JAX London 2014: A retrospective
Every rose has its thorns

The dark side of Java 8

Lukas Eder
creepy

We’ve mostly focused on the sweet new additions to Java 8 – but there are two sides to every release. Lukas Eder delves into the murkier side of the update.

This post was originally published over at jooq.org as part of a special series focusing on all things Java 8, including how take advantage of lambda expressions, extension methods, and other great stuff. You’ll find the source code on GitHub.

So far, we’ve been showing the thrilling parts of this new major release. But there are also caveats. Lots of them. Things that

  • … are confusing
  • … are wrong
  • … are omitted (for now)
  • … are omitted (for long)

There are always two sides to Java major releases. On the bright side, we get lots of new functionality that most people would say was overdue. Other languages, platforms have had generics long before Java 5. Other languages, platforms have had lambdas long before Java 8. But now, we finally have these features. In the usual quirky Java-way.

Lambda expressions were introduced quite elegantly. The idea of being able to write every anonymous SAM instance as a lambda expression is very compelling from a backwards-compatiblity point of view. So what are the dark sides to Java 8?

Overloading gets even worse

Overloading, generics, and varargs aren’t friends. We’ve explained this in a previous article, and also in this Stack Overflow question. These might not be every day problems in your odd application, but they’re very important problems for API designers and maintainers.

With lambda expressions, things get “worse”. So you think you can provide some convenience API, overloading your existing run() method that accepts a Callable to also accept the new Supplier type:

static <T> T run(Callable<T> c) throws Exception {
    return c.call();
}
 
static <T> T run(Supplier<T> s) throws Exception {
    return s.get();

What looks like perfectly useful Java 7 code is a major pain in Java 8, now. Because you cannot just simply call these methods with a lambda argument:

public static void main(String[] args)
throws Exception {
    run(() -> null);
    //  ^^^^^^^^^^ ambiguous method call
}

Tough luck. You’ll have to resort to either of these “classic” solutions:

run((Callable<Object>) (() -> null));
run(new Callable<Object>() {
    @Override
    public Object call() throws Exception {
        return null;
    }
});

So, while there’s always a workaround, these workarounds always “suck”. That’s quite a bummer, even if things don’t break from a backwards-compatibility perspective.

Not all keywords are supported on default methods

Default methods are a nice addition. Some may claim that Java finally has traits. Others clearly dissociate themselves from the term, e.g. Brian Goetz:

The key goal of adding default methods to Java was “interface evolution”, not “poor man’s traits.”

As found on the lambda-dev mailing list.

Fact is, default methods are quite a bit of an orthogonal and irregular feature to anything else in Java. Here are a couple of critiques:

They cannot be made final

Given that default methods can also be used as convenience methods in API:

public interface NoTrait {
 
    // Run the Runnable exactly once
    default final void run(Runnable r) {
        //  ^^^^^ modifier final not allowed
        run(r, 1);
    }
 
    // Run the Runnable "times" times
    default void run(Runnable r, int times) {
        for (int i = 0; i < times; i++)
            r.run();
    }
}

Unfortunately, the above is not possible, and so the first overloaded convenience method could be overridden in subtypes, even if that makes no sense to the API designer.

They cannot be made synchronized

Bummer! Would that have been difficult to implement in the language?

public interface NoTrait {
    default synchronized void noSynchronized() {
        //  ^^^^^^^^^^^^ modifier synchronized
        //  not allowed
        System.out.println("noSynchronized");
    }
}

Yes, synchronized is used rarely, just like final. But when you have that use-case, why not just allow it? What makes interface method bodies so special?

The default keyword

This is maybe the weirdest and most irregular of all features. The defaultkeyword itself. Let’s compare interfaces and abstract classes:

// Interfaces are always abstract
public /* abstract */ interface NoTrait {
 
    // Abstract methods have no bodies
    // The abstract keyword is optional
    /* abstract */ void run1();
 
    // Concrete methods have bodies
    // The default keyword is mandatory
    default void run2() {}
}
 
// Classes can optionally be abstract
public abstract class NoInterface {
 
    // Abstract methods have no bodies
    // The abstract keyword is mandatory
    abstract void run1();
 
    // Concrete methods have bodies
    // The default keyword mustn't be used
    void run2() {}
}

If the language were re-designed from scratch, it would probably do without any of abstract or default keywords. Both are unnecessary. The mere fact that there is or is not a body is sufficient information for the compiler to assess whether a method is abstract. I.e, how things should be:

public interface NoTrait {
    void run1();
    void run2() {}
}
 
public abstract class NoInterface {
    void run1();
    void run2() {}
}

The above would be much leaner and more regular. It’s a pity that the usefulness of default was never really debated by the EG. Well, it was debated but the EG never wanted to accept this as an option. I’ve tried my luck, with this response:

I don’t think #3 is an option because interfaces with method bodies are unnatural to begin with. At least specifying the “default” keyword gives the reader some context why the language allows a method body. Personally, I wish interfaces would remain as pure contracts (without implementation), but I don’t know of a better option to evolve interfaces.

Again, this is a clear commitment by the EG not to commit to the vision of “traits” in Java. Default methods were a pure necessary means to implement 1-2 other features. They weren’t well-designed from the beginning.

Other modifiers

Luckily, the static modifier made it into the specs, late in the project. It is thus possible to specifiy static methods in interfaces now. For some reason, though, these methods do not need (nor allow!) the default keyword, which must’ve been a totally random decision by the EG, just like you apparently cannot define static final methods in interfaces.

While visibility modifiers were discussed on the lambda-dev mailing list, but were out of scope for this release. Maybe, we can get them in a future release.

Few default methods were actually implemented

Some methods would have sensible default implementations on interface – one might guess. Intuitively, the collections interfaces, like List or Setwould have them on their equals() and hashCode() methods, because the contract for these methods is well-defined on the interfaces. It is also implemented in AbstractList, using listIterator(), which is a reasonable default implementation for most tailor-made lists.

It would’ve been great if these API were retrofitted to make implementing custom collections easier with Java 8. I could make all my business objects implement List for instance, without wasting the single base-class inheritance on AbstractList.

Probably, though, there has been a compelling reason related to backwards-compatibility that prevented the Java 8 team at Oracle from implementing these default methods. Whoever sends us the reason why this was omitted will get a free jOOQ sticker :-)

The wasn’t invented here – mentality

This, too, was criticised a couple of times on the lambda-dev EG mailing list. And while writing this blog series, I can only confirm that the new functional interfaces are very confusing to remember. They’re confusing for these reasons:

Some primitive types are more equal than others

The intlongdouble primitive types are preferred compared to all the others, in that they have a functional interface in the java.util.functionpackage, and in the whole Streams API. boolean is a second-class citizen, as it still made it into the package in the form of a BooleanSupplier or aPredicate, or worse: IntPredicate.

All the other primitive types don’t really exist in this area. I.e. there are no special types for byteshortfloat, and char. While the argument of meeting deadlines is certainly a valid one, this quirky status-quo will make the language even harder to learn for newbies.

The types aren’t just called Function

Let’s be frank. All of these types are simply “functions”. No one really cares about the implicit difference between a Consumer, a Predicate, aUnaryOperator, etc.

In fact, when you’re looking for a type with a non-void return value and two arguments, what would you probably be calling it? Function2? Well, you were wrong. It is called a BiFunction.

Here’s a decision tree to know how the type you’re looking for is called:

  • Does your function return void? It’s called a Consumer
  • Does your function return boolean? It’s called a Predicate
  • Does your function return an intlongdouble? It’s calledXXToIntYYXXToLongYYXXToDoubleYY something
  • Does your function take no arguments? It’s called a Supplier
  • Does your function take a single intlongdouble argument? It’s called an IntXXLongXXDoubleXX something
  • Does your function take two arguments? It’s called BiXX
  • Does your function take two arguments of the same type? It’s calledBinaryOperator
  • Does your function return the same type as it takes as a single argument? It’s called UnaryOperator
  • Does your function take two arguments of which the first is a reference type and the second is a primitive type? It’s called ObjXXConsumer(only consumers exist with that configuration)
  • Else: It’s called Function

Good lord! We should certainly go over to Oracle Education to check if the price for Oracle Certified Java Programmer courses have drastically increased, recently… Thankfully, with Lambda expressions, we hardly ever have to remember all these types!

Author
Lukas Eder
Lukas is a Java and SQL aficionado. He’s the founder and head of R&D at Data Geekery GmbH (datageekery.com), the company behind jOOQ (jooq.org) , the best way to write SQL in Java.
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