Every rose has its thorns

The dark side of Java 8

Lukas Eder

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

  • … 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

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
. 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
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”

run((Callable<Object>) (() -> null));
run(new Callable<Object>() {
    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

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++)

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

public interface NoTrait {
    default synchronized void noSynchronized() {
        //  ^^^^^^^^^^^^ modifier synchronized
        //  not allowed

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

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

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

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

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
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

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
  • Does your function return boolean? It’s
    called a Predicate
  • Does your function return
    an intlongdouble?
    calledXXToIntYYXXToLongYYXXToDoubleYY something
  • Does your function take no arguments? It’s called
  • 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
  • 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
 courses have drastically increased, recently…
Thankfully, with Lambda expressions, we hardly ever have to
remember all these types!

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