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Sorting with Comparator

A class implementing Comparable has only one natural sort order: that which is defined by its compareTo.

TreeSet<String> animals = new TreeSet<>(); // Natural sort order

To sort objects by you own criteria, create a Comparator for the object's class.

public class StringIgnoreCaseComparator implements Comparator<String> { }

The class that implements the Comparator interface must define the compare method.

public int compare(String o1, String o2) { 
  // Use String's own compareTo:
  return o1.toUpperCase().compareTo(o2.toUpperCase());
}

  • compare takes references to two objects to be compared, and returns an int.

  • compare typically calls property getters to determine how the first object compares with the second.

  • It returns a positive integer for greater than, a negative integer for less than, and zero for equal.

Both TreeSet and TreeMap have constructors allowing you to provide your own Comparator to perform the sorting.

Comparator<String> comp = new StringIgnoreCaseComparator();
Set<String> animals = new TreeSet<>(comp);

Drill

Run StringComparingApp to see the names sorted in "ASCIIbetical" order, in which lowercase letters sort after uppercase letters. Comment out the first Set declaration, and uncomment the Comparator and the second Set declaration to see how the TreeSet uses the custom Comparator.

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