Java Generics and Collections, 2nd Edition

Chapter 2. Comparison and Bounds

A Note for Early Release Readers

With Early Release ebooks, you get books in their earliest form—the author’s raw and unedited content as they write—so you can take advantage of these technologies long before the official release of these titles.

This will be the 3rd chapter of the final book. Please note that the GitHub repo will be made active later on.

If you have comments about how we might improve the content and/or examples in this book, or if you notice missing material within this chapter, please reach out to the editor at [e-mail].

Now that we have the basics, let’s look at some more advanced uses of generics. This chapter describes the interfaces Comparable<T> and Comparator<T>, which are used to support comparison on elements. These interfaces are useful if, for instance, you want to find the maximum element of a collection or to sort a list. Along the way, we will introduce bounds on type variables, an important feature of generics that is particularly useful in combination with the Comparable<T> interface.

Comparable<T>

The interface Comparable<T> declares a single instance method for comparing one object with another:

interface Comparable<T> {
  public int compareTo(T o);
}

The compareTo method returns an integer value that is negative, zero, or positive depending upon whether the receiver—this object—is less than, equal to, or greater than the argument. When a class implements Comparable, the ordering specified by this ...