Kotlin’s stdlib offers a number of useful helper methods for this. First, you can define a comparator using the compareBy() method and pass it to the sortedWith() extension method to receive a sorted copy of the list: val list: List<Foo> = … val sortedList = list.sortedWith(compareBy({ it.a }, { it.b }, { it.c })) Second, … Read more
For consistency I guess… when you see an enum type, you know for a fact that its natural ordering is the order in which the constants are declared. To workaround this, you can easily create your own Comparator<MyEnum> and use it whenever you need a different ordering: enum MyEnum { DOG(“woof”), CAT(“meow”); String sound; MyEnum(String … Read more
You just have to define that Animal implements Comparable<Animal> i.e. public class Animal implements Comparable<Animal>. And then you have to implement the compareTo(Animal other) method that way you like it. @Override public int compareTo(Animal other) { return Integer.compare(this.year_discovered, other.year_discovered); } Using this implementation of compareTo, animals with a higher year_discovered will get ordered higher. I … Read more
Use Comparable if you want to define a default (natural) ordering behaviour of the object in question, a common practice is to use a technical or natural (database?) identifier of the object for this. Use Comparator if you want to define an external controllable ordering behaviour, this can override the default ordering behaviour. See also: … Read more
The text below comes from Comparator vs Comparable Comparable A comparable object is capable of comparing itself with another object. The class itself must implements the java.lang.Comparable interface in order to be able to compare its instances. Comparator A comparator object is capable of comparing two different objects. The class is not comparing its instances, … Read more
It’s worth mentioning that the following expression: new Long(10).equals(new Integer(10)) is always false, which tends to trip everyone up at some point or another. So not only can you not compare arbitrary Numbers but you can’t even determine if they’re equal or not. Also, with the real primitive types (float, double), determining if two values … Read more
When your class implements Comparable, the compareTo method of the class is defining the “natural” ordering of that object. That method is contractually obligated (though not demanded) to be in line with other methods on that object, such as a 0 should always be returned for objects when the .equals() comparisons return true. A Comparator … Read more