, 4, 6),
      resultingSeq.collect(Collectors.toList()));
}

Also, there is a convenience toList() method if we type the resultingSeq variable to the jOOλ Seq type.

4.2. Merging Streams With prepend()

As expected, since an append() method exists, there is also a prepend() method in jOOλ:

@Test
public void given3Streams_whenPrepending_thenResultStreamContainsAllElements() {
    Stream<String> seq = Stream.of("foo", "bar");
    Stream<String> openingBracketSeq = Stream.of("[");
    Stream<String> closingBracketSeq = Stream.of("]");

    Stream<String> resultingStream = Seq.ofType(seq, String.class)
      .append(closingBracketSeq)
      .prepend(openingBracketSeq);

    Assert.assertEquals(
      Arrays.asList("[", "foo", "bar", "]"),
      resultingStream.collect(Collectors.toList()));
}

5. Conclusion

We saw that merging streams is relatively straightforward using JDK 8. When we need to do a lot of merging, it might be beneficial to use the StreamEx or jOOλ library for the sake of readability.

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.
Baeldung Pro – NPI EA (cat = Baeldung)
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Baeldung Pro comes with both absolutely No-Ads as well as finally with Dark Mode, for a clean learning experience:

>> Explore a clean Baeldung

Once the early-adopter seats are all used, the price will go up and stay at $33/year.

Partner – Microsoft – NPI EA (cat = Spring Boot)
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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.

Partner – Orkes – NPI EA (cat = Spring)
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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.

Partner – Orkes – NPI EA (tag = Microservices)
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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.

eBook – HTTP Client – NPI EA (cat=HTTP Client-Side)
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The Apache HTTP Client is a very robust library, suitable for both simple and advanced use cases when testing HTTP endpoints. Check out our guide covering basic request and response handling, as well as security, cookies, timeouts, and more:

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eBook – Java Concurrency – NPI EA (cat=Java Concurrency)
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Handling concurrency in an application can be a tricky process with many potential pitfalls. A solid grasp of the fundamentals will go a long way to help minimize these issues.

Get started with understanding multi-threaded applications with our Java Concurrency guide:

>> Download the eBook

eBook – Java Streams – NPI EA (cat=Java Streams)
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Since its introduction in Java 8, the Stream API has become a staple of Java development. The basic operations like iterating, filtering, mapping sequences of elements are deceptively simple to use.

But these can also be overused and fall into some common pitfalls.

To get a better understanding on how Streams work and how to combine them with other language features, check out our guide to Java Streams:

>> Join Pro and download the eBook

eBook – Persistence – NPI EA (cat=Persistence)
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eBook – Java Streams – NPI (cat=Java Streams)
announcement - icon

Since its introduction in Java 8, the Stream API has become a staple of Java development. The basic operations like iterating, filtering, mapping sequences of elements are deceptively simple to use.

But these can also be overused and fall into some common pitfalls.

To get a better understanding on how Streams work and how to combine them with other language features, check out our guide to Java Streams:

>> Join Pro and download the eBook

eBook Jackson – NPI EA – 3 (cat = Jackson)