4.12 Java4070: the comparable interface, part 1  (Page 4/6)

The incoming parameter to the fillIt method in Listing 4 is a reference to an object instantiated from the ArrayList class. That reference is passed to the fillIt method as type Collection , which is legal because the ArrayList class implements both the Collection interface and the List interface.

Casting to type List

The code in Listing 4 uses a cast to temporarily convert the incoming reference from type Collection to type List . Because the version of the add method that is used in Listing 4 is declared in the List interface, and because the ArrayList class correctly implements the List interface, that version of the add method can be called on the reference to the ArrayList object when it is treated as the interface type List . Hopefully this is review material for you at this point. If not, you may need togo back and study some of my earlier modules.

The List contract for the add method

Listing 4 also illustrates part of the contract for this version of the add method in the List interface. This version of the add method makes it possible to specify the position of each element added to the ArrayList object.

(A List is an ordered collection because the user has control over the location of each element in the collection relative to theother elements in the collection.)

Controlling the locations of the elements

In Listing 4 , the elements are added to the ArrayList object in increasing element order during the first five invocations of the add method. However, the sixth invocation of the add method adds a new element at index position 3.

Add method actually does an insert

A portion of the contract for this version of the add method in the List interface is as follows:

"Inserts the specified element at the specified position in this list (optional operation). Shifts the element currently at that position (if any)and any subsequent elements to the right (adds one to their indices)."

Thus, the new element is inserted at that position, and the other elements are pushed up, as required, toward higher index values to make room for the newelement.

The Vector class

Here is an interesting side note. The Java Vector class has been around longer than the Collections Framework. Somewhere along the way, the Vector class was upgraded to cause it to become a concrete implementation of the Collection interface and the List interface.

As a result of the upgrade, the Vector class now provides an implementation of the add method described above. Except for the order of the parameters, that add method appears to have the same behavior as the older method named:

insertElementAt(Object elem, int index)

You can insert elements into a Vector object by calling the add method on that object while treating it as type List . However, since the older insertElementAt method is not declared in the List interface, you cannot insert an element into the Vector object by calling the insertElementAt method while treating it as a List . In order to call that method, you must treat it as type Vector .

More on the List contract

Another portion of the contract for a List object is that the iterator method