Comparable : Java Glossary

java.lang.Comparable is used to define the natural sort order of a class. In contrast, java.lang.Comparator is used to define an auxiliary sort order for a class.

• Use the ComparatorCutter to mindlessly crank out complete code for Comparables and Comparators. The code will help teach you the structure so that you can write them without such training wheels.
• How do you remember that java.util. Comparable. compareTo defines a natural order where java.util. Comparator. compare defines an auxiliary order? I think about it this way:
  “Does it makes sense to compare two Oranges? If so they are Comparable. I will need a method that compares this Orange Object to (compareTo) another. If I want do define an alternate order, I need to invent some sort of independent machine, a Comparator to do it. The comparison is not inherent to Oranges, so there is no this. Instead, I compare two Orange Objects.”

Normally you also redefine equals when you implement Comparable even though it is not part of the Comparable interface. There are no generics involved in overriding equals.

To implement Comparable you must write only one method: compareTo. It is typically added to an existing class to describe the natural order of that class. compareTo compares this object with another.

public int compareTo( Object o );

Comparable Using Generics

Comparable Gotcha

// base class
class SalesTaxItem implements Comparable<SalesTaxItem>
{
...
}

and you try to extend it with:

you will get this error message. Comparable cannot be inherited with different arguments

The best way I know of to bypass the problem is to write both classes without the implements Comparable. Then write two Comparators instead.

Descending/Inverse/Reverse Order with reverseOrder

Sorting ascending order means sorting with the small elements first then the big. This is usual ordering. Descending order means sorting with the big elements first then the small.

If you have a Comparator or Comparable of some kind, you can convert it into one that sorts into the reverse of the usual order, e. g. if the original sorts alphabetically, the new one will sort in reverse alphabetical order. Here is how you use it:

If you don’t have a suitable base Comparator, just write an ordinary Comparator from scratch and reverse the operands to each compare inside it, or return - result instead of result.

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