Bounded types

Definition

A bounded type parameter limits which types are allowed, using extends. Inside the method you can then call the methods of the bound, such as compareTo.

Example code

A max method that only accepts things that can be compared

import java.util.List;

class Util {
    static <T extends Comparable<T>> T max(List<T> items) {
        T best = items.get(0);
        for (T item : items) {
            if (item.compareTo(best) > 0) best = item;   // allowed because T is Comparable
        }
        return best;
    }
}

class Demo {
    public static void main(String[] args) {
        System.out.println(Util.max(List.of(3, 9, 4)));
        System.out.println(Util.max(List.of("pear", "apple", "zebra")));
        // Util.max(List.of(new Object()));            // compile error: Object is not Comparable
    }
}

// Output:
// 9
// zebra