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Sneaking features through the back door

15/04/2017 Leave a comment

Sometimes programming language developers decide that certain practices are bad, so bad that they try to prevent their use through the language they develop.

For example: In both Java and C# multiple inheritance is not allowed. The language standard prohibits it, so trying to specify more than one base class will result in compiler error.

Another blocking “feature” these languages share, is a syntax preventing creation of derivative classes all together.

For Java, it is declaring a class to be final which might be a bit confusing for new users, since the same keyword is used to declare constants.

As an example, this will not compile:

public final class YouCanNotExtendMe {
    ...
}

public class TryingAnyway extends YouCanNotExtendMe {
    ...
}

For C# just replace final with sealed.

This can also be applied to specific methods instead of the entire class, to prevent overriding, in both languages.

While application developers may not find many uses for this feature, it shows up even in the Java standard library. Just try extending the built-in String class.

But, language features are tools in a tool box.

Each one can be both useful and misused or abused. It depends solely on the developer using the tools.

And that is why as languages evolve over the years, on some occasions their developers give up fighting the users and add some things they left out at the beginning.

Usually in a sneaky, roundabout way, to avoid admitting they were wrong or that they succumbed to peer pressure.

In this post, I will show two examples of such features, one from Java, and one from C#.

C# Extension methods

In version 3.0 of C# a new feature was added to the language: “extension methods”.

Just as their name suggests, they can be used to extend any class, including a sealed class. And you do not need access to the class implementation to use them. Just write your own class with a static method (or as many methods as you want), that has the first parameter denoted by the keyword this and of the type you want to extend.

Microsoft’s own guide gives an example of adding a method to the built in sealed String type.

Those who know and use C# will probably argue that there are two key differences between extension methods and derived classes:

  1. Extension methods do not create a new type.
    Personally, I think that will only effect compile time checks, which can be replaced with run time checks if not all instances of the ‘base’ class can be extended.
    Also, a creative workaround may be possible with attributes.
  2. Existing methods can not be overridden by extension methods.
    This is a major drawback, and I can not think of a workaround for it.
    But, you can still overload methods. And who knows what will be added in the future…

So it may not be complete, but a way to break class seals was added to the language after only two major iterations.

Multiple inheritance in Java through interfaces

Java has two separate mechanisms to support polymorphism: inheritance and interfaces.

A Java class can have only one base class it inherits from, but can implement many interfaces, and so can be referenced through these interface types.

public interface IfaceA {
    void methodA();
}

public interface IfaceB {
    void methodB();
}

public class Example implements IfaceA, IfaceB {
    @override
    public void methodA() {
        ...
    }

    @override
    public void methodB() {
        ...
    }
}

Example var0 = new Example();
IfaceA var1 = var0;
IfaceB var2 = var0;

But, before Java 8, interfaces could not contain any code, only constants and method declarations, so classes could not inherit functionality from them, as they could by extending a base class.

Thus while interfaces provided the polymorphic part of multiple inheritance, they lacked the functionality reuse part.

In Java 8 all that changed with addition of default and static methods to interfaces.

Now, an interface could contain code, and any class implementing it would inherit this functionality.

It appears that Java 9 is about to take this one step further: it will add private methods to interfaces!

Before this, everything in an interface had to be public.

This essentially erases any differences between interfaces and abstract classes, and allows multiple inheritance. But, being a back door feature, it still has some limitations compared to true multiple inheritance that is available in languages like Python and C++:

  • You can not inherit any collection of classes together. Class writer must allow joined inheritance by implementing the class as interface.
  • Unlike regular base classes, interfaces can not be instantiated on their own, even if all the methods of an interface have default implementations.
    This can be easily worked around by creating a dummy class without any code that implements the interface.
  • There are no protected methods.
    Maybe Java 10 will add them…

But basically, after 8 major iterations of the language, you can finally have full blown multiple inheritance in Java.

Conclusion

These features have their official excuses:
Extension methods are supposed to be “syntactic sugar” for “helper” and utility classes.
Default method implementation is suppose to allow extending interfaces without breaking legacy code.

But whatever the original intentions and reasoning were, the fact remains: you can have C# code that calls instance methods on objects that are not part of the original object, and you can now have Java classes that inherit “is a” type and working code from multiple sources.

And I don’t think this is a bad thing.
As long as programmers use these tools correctly, it will make code better.
Fighting your users is always a bad idea, more so if your users are developers themselves.

Do you know of any other features like this that showed up in other languages?
Let me know in the comments or by email!

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