Java Sort object list based on value order (e.g. d, s, a, q, c)

Question

I have Events class arraylist where Events class is like

class Events {
Date eventDate;
String eventType;
}

Now I want to sort this array based on the newest eventDate first. If two or more events are on the same date then I have sort them in following event type order

1. Maths 2. Science 3. History 4. Algebra .

So if my list is

{ "01/01/2010  History", "01/01/2010 Algebra", "01/01/2010 Maths", "01/01/2010 Science"}

Then I want to sort it like

{ "01/01/2010  Maths", "01/01/2010 Science", "01/01/2010 History", "01/01/2010 Algebra"}

Please suggest how can I do this?

TIA, Hanumant.

Answer 1

你将需要实现你自己的比较如图所示这里 。

Answer 2

Not tested, but it should give you the idea:

class Events
implements Comparable
{
    Date eventDate;
    String eventType;

    int eventScore()
    {
        if (eventType.equals("Maths"))
            return 0;
        else if (eventType.equals("Science"))
            return 1;
        else if (eventType.equals("History"))
            return 2;
        else if (eventType.equals("Alegbra"))
            return 3;
        return 4;
    }

    public int compareTo(Object o)
    {
        Events other = (Events)o;
        if (other.eventDate.before(this.eventDate))
            return -1;
        else if (other.eventDate.after(this.eventDate))
            return 1;
        return other.eventScore() < this.eventScore() ? -1 : 1;
    }
}

Answer 3

Your design leaves a little something to be desired. Try:

class Event implements Comparable {
    private Date date;
    private Event.Type type;

    enum Type {
        MATHS,     // MATH / MATHEMATICS?
        SCIENCE,
        HISTORY,
        ALGEBRA
    }

    public int compareTo(Event other) {
        int comparison = other.date.compareTo(date);
        if (0 == comparison) {
            comparison = type.compareTo(other.type);
        }
        return comparison;
    }
}

Then given a Collection<Event> events you can just Collections.sort(events) .

Answer 4

因为这听起来像HW，你应该为Events类实现Comparable接口。