10.3 Writing a Max Function

We've seen that Java provides a nice Collections.max function for us. Collections also includes other handy functions like min, sort, and reverse.

In this chapter we'll see how we can write such functions ourselves.

Generic Functions (Warumup)

As a warmup suppose we want a function which picks a random item from an array. Some code is shown below:

public class RandomPicker {
    public static <T> T randomItem(T[] items) {
        Random random = new Random();
        int randomIndex = random.nextInt(x.length);
        return x[randomIndex];
    }
}

And a sample program that uses this function is given below:

public class RandomPickerDemo {
   public static void main(String[] args) {
       String[] x = {"hi", "little", "cat"};
       System.out.println(RandomPicker.pickRandom(x));
   }
}

Next, we'll try to make it so that the pickRandom function can work on any type of array.

Approach One: Generics

We can try to use Generics, as shown in the code below:

public class RandomPicker<T> {
   public static T pickRandom(T[] x) {
       Random random = new Random();
       int randomIndex = random.nextInt(x.length);
       return x[randomIndex];
   }
}

However, this code doesn't really make sense as written. The pickRandom function is static, but we have to instantiate a RandomPicker object to specify the generic type. That is, we'd need to do something like:

String[] x = {"hi", "little", "cat"};
RandomPicker<String> stringPicker = new RandomPicker<>();
System.out.println(stringPicker.pickRandom(x));

but we can't call static methods using an instance.

A fix for this is to make the pickRandom function non-static, as shown below:

public class RandomPicker<T> {
   public T pickRandom(T[] x) {
       Random random = new Random();
       int randomIndex = random.nextInt(x.length);
       return x[randomIndex];
   }
}

If we do this our code will work fine, but it's still very awkward to have to instantate a picker object.

Approach Two: Generic Static Methods

The correct and better approach is to make our static method generic. This is shown below:

public class RandomPicker {
   public static <T> T pickRandom(T[] x) {
       Random random = new Random();
       int randomIndex = random.nextInt(x.length);
       return x[randomIndex];
   }
}

This syntax is confusing (and it'll get worse later!). The way you can read public static <T> T pickRandom(T[] x) is: "I am declaring a public static function that works on objects of type , it returns a T, it is called pickRandom, and it takes an array of Ts as its input.

After noting to Java that our static method is generic, we can call it without instantiating a RandomPicker object as shown below:

public class RandomPickerDemo {
   public static void main(String[] args) {
       String[] x = {"hi", "little", "cat"};
       System.out.println(RandomPicker.pickRandom(x));
   }
}

Note that unlike generic classes, we don't need to specify the generic type when calling a generic static method (i.e. there's no <String> after pickRandom). The type is automatically inferred.

One minor annoyance is that our pickRnadom function will not work with arrays of primitive types, i.e. we can use int[], double[], float[].

public class RandomPickerDemo {
   public static void main(String[] args) {
       int[] x = {1, 2, 3, 4, 5, 6, 7, 8};
       System.out.println(RandomPicker.pickRandom(x));
   }
}

There's no way around this, and in fact real Java libraries often have multiple functions, one for each primitive type. For example, there exists Arrays.sort(int[]), Arrays.sort(double[]), Arrays.sort(float[]), etc. One day maybe Project Valhalla will fix this.

Writing our Max Function, Type Bounds

Using the idea of a generic static function, we can write our max function as follows:

public class Maximizer {
   public static <T> T max(T[] items) {
       T maxItem = items[0];
       for (int i = 0; i < items.length; i += 1) {
           int cmp = items[i].compareTo(maxItem);
           if (cmp > 0) {
               maxItem = items[i];
           }
       }
       return maxItem;
   }
}

There's one last problem, Java doesn't know that T has a compareTo method. We can fix this by adding a type bound to our generic type, as shown below:

public class Maximizer {
   public static <T extends Comparable<T>> T max(T[] items) {
       T maxItem = items[0];
       for (int i = 0; i < items.length; i += 1) {
           int cmp = items[i].compareTo(maxItem);
           if (cmp > 0) {
               maxItem = items[i];
           }
       }
       return maxItem;
   }
}

This tells Java that T, whatever it is, has to implement Comparable<T>. If you try to pass an arary of objects into max that does not implement Comparable<T>, you'll get a compile-time error.

The way to read public static <T extends Comparable<T>> T max(T[] items) is: "I am declaring a public static function that works on objects of type , it returns a T, it is called max, and it takes an array of Ts as its input. Additionally, T has to implement Comparable."

Bonus: An Even Better Type Bound

In real industrial strength Java code, the correct way to specify our method is:

public class Maximizer {
   public static <T extends Comparable<? super T>> T max(T[] items) {
		...
   }
}