Format a duration in Java, #java #duration

DurationToString.java

public class DurationToString
{
    private final Duration duration;
    
    private final TimeUnit unit;

    private DurationToString(Duration d, TimeUnit u)
    {
        this.duration = d;
        this.unit = u;
    }
    
    public static DurationToString from(Duration d)
    {
        Objects.requireNonNull(d);
        return new DurationToString(d, TimeUnit.SECONDS);
    }
    
    public static DurationToString from(Duration d, TimeUnit u)
    {
        Objects.requireNonNull(d);
        Objects.requireNonNull(u);
        if (u.compareTo(TimeUnit.MILLISECONDS) < 0)
            u = TimeUnit.MILLISECONDS;
        else if (u.compareTo(TimeUnit.MINUTES) > 0)
            u = TimeUnit.MINUTES;
        return new DurationToString(d, u);
    }
    
    @Override
    public String toString()
    {
        final StringBuilder sb = new StringBuilder(); 
        
        Duration d = this.duration;
        
        final long days = d.toDays();
        if (days > 0) {
            sb.append(days);
            sb.append("days ");
            d = d.minusDays(days);
        }
        
        final long hours = d.toHours();
        if (hours > 0) {
            sb.append(hours);
            sb.append("h");
            d = d.minusHours(hours);
        }
        
        final long minutes = d.toMinutes();
        if (minutes > 0 || this.unit == TimeUnit.MINUTES) {
            sb.append(minutes);
            sb.append("m");
            d = d.minusMinutes(minutes);
        }
        if (this.unit == TimeUnit.MINUTES) {
            return sb.toString();
        }
        
        final long seconds = d.getSeconds();
        sb.append(seconds);
        sb.append("s");
        if (this.unit == TimeUnit.SECONDS) {
            return sb.toString();
        }
        
        assert(this.unit == TimeUnit.MILLISECONDS);
        
        final long millis = d.getNano() / 1_000_000;
        if (millis > 0) {
            sb.append(millis);
            sb.append("ms");
        }
        
        return sb.toString();
    }
}