Why There Is Interface Pollution in Java 8

Edwin DalorzoI was reading this interesting post about The Dark Side of Java 8. In it, Lukas Eder, the author, mentions how bad it is that in the JDK 8 the types are not simply called functions. For instance, in a language like C#, there is a set of predefined function types accepting any number of arguments with an optional return type (Func and Action each one going up to 16 parameters of different types T1, T2, T3, …, T16), but in the JDK 8 what we have is a set of different functional interfaces, with different names and different method names, and whose abstract methods represent a subset of well know function signatures/function arities (i.e. nullary, unary, binary, ternary, etc).

The Type Erasure Issue

So, in a way, both languages suffer from some form of interface pollution (or delegate pollution in C#). The only difference is that in C# they all have the same name. In Java, unfortunately, due to type erasure, there is no difference between Function<T1,T2> and Function<T1,T2,T3> or Function<T1,T2,T3,...Tn>, so evidently, we couldn’t simply name them all the same way and we had to come up with creative names for all possible types of function combinations.

Don’t think the expert group did not struggle with this problem. In the words of Brian Goetz in the lambda mailing list:

[…] As a single example, let’s take function types. The lambda strawman offered at devoxx had function types. I insisted we remove them, and this made me unpopular. But my objection to function types was not that I don’t like function types — I love function types — but that function types fought badly with an existing aspect of the Java type system, erasure. Erased function types are the worst of both worlds. So we removed this from the design.

But I am unwilling to say “Java never will have function types” (though I recognize that Java may never have function types.) I believe that in order to get to function types, we have to first deal with erasure. That may, or may not be possible. But in a world of reified structural types, function types start to make a lot more sense […]

So, how does this affect us as developers? The following is a categorization of some of the most important new functional interfaces (and some old ones) in the JDK 8 organized by function return type and the number of expected arguments in the interface method.

Functions with Void Return Type

In the realm of functions with a void return type, we have the following:

Type of Function Lambda Expression Known Functional Interfaces
				() -> doSomething()
				foo  -> System.out.println(foo)
				(console,text) -> console.print(text)
				(sender,host,text) -> sender.send(host, text)
Define your own

Functions with Some Return Type T

In the realm of functions with a return type T, we have the following:

Type of Function Lambda Expression Known Functional Interfaces
				() -> "Hello World"
				n -> n + 1
				n -> n >= 0
				(a,b) -> a > b ? 1 : 0
				(x,y) -> x + y
				(x,y) -> x % y == 0
				(x,y,z) -> 2 * x + Math.sqrt(y) - z
Define your own

An advantage of this approach is that we can define our own interface types with methods accepting as many arguments as we would like, and we could use them to create lambda expressions and method references as we see fit. In other words, we have the power to pollute the world with yet even more new functional interfaces. Also we can create lambda expressions even for interfaces in earlier versions of the JDK or for earlier versions of our own APIs that defined SAM types like these. And so now we have the power to use Runnable and Callable as functional interfaces.

However, these interfaces become more difficult to memorize since they all have different names and methods.

Still, I am one of those wondering why they didn’t solve the problem as in Scala, defining interfaces like Function0, Function1, Function2, …, FunctionN. Perhaps, the only argument I can come up with against that is that they wanted to maximize the possibilities of defining lambda expressions for interfaces in earlier versions of the APIs as mentioned before.

Lack of Value Types

So, evidently type erasure is one driving force here. But if you are one of those wondering why we also need all these additional functional interfaces with similar names and method signatures and whose only difference is the use of a primitive type, then let me remind you that in Java we also lack of value types like those in a language like C#. This means that the generic types used in our generic classes can only be reference types, and not primitive types.

In other words, we can’t do this:

List<int> numbers = asList(1,2,3,4,5);

But we can indeed do this:

List<Integer> numbers = asList(1,2,3,4,5);

The second example, though, incurs in the cost of boxing and unboxing of the wrapped objects back and forth from/to primitive types. This can become really expensive in operations dealing with collections of primitive values. So, the expert group decided to create this explosion of interfaces to deal with the different scenarios. To make things “less worse” they decided to only deal with three basic types: int, long and double.

Quoting the words of Brian Goetz in the lambda mailing list:

More generally: the philosophy behind having specialized primitive streams (e.g., IntStream) is fraught with nasty tradeoffs. On the one hand, it’s lots of ugly code duplication, interface pollution, etc. On the other hand, any kind of arithmetic on boxed ops sucks, and having no story for reducing over ints would be terrible. So we’re in a tough corner, and we’re trying to not make it worse.

Trick #1 for not making it worse is: we’re not doing all eight primitive types. We’re doing int, long, and double; all the others could be simulated by these. Arguably we could get rid of int too, but we don’t think most Java developers are ready for that. Yes, there will be calls for Character, and the answer is “stick it in an int.” (Each specialization is projected to ~100K to the JRE footprint.)

Trick #2 is: we’re using primitive streams to expose things that are best done in the primitive domain (sorting, reduction) but not trying to duplicate everything you can do in the boxed domain. For example, there’s no IntStream.into(), as Aleksey points out. (If there were, the next question(s) would be “Where is IntCollection? IntArrayList? IntConcurrentSkipListMap?) The intention is many streams may start as reference streams and end up as primitive streams, but not vice versa. That’s OK, and that reduces the number of conversions needed (e.g., no overload of map for int -> T, no specialization of Function for int -> T, etc.)

We can see that this was a difficult decision for the expert group. I think few would agree that this is cool, and most of us would most likely agree it was necessary.

The Checked Exceptions Issue

There was a third driving force that could have made things even worse, and it is the fact that Java supports two type of exceptions: checked and unchecked. The compiler requires that we handle or explicitly declare checked exceptions, but it requires nothing for unchecked ones. So, this creates an interesting problem, because the method signatures of most of the functional interfaces do not declare to throw any exceptions. So, for instance, this is not possible:

Writer out = new StringWriter();
Consumer<String> printer = s -> out.write(s); //oops! compiler error

It cannot be done because the write operation throws a checked exception (i.e. IOException) but the signature of the Consumer method does not declare it throws any exception at all. So, the only solution to this problem would have been to create even more interfaces, some declaring exceptions and some not (or come up with yet another mechanism at the language level for exception transparency). Again, to make things “less worse” the expert group decided to do nothing in this case.

In the words of Brian Goetz in the lambda mailing list:

Yes, you’d have to provide your own exceptional SAMs. But then lambda conversion would work fine with them.

The EG discussed additional language and library support for this problem, and in the end felt that this was a bad cost/benefit tradeoff.

Library-based solutions cause a 2x explosion in SAM types (exceptional vs not), which interact badly with existing combinatorial explosions for primitive specialization.

The available language-based solutions were losers from a complexity/value tradeoff. Though there are some alternative solutions we are going to continue to explore — though clearly not for 8 and probably not for 9 either.

In the meantime, you have the tools to do what you want. I get that you prefer we provide that last mile for you (and, secondarily, your request is really a thinly-veiled request for “why don’t you just give up on checked exceptions already”), but I think the current state lets you get your job done.

So, it’s up to us, the developers, to craft yet even more interface explosions to deal with these in a case-by-case basis:

interface IOConsumer<T> {
   void accept(T t) throws IOException;

static<T> Consumer<T> exceptionWrappingBlock(IOConsumer<T> b) {
   return e -> {
	try { b.accept(e); }
	catch (Exception ex) { throw new RuntimeException(ex); }

In order to do:

Writer out = new StringWriter();
Consumer<String> printer = exceptionWrappingBlock(s -> out.write(s));

Probably, in the future (maybe JDK 9) when we get Support for Value Types in Java and Reification, we will be able to get rid of (or at least no longer need to use anymore) these multiple interfaces.

In summary, we can see that the expert group struggled with several design issues. The need, requirement or constraint to keep backwards compatibility made things difficult, then we have other important conditions like the lack of value types, type erasure and checked exceptions. If Java had the first and lacked of the other two the design of JDK 8 would probably have been different. So, we all must understand that these were difficult problems with lots of tradeoffs and the EG had to draw a line somewhere and make a decisions.

So, when we find ourselves in the dark side of Java 8, probably we need to remind ourselves that there is a reason why things are dark in that side of the JDK :-)

Further Reading

  • Related Answer in Stackoverflow
  • Lambda Straw-Man Proposal
  • Neal Gafter on The Future of Java
  • 14 thoughts on “Why There Is Interface Pollution in Java 8

    1. Thanks for digging up that explanation by Brian Goetz, I knew that something like this had been written once, but I couldn’t find it. It’s funny to agree that such an uncool solution, as you put it, was actually the best thing they could’ve done, given all the constraints :-)

      • I have been reading the mailing list for like a year and a half and kept references to the most interesting discussions on decisions like this. Actually I just updated the post to complement with references to other threads in the mailing list discussing other driving forces like type erasure and checked/unchecked exceptions. Simply put, the point I am trying to make is that there are good explanations for the dark side to exist :-)


          No doubt, the EG really consists of experts. I really respect their work. Keeping JDK 8 backwards-compatible with JDK 1-7 is really not the kind of job that I envy :-)

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    8. The main problem of Java in this domain is verbosity. We do not need functions of arity > 1. If you want a function of 5 arguments, you can write:

      Function<Integer, Function<Double, Function<String, Function<List, String> f = a -> b -> c -> d -> e -> … implementation using a, b, c, d and e to produce a string.

      This is clean an straightforward (but verbose).

      • Definitely, we can add currying and tons and tons of more type inference to the list of other functional programming features we are missing in Java now that it has become a little bit more functional.

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