How to Find All Maximum Values in a Java Stream?

How to Retrieve All Maximum Values in a Java Stream

In Java 8, the max function can be used on lambda and streams to identify the maximum value in a dataset. However, when multiple elements have the same maximum value, max arbitrarily chooses and returns one of them.

Expected Behavior:

The goal is to retrieve a list of all elements that share the maximum value, not just a single representative.

Solution:

Since the API lacks an explicit method for this functionality, you can implement a custom solution using intermediate storage and collection manipulation. Here are two approaches:

1. Two-Pass Solution (For Collections):

• Find the maximum value using max and store it.
• Filter the collection to include only elements that match the maximum value.

List<String> list = ... ;

int longest = list.stream()
                  .mapToInt(String::length)
                  .max()
                  .orElse(-1);

List<String> result = list.stream()
                          .filter(s -> s.length() == longest)
                          .collect(toList());

2. Single-Pass Solution (For Streams):

• Employ a custom collector that maintains a list of all elements with the current maximum value during stream processing.
• Return the collected list as the result.

static <T> Collector<T, ?, List<T>> maxList(Comparator<? super T> comp) {
    // Implementation goes here (provided in the original response)
}

Stream<String> input = ... ;

List<String> result = input.collect(maxList(comparing(String::length)));

This custom collector tracks intermediate results and updates its state based on each element's value compared to the current maximum. By leveraging this approach, you can obtain a list containing all maximum values from your stream or collection.

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