1. Introduction

Let's look at how to find max and min values from a List or array using the Java 8 Stream API.

2. Finding the Max Value Using Streams

2.1 Finding the Max Value in a List of Numbers

@Test
public void 숫자_list_max_값_찾기() {
        List<Integer> intList = Arrays.asList(2, 3, 6, 4, 10, 23);
        Integer maxValue = intList.stream()
                .mapToInt(x -> x)
                .max()
                .orElseThrow(NoSuchElementException::new);
//                .orElseThrow(() -> new NoSuchElementException());
        
        assertThat(maxValue).isEqualTo(23);
}

• stream() : to find the max value in a list of numbers
• mapToInt() : converts the Stream into an Int primitive stream
• max()
  • it is a method provided by IntStream
  • returns the result as an Optional
• orElseThrow()
  • throws a NoSuchElementException exception when there is no result

You can also use max() provided by Stream without converting to IntStream.

@Test
public void 숫자_list_max_값_찾기2() {
        List<Integer> intList = Arrays.asList(2, 3, 6, 4, 23, 10);
        Integer maxValue = intList.stream()
                .max(Comparator.comparing(x -> x))
                .orElseThrow(NoSuchElementException::new);

        assertThat(maxValue).isEqualTo(23);
}

• max()
  • takes a Comparator as an argument value.
  • to easily create a Comparator, the Comparator.comparing() method was used

2.2 Finding the Max Value in an Array of Numbers

@Test
public void 숫자_array_max_값_찾기() {
        int[] intArr = {3, 2, 6, 10, 234};
        Integer maxValue = Arrays.stream(intArr)
                .max()
                .getAsInt();
        assertThat(maxValue).isEqualTo(234);
}

• Arrays.stream()
  • converts a primitive array into an IntStream stream
• max()
  • returns the result as an Optional object
• getAsInt()
  • returns an int value.
  • if there is no max value, a NoSuchElementException exception can occur

2.3 Finding the Object with the Maximum Value of a Specific Field in a List of Objects

In this example, let's look at how to find the maximum value of a specific field in objects. This is an example of finding the oldest student among students.

@Test
public void 객체_list에서_나이가_max_값_찾기() {
        int max = 5;
        List<Student> students = IntStream.range(0, max)
                .mapToObj(i -> new Student("name" + i, i + 10))
                .peek(i -> log.info("{}", i))
                .collect(Collectors.toList());

//        Comparator<Student> comparatorByAge = (x1, x2) -> Integer.compare(x1.getAge(), x2.getAge());
        Comparator<Student> comparatorByAge = Comparator.comparingInt(Student::getAge);

        Student studentWithMaxAge = students.stream()
                .max(comparatorByAge)
                .orElseThrow(NoSuchElementException::new);

        assertThat(studentWithMaxAge.getAge()).isEqualTo(14);
}

• Creating a list of Student objects
  • instead of a for loop, IntStream and mapToObj() are used to create a List of student objects
• Finding the oldest student among the students
  • to find the maximum age value among the objects, comparatorByAge is passed as the argument value to the max() method.

2.4 Finding the Longest String in a String Array

@Test
public void array_str에서_가장_긴_string의_길이_찾기() {
        String[] lines = {"Hello", "My", "World11"};
        int maxWidth = Arrays.stream(lines).mapToInt(String::length).max().getAsInt();
        assertThat(maxWidth).isEqualTo(7);
}

• mapToInt()
  • when converting to an IntStream, it converts to the length of the string and finds the max value

3. Wrapping Up

In this post, we looked at how to find the maximum value in a List or Array using the max(), min() methods of the Java 8 Stream API.

For the example, please refer to the Github source.

4. References

• https://stackoverflow.com/questions/52443550/how-to-find-max-length-in-list-of-string-using-streams-in-java
• https://www.baeldung.com/java-collection-min-max
• https://codechacha.com/ko/java8-stream-max-min/