Comparator : Java Glossary

• 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.”

public final int compare( Object o1, Object o2 );
public boolean equals( Object obj );

Collections.sort( myArrayList, new Comparator()
                     {
                     public int compare( Object a, Object b )
                        {
                        return( (String)a ).compareToIgnoreCase( (String) b );
                        }
                     } );

Tips

• If you make your Comparators nested static classes of the class they sort, they will be easy to find and easy to associate with the correct class.
• String.CASE_INSENSITIVE_ORDER is a Comparator you can use without writing any code. Unfortunately there is no String.CASE_SENSITIVE_ORDER. The reason is String’s Comparable implementation provides it naturally without a Comparator. You can use the following code to sort by natural Comparable order when a Comparator is required.
• Beware of using subtraction in writing Comparators rather than < > and Double. compareTo if there is any possibility you will be using numbers large enough to overflow and give the wrong sign comparing them with subtraction. The other problem with subtraction is it might give a long or double result which is not an integer value suitable for return.

Descending/Inverse/Reverse Order with reverseOrder

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.

Comparator Using Generics

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