4.14 Java4080: the comparable interface, part 2  (Page 3/5)

The compareTo method

The documentation for the TreeSet class states the following:

"... the Set interface is defined in terms of the equals operation, but a TreeSet instance performs all key comparisons using its compareTo (or compare ) method ..."

What this means is that insofar as the handling of duplicate elements is concerned, (with the possible exception given below involving a Comparator ), in order for a reference to an object to be included in a TreeSet collection, the class from which that object is instantiated must implement the Comparable interface.

A possible exception

Note that one of the constructors for the TreeSet class makes it possible to instantiate a new object by passing a parameter that is a referenceto an object that implements the Comparator interface.

The Comparator interface declares a method named compare , which compares its two arguments for order. The text in the above excerpt fromthe Oracle documentation suggests that when this parameterized constructor is used, it may not be necessary for the objects included in the TreeSet collection to implement the Comparable interface.

I won't discuss that possibility in this module, but I will discuss it in a future module that discusses the use of the Comparator interface. For purposes of this module, I will concentrate on the use of a TreeSet collection that does not receive a reference to a Comparator object when it is instantiated.

The SortedSet interface

The TreeSet class also implements the SortedSet interface. The documentation for the SortedSet interface states the following:

"A set that further guarantees that its iterator will traverse the set in ascending element order, sorted according to the natural ordering of itselements (see Comparable ), or by a Comparator provided at sorted set creation time."

Natural ordering of the elements

The key term to note in the above quotation is the term natural ordering of its elements . This takes us back to the Comparable interface, for which the documentation states:

"This interface imposes a total ordering on the objects of each class that implements it. This ordering is referred to as the class's naturalordering, and the class's compareTo method is referred to as its natural comparison method."

Conclusion regarding traversal

The conclusion is, in order for the iterator to be able to traverse the set according to the natural ordering of its elements, the elements stored in an object that implements the SortedSet interface must be instantiated from a class that implements the Comparable interface (unless a Comparator is provided when the SortedSet object is instantiated.)

The bottom line

The bottom line is, because the class named MyClass in Listing 1 does not implement the Comparable interface, objects of that class are not eligible for use with a TreeSet collection (unless a Comparator is provided when the TreeSet object is instantiated).

A Comparator was not provided when the TreeSet object was instantiated in Listing 1 . Therefore, the attempt in Listing 2 , to add a MyClass object to the TreeSet collection resulted in a ClassCastException being thrown at runtime. The runtime error reads partially as follows: