Binary Trees - Building Java Programs: A Back to Basics Approach - page 1021

return false;
} else {
int compare = value.compareTo(root.data);
if (compare == 0) {
return true;
} else if (compare < 0) {
return contains(root.left, value);
} else {
return contains(root.right, value);
}
}
}
You have to make one final change. Java knows only that these data values are of
some generic type E . As a result, the calls on compareTo in your add and contains
methods generate compiler errors. You could fix this problem with a cast. For example,
you could replace the line:
int compare = value.compareTo(root.data);
with:
int compare = ((Comparable<E>) value).compareTo(root.data);
A better approach is to modify the class header to include this information. You
want to add the constraint that the class E implements the Comparable interface. You
specify that constraint by modifying the header as follows:
public class SearchTree<E extends Comparable<E>> {
...
}
It's odd that Java has you use the keyword extends because you want it to imple-
ment the interface, but that's how generics work in Java. If you are defining a class,
you make a distinction between the cases in which the new class extends another
class and those in which it implements an interface. But in generic declarations, you
use the word extends for both kinds of extension.
This is a fairly quick explanation of a complex topic. We don't have time to
explain the details of programming with generics in detail in this topic, although
there are many excellent online tutorials that you can find by entering “Java generics”
into your favorite web browser.
The complete code for the SearchTree class may be found at http://www.
buildingjavaprograms.com.