如何使用索引遍历Stream

1. Overview

Java 8 Streams are not collections and elements cannot be accessed using their indices, but there are still a few tricks to make this possible.

In this short article, we’re going to look at how to iterate over a Stream using IntStream, StreamUtils, EntryStream, and Vavr‘s Stream.

2. Using Plain Java

We can navigate through a Stream using an Integer range, and also benefit from the fact that the original elements are in an array or a collection accessible by indices.

Let’s implement a method which iterates with indices and demonstrates this approach.

Simply put, we want to get an array of Strings and only select even indexed elements:

public List<String> getEvenIndexedStrings(String[] names) {
    List<String> evenIndexedNames = IntStream
      .range(0, names.length)
      .filter(i -> i % 2 == 0)
      .mapToObj(i -> names[i])
      .collect(Collectors.toList());
    
    return evenIndexedNames;
}

Let’s now test out the implementation:

@Test
public void whenCalled_thenReturnListOfEvenIndexedStrings() {
    String[] names 
      = {"Afrim", "Bashkim", "Besim", "Lulzim", "Durim", "Shpetim"};
    List<String> expectedResult 
      = Arrays.asList("Afrim", "Besim", "Durim");
    List<String> actualResult 
      = StreamIndices.getEvenIndexedStrings(names);
   
    assertEquals(expectedResult, actualResult);
}

3. Using StreamUtils

Another way to iterate with indices can be done using zipWithIndex() method of StreamUtils from the proton-pack library (the latest version can be found here).

First, you need to add it to your pom.xml:

<dependency>
    <groupId>com.codepoetics</groupId>
    <artifactId>protonpack</artifactId>
    <version>1.16</version>
</dependency>

Now, let’s look at the code:

public List<Indexed<String>> getEvenIndexedStrings(List<String> names) {
    List<Indexed<String>> list = StreamUtils
      .zipWithIndex(names.stream())
      .filter(i -> i.getIndex() % 2 == 0)
      .collect(Collectors.toList());
    
    return list;
}

The following tests this method and passes successfully:

@Test
public void whenCalled_thenReturnListOfEvenIndexedStrings() {
    List<String> names = Arrays.asList(
      "Afrim", "Bashkim", "Besim", "Lulzim", "Durim", "Shpetim");
    List<Indexed<String>> expectedResult = Arrays.asList(
      Indexed.index(0, "Afrim"), 
      Indexed.index(2, "Besim"), 
      Indexed.index(4, "Durim"));
    List<Indexed<String>> actualResult 
      = StreamIndices.getEvenIndexedStrings(names);
    
    assertEquals(expectedResult, actualResult);
}

4. Using StreamEx

We can also iterate with indexes using filterKeyValue() of EntryStream class from StreamEx library (the latest version can be found here). First, we need to add it to our pom.xml:

<dependency>
    <groupId>one.util</groupId>
    <artifactId>streamex</artifactId>
    <version>0.8.1</version>
</dependency>

Let’s see a simple application of this method using our previous example:

public List<String> getEvenIndexedStringsVersionTwo(List<String> names) {
    return EntryStream.of(names)
      .filterKeyValue((index, name) -> index % 2 == 0)
      .values()
      .toList();
}

We’ll use a similar test to test this:

@Test
public void whenCalled_thenReturnListOfEvenIndexedStringsVersionTwo() {
    String[] names 
      = {"Afrim", "Bashkim", "Besim", "Lulzim", "Durim", "Shpetim"};
    List<String> expectedResult 
      = Arrays.asList("Afrim", "Besim", "Durim");
    List<String> actualResult 
      = StreamIndices.getEvenIndexedStrings(names);
   
   assertEquals(expectedResult, actualResult);
}

5. Iteration Using **Vavre‘s Stream**

Another plausible way of iteration is using zipWithIndex() method of Vavr (previously known as Javaslang)’s Stream implementation:

<dependency>    
     <groupId>io.vavr</groupId>    
     <artifactId>vavr</artifactId>    
    <version>0.10.4</version>    
</dependency>

public List<String> getOddIndexedStringsVersionTwo(String[] names) {
    return Stream
      .of(names)
      .zipWithIndex()
      .filter(tuple -> tuple._2 % 2 == 1)
      .map(tuple -> tuple._1)
      .toJavaList();
}

We can test this example with the following method:

@Test
public void whenCalled_thenReturnListOfOddStringsVersionTwo() {
    String[] names 
      = {"Afrim", "Bashkim", "Besim", "Lulzim", "Durim", "Shpetim"};
    List<String> expectedResult 
      = Arrays.asList("Bashkim", "Lulzim", "Shpetim");
    List<String> actualResult 
      = StreamIndices.getOddIndexedStringsVersionTwo(names);

    assertEquals(expectedResult, actualResult);
}

If you want to read more about Vavr, check this article.

6. Conclusion

In this quick tutorial, we saw four approaches on how to iterate through streams using indices. Streams have gotten a lot of attention and being able to also iterate through them with indices can be helpful.

There are a lot of features that are included in Java 8 Streams, some of which are already covered on Baeldung.

The code for all the examples explained here, and much more can be found over on GitHub.

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