Interfaces and Inner Classes - Absolute Java - page 722

Display 13.5 Sorting Method for Array of Comparable (part 2 of 2)

40 /**
41 Precondition: i and j are legal indices for the array a.
42 Postcondition: Values of a[i] and a[j] have been interchanged.
43 */
44 private static void interchange( int i, int j, Comparable[] a)
45 {
46 Comparable temp;
47 temp = a[i];
48 a[i] = a[j];
49 a[j] = temp; //original value of a[i]
50 }

51 }

EXAMPLE: (continued)

Display 13.6 shows a demonstration of using the sorting method given in Display 13.5. To
understand why the demonstration works, you need to be aware of the fact that both of
the classes Double and String implement the Comparable interface.
If you were to check the full documentation for the class Double , you would see
that Double implements the Comparable interface and so has a compareTo method.
Moreover, for objects o1 and o2 of Double ,

o1.compareTo(o2) < 0 //o1 "comes before" o2
means the same thing as

o1.doubleValue() < o2.doubleValue()
So, the implementation of the Comparable interface for the class Double is really
just the ordinary less-than relationship on the double values corresponding to the
Double objects.
Similarly, if you were to check the full documentation for the class String ,
you would see that String implements the Comparable interface and so has a
compareTo method. Moreover, the implementation of the compareTo method for
the class String is really just the ordinary lexicographic relationship on the strings.
This Programming Example uses the standard library classes Double and String
for the base type of the array. You can do the same thing with arrays whose base
class is a class you defined, so long as the class implements the Comparable interface
(including the standard semantics, which we discussed earlier in the Pitfall “Interface
Semantics Are Not Enforced”).
This Programming Example does point out one restriction on interfaces. They can
apply only to classes. A primitive type cannot implement an interface. So, in Display 13.6,
we could not sort an array with base type double using the sorting method for an array of
Comparable . We had to settle for sorting an array with base type Double . This is a good
example of using a wrapper class with its “wrapper class personality.”