Sorting Java objects using multiple keys

andras picture andras · Nov 7, 2011 · Viewed 14.3k times · Source

I've got a collection of Duck objects and I'd like to sort them using multiple keys.

class Duck {
    DuckAge age; //implements Comparable
    DuckWeight weight; //implements Comparable
    String name;
}
List<Duck> ducks = Pond.getDucks();

eg. I want to sort them primarily by their weights, and secondarily by their age. If two ducks have the exact same weight and the exact same age, then let's differentiate them using their names as a tertiary key. I might do something like this:

Collections.sort(ducks, new Comparator<Duck>(){
    @Override
    public int compare(Duck d1, Duck d2){
        int weightCmp = d1.weight.compareTo(d2.weight);
        if (weightCmp != 0) {
            return weightCmp;
        }
        int ageCmp = d1.age.compareTo(d2.age);
        if (ageCmp != 0) {
            return ageCmp;
        }
        return d1.name.compareTo(d2.name);
    }
});

Well I do this quite frequently, but this solution doesn't smell right. It doesn't scale well, and it's easy to mess up. Surely there must be a better way of sorting Ducks using multiple keys! Does anybody know of a better solution?

EDIT removed unnecessary else branches

Answer

JB Nizet picture JB Nizet · Nov 7, 2011

Guava is more elegant:

return ComparisonChain.start()
     .compare(d1.weight, d2.weight)
     .compare(d1.age, d2.age)
     .compare(d1.name, d2.name)
     .result();

Apache commons-lang has a similar construct, CompareToBuilder.