Java Generics and Collections, 2nd Edition

Chapter 3. Comparison and Bounds

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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 ...