4.22 Java4120: the comparator interface, part 4  (Page 5/6)

Sorting the array into natural order

The code in Listing 8 is also new to this module. This code uses one of the overloaded sort methods of the Arrays class to sort the contents of the array into natural order .

Here is part of what Oracle has to say about the Arrays class:

"This class contains various methods for manipulating arrays (such as sorting and searching)."

The class contains many overloaded versions of the sort method. Here is part of what Oracle has to say about the version of the sort method used in Listing 8 above:

"Sorts the specified array of objects into ascending order, according to the natural ordering of its elements. All elements in the array mustimplement the Comparable interface."

The Comparable interface and polymorphic behavior

Although the declared type of the array is Object , the array actually contains references to String objects.

The String class implements the Comparable interface. It is not necessary to cast the array to type String before passing it to the Sort method. (The Sort method declares the incoming parameter as type Object .)

The sort method treats the array elements as type Comparable and uses the compareTo method declared in that interface to perform any necessary comparisons required to carry out the sorting operation.

This is another example of the usefulness of polymorphism as implemented through the use of the Java interface. (The Comparable interface and the compareTo method declared in that interface were discussed in detail in an earlier module.)

Display the sorted array data

The code in Listing 9 displays the contents of the array after those contents are sorted into natural order by the sort method in Listing 8 above.

The output produced by Listing 9 above is:

The natural order for String objects

I discussed the concept of natural ordering in a previous module with particular emphasis of the natural order for strings. You will recognize that the strings shown in the above output have been sorted into natural order according to the definition of the compareTo method of the String class.

Sort the array with a Comparator

The code in Listing 10 is also new to this module. This code uses a different version of the overloaded sort method of the Arrays class to sort the array using the rules defined in the compare method of a Comparator object (passed as a parameter to the sort method).

What does Oracle have to say about this?

Here is part of what Oracle has to say about this version of the sort method of the Arrays class:

"Sorts the specified array of objects according to the order induced by the specified comparator. All elements in the array must be mutuallycomparable by the specified comparator (that is, c.compare(e1, e2) must not throw a ClassCastException for any elements e1 and e2 in the array)."