Java 8 Stream findFirst() vs. findAny()

Table of Contents

1. Overview
2. Using Stream.findAny()
3. Using Stream.findFirst()
4. Conclusion

1. Overview

The Java 8 Stream API introduced two methods that are often misunderstood: findAny() and findFirst().

In this quick tutorial, we’ll look at the difference between these two methods and when to use them.

2. Using Stream.findAny()

As the name suggests, the findAny() method allows us to find any element from a Stream. We use it when we’re looking for an element without paying an attention to the encounter order:

The method returns an Optional instance, which is empty if the Stream is empty:

@Test
public void createStream_whenFindAnyResultIsPresent_thenCorrect() {
    List<String> list = Arrays.asList("A","B","C","D");

    Optional<String> result = list.stream().findAny();

    assertTrue(result.isPresent());
    assertThat(result.get(), anyOf(is("A"), is("B"), is("C"), is("D")));
}

In a non-parallel operation, it will most likely return the first element in the Stream, but there is no guarantee for this.

For maximum performance when processing the parallel operation, the result cannot be reliably determined:

@Test
public void createParallelStream_whenFindAnyResultIsPresent_thenCorrect()() {
    List<Integer> list = Arrays.asList(1, 2, 3, 4, 5);
    Optional<Integer> result = list
      .stream().parallel()
      .filter(num -> num < 4).findAny();

    assertTrue(result.isPresent());
    assertThat(result.get(), anyOf(is(1), is(2), is(3)));
}

3. Using Stream.findFirst()

The findFirst() method finds the first element in a Stream. So, we use this method when we specifically want the first element from a sequence.

When there is no encounter order, it returns any element from the Stream. According to the java.util.streams package documentation, “Streams may or may not have a defined encounter order. It depends on the source and the intermediate operations.”

The return type is also an Optional instance, which is empty if the Stream is empty too:

@Test
public void createStream_whenFindFirstResultIsPresent_thenCorrect() {

    List<String> list = Arrays.asList("A", "B", "C", "D");

    Optional<String> result = list.stream().findFirst();

    assertTrue(result.isPresent());
    assertThat(result.get(), is("A"));
}

The behavior of the findFirst method does not change in the parallel scenario. If the encounter order exists, it will always behave deterministically.