Using Java Comparator for sorting using multiple fields based on condition

Question

I have a question on effective use of Java Comparator.

class MyClass {
    //Active State: OPEN, PENDING, RUNNING
    private String state;
    private Date startDate;
    private Date endDate;
}

Here the possible values of state field are OPEN, PENDING, RUNNING, CLOSED, CANCELLED etc., out of which OPEN, PENDING and RUNNING are the Active state. Now I want to write comparator which sorts the List<MyClass>, so that the active ones comes first and are sorted by startDate followed by non-active ones which are sorted based on endDate.

static final Set<String> ACTIVE;// this set contains OPEN, PENDING, RUNNING

List<MyClass> myList;//This is my list
...
Collections.sort(myList, new Comparator<MyClass>() {
    @Override
    public int compare(MyClass o1, MyClass o2) {
        int c;
        boolean isO2 = ACTIVE.contains(o2.getState());
        boolean isO1 = ACTIVE.contains(o1.getState());
        if (isO2 && isO1) {
            c = DateTimeComparator.getInstance().compare(o2.getStartDate(), o1.getStartDate());
        } else if (isO2) {
            c = 1;
        } else if (isO1) {
            c = -1;
        } else {
            c = DateTimeComparator.getInstance().compare(o2.getEndDate(), o1.getEndDate());
        }
        return c;
    }
});

My question is whether my above implementation of having single comparator good? or are there better ways to do it? Most likely I have to stick with Java 7 but solutions with Java 8 are welcome too.

Question: why is your active state a String? Wouldn't an enum be more appropriate here? — Hovercraft Full Of Eels
– Hovercraft Full Of Eels, Commented Jan 4, 2017 at 2:01
BTW: I don't have issue with your implementation but you'd best wait for someone more expert in this to fully weigh in. — Hovercraft Full Of Eels
– Hovercraft Full Of Eels, Commented Jan 4, 2017 at 2:07
@HovercraftFullOfEels, Unfortunately I can not make any change in MyClass as we don't own it, just simply using it. May be I need to ask the corresponding team to make it enum. — Learner
– Learner, Commented Jan 4, 2017 at 2:17

pscuderi · Accepted Answer · 2017-01-04 19:01:03Z

4

In Java 8, I think it would be a bit cleaner to use Comparator::comparing. For example:

Comparator<MyClass> comparator = Comparator.nullsFirst(Comparator.comparing((MyClass myClass) -> !isActive(myClass))
    .thenComparing((MyClass myClass) -> isActive(myClass) ? myClass.startDate : myClass.endDate, Comparator.nullsFirst(DateTimeComparator.getInstance())));

private static boolean isActive(MyClass myClass)
{
    switch (myClass.state)
    {
    case "OPEN":
    case "PENDING":
    case "RUNNING":
        return true;
    default:
        return false;
    }
}

In Java 7, assuming you have Guava on the classpath, you could use Ordering. For example:

Comparator<MyClass> comparator = Ordering.natural().reverse().onResultOf(new Function<MyClass, Boolean>() {
        @Override
        public Boolean apply(MyClass myClass) {
            return isActive(myClass);
        }
    })
    .compound(Ordering.from(DateTimeComparator.getInstance()).nullsFirst().onResultOf(new Function<MyClass, Date>() {
        @Override
        public Date apply(MyClass myClass) {
            return isActive(myClass) ? myClass.startDate : myClass.endDate;
        }
    }))
    .nullsFirst();

edited Jan 4, 2017 at 19:01

answered Jan 4, 2017 at 2:40

pscuderi

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7 Comments

Learner Over a year ago

Is there an equivalent of this in Java 7? This is good though. +1

Learner Over a year ago

How will the dates the compared here? In my solution I am using DateTimeComparator. What will he be used here if the dates are joda time?

pscuderi Over a year ago

You can pass your DateTimeComparator as the second parameter to thenComparing. See the updated code.

pscuderi Over a year ago

I don't think there's a particularly elegant solution in Java 7 without the use of Guava or another library.

pscuderi Over a year ago

I added an example in Java 7, though unfortunately it requires Guava.

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