Why should a Java class implement comparable?

Question

Why is Java Comparable used? Why would someone implement Comparable in a class? What is a real life example where you need to implement comparable

Answer 1

Comparable defines a natural ordering. What this means is that you're defining it when one object should be considered "less than" or "greater than".

Suppose you have a bunch of integers and you want to sort them. That's pretty easy, just put them in a sorted collection, right?

TreeSet m = new TreeSet(); 
m.add(1);
m.add(3);
m.add(2);
for (Integer i : m)
... // values will be sorted

But now suppose I have some custom object, where sorting makes sense to me, but is undefined. Let's say, I have data representing districts by zipcode with population density, and I want to sort them by density:

public class District {
  String zipcode; 
  Double populationDensity;
}

Now the easiest way to sort them is to define them with a natural ordering by implementing Comparable, which means there's a standard way these objects are defined to be ordered.:

public class District implements Comparable{
  String zipcode; 
  Double populationDensity;
  public int compareTo(District other)
  {
    return populationDensity.compareTo(other.populationDensity);
  }
}

Note that you can do the equivalent thing by defining a comparator. The difference is that the comparator defines the ordering logic outside the object. Maybe in a separate process I need to order the same objects by zipcode - in that case the ordering isn't necessarily a property of the object, or differs from the objects natural ordering. You could use an external comparator to define a custom ordering on integers, for example by sorting them by their alphabetical value.

Basically the ordering logic has to exist somewhere. That can be -

• in the object itself, if it's naturally comparable (extends Comparable -e.g. integers)

• supplied in an external comparator, as in the example above.