Here we use a parametrized interface Comprable<T>. By doing so we restrict the type of the parameter in compareTO method to T. This means that Vehicle can be compared only to another Vehicle, since it implements Comprable<Vehicle>. The class String in Java 5.0 implements Comprable<String>, therefore a String can be compared only to another String.

Here is the class Vehicle which implements Comprable<Vehicle>:

public class Vehicle implements Comparable<Vehicle> {
    protected String model;
    protected int maxPassengers;
    protected double maxSpeed;  // mph

    public Vehicle(String m, int passengers, double speed) {
        model = m;
        maxPassengers = passengers;
        maxSpeed = speed;
    }

    public int compareTo(Vehicle v) {
        return (int) (maxSpeed - v.maxSpeed);
    }

    // takes distance in miles, returns time in hours
    public double time(double distance) {
        return distance/maxSpeed;
    }


    public void print() {
        System.out.println("this is a " + model + ", it can carry " +
                           maxPassengers + " and can travel at " +
                           maxSpeed + " mph");
    }
}

public class TestGenerics {
    public static void main(String [] args) {
	Vehicle [] vehicles = {new Car("Dodge",4,80.0,"automatic"),
			       new Airplane("Airbus",200,1000,30000),
			       new Vehicle ("Broom",1,100)};
	Vehicle v = min(vehicles);
	v.print();

	String [] strings = {"orange", "banana", "apple", "lemon", "kiwi"};
	String s = min(strings);
	System.out.println(s);
    }

    public static <T extends Comparable> T min(T [] array) {
	T min = array[0];
	for (int i = 1; i < array.length; ++i) {
	    if (min.compareTo(array[i]) > 0) {
		min = array[i];
	    }
	}
	return min;
    }
}