Sorting Java objects using multiple keys

Question

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.

List<Duck> ducks = new ArrayList<Duck>();
Collections.sort(ducks, new Comparator<Duck>() {

  @Override
  public int compare(Duck o1, Duck o2) {

    return new org.apache.commons.lang.builder.CompareToBuilder().
        append(o1.weight, o2.weight).
        append(o1.age, o2.age).
        append(o1.name, o2.name).
        toComparison();
  }
});

Java 8 solution:

Comparator<Duck> cmp = Comparator.comparing(Duck::getWeight)
    .thenComparing(Duck::getAge)
    .thenComparing(Duck::getName);

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)
    .thenComparing(duck-> duck.name);

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.

Firstly, your solution isn't that slow.

If you really want another method, then give each duck a "score" which is essentially a single number that is the sum of their three characteristics, but with a huge weighting (excuse the almost unavoidable pun) for weight, a lesser one for age; and a very small one for the name.

You can allocate ~10 bits for each characteristic, so for each characteristic you have to be in the range 0..1023.

score = ( (weight << 10) + age) << 10 + name;

This is probably completely unneeded, but whatever :)

You can use the CompareToBuilder from Apache Commons Lang. (It explains comparable, but works for Comparator too).