Stream map in order to find value of latest key

Question

I have a Map<Element, Attributes> consisting of instances of the following (example) class and enum, where I want to get the value of the most recent key via stream() . The most recent key can be determined by the property creationTime of the class Element and the corresponding value in the Map is just an enum value:

import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;

public class Element implements Comparable<Element> {

    String abbreviation;
    LocalDateTime creationTime;

    public Element(String abbreviation, LocalDateTime creationTime) {
        this.abbreviation = abbreviation;
        this.creationTime = creationTime;
    }

    public String getAbbreviation() {
        return abbreviation;
    }

    public void setAbbreviation(String abbreviation) {
        this.abbreviation = abbreviation;
    }

    public LocalDateTime getCreationTime() {
        return creationTime;
    }

    public void setCreationTime(LocalDateTime creationTime) {
        this.creationTime = creationTime;
    }

    /*
     * (non-Javadoc)
     * 
     * @see java.lang.Comparable#compareTo(java.lang.Object)
     */
    @Override
    public int compareTo(Element otherElement) {
        return this.creationTime.compareTo(otherElement.getCreationTime());
    }

    @Override
    public String toString() {
        return "[" + abbreviation + ", " + creationTime.format(DateTimeFormatter.ISO_LOCAL_DATE_TIME) + "]";
    }
}

Please not that an Element implements Comparable<Element> just using the built-in comparison of LocalDateTime .

public enum Attributes {

    DONE,
    FIRST_REGISTRATION,
    SUBSEQUENT_REGISTRATION
}

My current approach is just able to filter the keySet and find the most recent key, which I then use to simply get the value in a new line of code. I was wondering if it is possible in a single stream().filter(...) statement:

Map<Element, Attributes> results = new TreeMap<>();

// filling the map with random elements and attributes

Element latestKey = results.keySet().stream().max(Element::compareTo).get();
Attributes latestValue = results.get(latestKey);

Can we get a value by filtering the keySet of a Map in a single stream() statement like

Attributes latestValue = results.keySet().stream()
                .max(Element::compareTo)
                // what can I use here?
                .somehowAccessTheValueOfMaxKey()
                .get()

?

Additional information I do not need a default value like null , because the Map will only be checked if it contains at least one key-value pair, which means there will always be a most recent element-attribute pair, a single one, at least.

Answer 1

You can find the max Entry instead of the max key:

Attributes latestValue =
    results.entrySet()
           .stream()
           .max(Comparator.comparing(Map.Entry::getKey))
           .map(Map.Entry::getValue)
           .get();

Answer 2

Attributes latestValue = results.keySet().stream()
            .max(Element::compareTo)
            .map(results::get)
            .get()

Answer 3

You can also use Collectors.toMap with TreeMap as a map factory

Attributes value = results.entrySet().stream()
       .collect(toMap(Map.Entry::getKey, Map.Entry::getValue, (v1, v2) -> v1, TreeMap::new))
       .lastEntry().getValue();