Sorting ArrayList of Objects by Object attribute

Another good way of doing this that is a bit more flexible if there is more than one property of an object that you may wish to sort by is to use Guava's Ordering class with its onResultOf(Function) option. This is ideally suited for sorting by properties since a Function can be used to retrieve and return a specific property of an object. For a simple example, imagine a class Person with String getFirstName() and String getLastName() methods.

List<Person> people = ...;
Collections.sort(people, Ordering.natural().onResultOf(
    new Function<Person, String>() {
      public String apply(Person from) {
        return from.getFirstName();
      }
    }));

The above will sort the list by first name.

To make it read nicer, you may want to define the functions you might want to use as public static final fields on the Person class. Then you could sort by last name like this:

Collections.sort(people, Ordering.natural().onResultOf(Person.GET_LAST_NAME));

As a fun aside note, this should all be a lot easier in Java 7 with lambda expressions and method references. As the plan stands now, you'll probably be able to write something like this without having to define any clumsy anonymous inner classes or static final fields:

people.sort(Ordering.natural().onResultOf(Person#getLastName()));

There could even be something like this:

people.sortBy(Person#getLastName());