andras andras - 1 year ago 77
Java Question

Sorting Java objects using multiple keys

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>(){
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;

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

Answer Source

Java 8 solution:

Comparator<Duck> cmp = Comparator.comparing(Duck::getWeight)

Hooray for lambdas, method references, and default methods:)! Too bad we have to define getters, or use explicit lambdas, like so:

Comparator<Duck> cmp = Comparator
    .comparing((Duck duck)-> duck.weight)
    .thenComparing((Duck duck)-> duck.age)

Type inference won't work with implicit lambdas, so you have to specify the argument type of the first two lambdas. More details in this answer by Brian Goetz.

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