A simple printer for the Java "Duration" type

DurationPrinter.java

// Licensed under "The Unlicense" - https://unlicense.org/

package backup.helpers;

import org.jetbrains.annotations.NotNull;

import java.time.Duration;
import java.time.temporal.ChronoUnit;
import java.util.*;

public abstract class DurationPrinter {

    private final static Map<ChronoUnit, StringSet> stringsOfChronoUnit;

    static {
        Map<ChronoUnit, StringSet> tmp = new HashMap<>();
        tmp.put(ChronoUnit.DAYS, new StringSet("day", "days"));
        tmp.put(ChronoUnit.HOURS, new StringSet("hour", "hours"));
        tmp.put(ChronoUnit.MINUTES, new StringSet("min", "mins"));
        tmp.put(ChronoUnit.SECONDS, new StringSet("s", "s"));
        tmp.put(ChronoUnit.MILLIS, new StringSet("ms", "ms"));
        stringsOfChronoUnit = Collections.unmodifiableMap(tmp);
    }

    private static class StringSet {

        private final String usual;
        private final String mono;

        public StringSet(@NotNull String mono, @NotNull String usual) {
            this.mono = mono;
            this.usual = usual;
        }

        public String of(int count) {
            if (count == 1) {
                return mono;
            } else {
                return usual;
            }
        }
    }

    private static void stringify(@NotNull List<String> res, int count, @NotNull ChronoUnit unit) {
        if (count == 0 && res.isEmpty() && !ChronoUnit.MILLIS.equals(unit)) {
            // suppress adding anything unless something already exists and this is not "seconds" (because
            // we print at least seconds)
        } else {
            StringSet ss = stringsOfChronoUnit.get(unit);
            if (!res.isEmpty()) {
                res.add(", "); // separator
            }
            res.add(Integer.toString(count));
            res.add(" ");
            if (ss == null) {
                // missing chrono unit!?!
                res.add(unit.toString());
            } else {
                res.add(ss.of(count));
            }
        }
    }

    // ---
    // Simple helper to print duration
    // https://stackoverflow.com/questions/3471397/how-can-i-pretty-print-a-duration-in-java
    // ---

    public static String formatDuration(Duration duration) {
        List<String> parts = new ArrayList<>(10);
        stringify(parts, (int) (duration.toDaysPart()), ChronoUnit.DAYS);
        stringify(parts, duration.toHoursPart(), ChronoUnit.HOURS);
        stringify(parts, duration.toMinutesPart(), ChronoUnit.MINUTES);
        stringify(parts, duration.toSecondsPart(), ChronoUnit.SECONDS);
        stringify(parts, duration.toMillisPart(), ChronoUnit.MILLIS);
        return String.join("", parts);
    }

}

DurationPrinterTest.java

// Licensed under "The Unlicense" - https://unlicense.org/

package backup;

import backup.helpers.DurationPrinter;
import org.junit.jupiter.api.Test;

import java.time.Duration;
import java.time.temporal.ChronoUnit;
import java.util.SortedMap;
import java.util.TreeMap;

import static org.assertj.core.api.Assertions.assertThat;

public class TestDurationPrinter {

    @Test
    void testDurationPrinter() {
        SortedMap<Duration,String> durs = new TreeMap<>();
        durs.put(Duration.of(0, ChronoUnit.MILLIS),"0 ms");
        durs.put(Duration.of(1000, ChronoUnit.MILLIS),"1 s, 0 ms");
        durs.put(Duration.of(1500, ChronoUnit.MILLIS),"1 s, 500 ms");
        durs.put(Duration.of(60000, ChronoUnit.MILLIS),"1 min, 0 s, 0 ms");
        durs.put(Duration.of(120000, ChronoUnit.MILLIS),"2 mins, 0 s, 0 ms");
        durs.put(Duration.of(122222, ChronoUnit.MILLIS),"2 mins, 2 s, 222 ms");
        durs.put(Duration.of(5677821, ChronoUnit.MILLIS),"1 hour, 34 mins, 37 s, 821 ms");
        durs.put(Duration.of(861862066L, ChronoUnit.MILLIS),"9 days, 23 hours, 24 mins, 22 s, 66 ms");
        durs.put(Duration.of(9877461771L, ChronoUnit.MILLIS),"114 days, 7 hours, 44 mins, 21 s, 771 ms");
        durs.keySet().forEach( dur -> assertThat(DurationPrinter.formatDuration(dur)).isEqualTo(durs.get(dur)));
        /*
        for (long l = 0; l < 9877461771L; l += 533) {
            System.out.println(l + " == " + DurationPrinter.formatDuration(Duration.of(l, ChronoUnit.MILLIS)));
        }
         */
    }
}