Finding Max/Min of a List or Collection

1. Overview

This tutorial is a quick intro on how to find the min and max values from a given list or collection with the powerful Stream API in Java 8.

2. Find Max in a List of Integers

We can use the max() method provided through the java.util.Stream interface, which accepts a method reference:

@Test
public void whenListIsOfIntegerThenMaxCanBeDoneUsingIntegerComparator() {
    // given
    List<Integer> listOfIntegers = Arrays.asList(1, 2, 3, 4, 56, 7, 89, 10);
    Integer expectedResult = 89;

    // then
    Integer max = listOfIntegers
      .stream()
      .mapToInt(v -> v)
      .max().orElseThrow(NoSuchElementException::new);

    assertEquals("Should be 89", expectedResult, max);
}

Let's take a closer look at the code:

1. Calling stream() method on the list to get a stream of values from the list
2. Calling mapToInt(value -> value) on the stream to get an Integer Stream
3. Calling max() method on the stream to get the max value
4. Calling orElseThrow() to throw an exception if no value is received from max()

3. Find Min With Custom Objects

In order to find the min/max on custom objects, we can also provide a lambda expression for our preferred sorting logic.

Let's first define the custom POJO:

class Person {
    String name;
    Integer age;
      
    // standard constructors, getters and setters
}

We want to find the Person object with the minimum age:

@Test
public void whenListIsOfPersonObjectThenMinCanBeDoneUsingCustomComparatorThroughLambda() {
    // given
    Person alex = new Person("Alex", 23);
    Person john = new Person("John", 40);
    Person peter = new Person("Peter", 32);
    List<Person> people = Arrays.asList(alex, john, peter);

    // then
    Person minByAge = people
      .stream()
      .min(Comparator.comparing(Person::getAge))
      .orElseThrow(NoSuchElementException::new);

    assertEquals("Should be Alex", alex, minByAge);
}

Let's have a look at this logic:

1. Calling stream() method on the list to get a stream of values from the list
2. Calling min() method on the stream to get the minimum value. We pass a lambda function as a comparator, and this is used to decide the sorting logic for deciding the minimum value.
3. Calling orElseThrow() to throw an exception if no value is received from min()

4. Find Min/Max in an ArrayList Along With the Index Number

To determine the minimum or maximum value in an ArrayList, we can either use the method we saw earlier or the min() and max() methods of the Java Collections class. Those methods return the minimum and maximum element of a given collection, respectively.

Further, we can use the indexOf() method of the ArrayList class to get the index of an element in a list. This method returns the index of the first occurrence of an element in the list.

Let's take this example:

List<Integer> listOfIntegers = Arrays.asList(11, 13, 9, 20, 7, 3, 30);
Integer expectedMinValue = 3;
Integer expectedMinIndex = 5;

Integer minValue = Collections.min(listOfIntegers);
Integer minIndex = listOfIntegers.indexOf(minValue);

assertEquals(minValue, expectedMinValue);
assertEquals(minIndex, expectedMinIndex);

First, we defined a list of Integers to store our values. Then we used Collections.min() to get the minimum value of the list, which in our case, equals three. Finally, we used listOfIntegers.indexOf() to get the index of this value in the list, which in our case, equals five.

5. Conclusion

In this quick article, we explored how the max() and min() methods from the Java 8 Stream API can be used to find the maximum and minimum value from a List or Collection.

As always, the code is available over on GitHub.

res – REST with Spring (eBook) (everywhere)

Learning to build your API
with Spring?

Download the E-book