Message Dispatching

Video Materials

The term dispatch refers to how a language decides which polymorphic operation (a method or function) a message should trigger.

Consider polymorphic functions in Java, also known as method overloading, where multiple methods use the same name but have different parameters. Here’s an example for calculating the rounded sum of an array of numbers:

Java

public int roundedSum(int[] a){
    int sum = 0;
    for (int i : a) {
        sum += i;
    }
    return sum
}

public int roundedSum(double[] a){
    double sum = 0;
    for (double i : a) {
        sum += i;
    }
    return Math.round(sum);
}

How does the computer know which version to invoke at runtime? It should not be a surprise that it is determined by the arguments - if an integer array is passed, the first is invoked, if a float array is passed, the second.

Python works a bit differently. In Python, method overloading is not allowed, so there cannot be two methods with the same name within a class. To achieve the same effect, optional parameters are used. In addition, because Python is dynamically typed, we could instead write our function to accept values of multiple types:

Python

def rounded_sum(a: List[Union[int, float]]) -> int:
    sum_value: float = 0.0
    for i in a:
        sum_value += i
    return round(sum_value)

As we can see, that function will accept a list of either integer values or floating-point values, and it can properly handle them in either case. In Python, the name of the method is the only thing that is used to determine which piece of code should be executed, not the arguments.

Object-Oriented Polymorphism

However, inheritance can cause some challenges in selecting the appropriate polymorphic form. Consider the following fruit implementations that feature a blend() method:

public class Fruit {

    public String blend() {
        return "A pulpy mess, I guess";
    }
}

public class Banana extends Fruit {

    @Override
    public String blend() {
        return "Yellow mush";
    }
}

public class Strawberry extends Fruit {

    @Override
    public String blend() {
        return "Gooey red sweetness!";
    }
}

class Fruit:
    
    def blend(self) -> str:
        return "A pulpy mess, I guess"

    
class Banana(Fruit):
    
    def blend(self) -> str:
        return "Yellow mush"
    

class Strawberry(Fruit):
    
    def blend(self) -> str:
        return "Gooey red sweetness!"

Let’s add some fruit instances to a list, and invoke their blend() methods:

LinkedList<Fruit> toBlend = new LinkedList<>();
toBlend.add(new Fruit());
toBlend.add(new Banana());
toBlend.add(new Strawberry());
for(Fruit f : toBlend){
    System.out.println(f.blend());
}

to_blend: List[Fruit] = list()
to_blend.append(Fruit())
to_blend.append(Banana())
to_blend.append(Strawberry())
for f in to_blend:
    print(f.blend())

What will be printed? If we look at the declared types, we’d expect each of them to act like a Fruit instance, so in that case the output would be just three lines of A pulpy mess, I guess?

However, that is not correct! This is the powerful aspect of polymorphic method dispatch. In both Java and Python, we don’t look at the declared type of the object, but the actual underlying type of the instance itself. So, if the object was created as a Banana or Strawberry, then it will use the overridden methods from those child classes instead of the parent Fruit class. So, the actual output we’ll get is:

A pulpy mess, I guess
Yellow mush
Gooey red sweetness!

In both Java and Python, we see an example of method overriding. If we include a method of the same name in the child class (and the same set of parameters, in the case of Java), we can override the method that exists in the parent class. In Java, we must use the @Override decorator, but Python doesn’t require anything special.

Abstract vs. Interface

Of course, we can also update this example to either use an abstract class or an interface. There are some pros and cons to either option, but here’s a good rule of thumb to start with:

• Use inheritance without making the parent class abstract only if it makes sense for the parent class to be instantiated itself. So, it might make sense to have a parent Car class and a subclass SportsCar that are both able to be instantiated.
• Use inheritance with abstract classes if the parent class should not be instantiated. For example, when modeling the animal kingdom with a parent Canine class and subclasses Dog and Wolf, it might be best if the parent class cannot be instantiated directly.
• Use interfaces when you want to design a set of methods or behaviors that a class should implement, but which may not otherwise create strong a relationship between the classes. For example, we could create an IUpdatable interface to require several classes to implement a method called update, but the classes themselves might not be related otherwise.

Finally, remember that there are not really any correct answers here - each option comes with trade-offs, and it is up to you as a developer to help determine which is best. Therefore, it is very helpful to have experience with all three approaches so you understand how each one can be used.