An Ingenious Workaround to Emulate an Application of Union Types in Java

Before I move on with the actual article, I’d like to give credit to Daniel Dietrich, author of the awesome Javaslang library, who has had the idea before me:

Contravariant Generic Bounds

It all started with a tweet:

I wanted to do something like pattern-matching a common super type of a set of types, along the lines of:

<T super T1 | T2 | ... | TN>

Note that what I really wanted is support for union types, not intersection types as I originally claimed.

Why did I want to do that? Because it would be a nice addition to the jOOλ library, which features typesafe tuples for Java:

class Tuple3<T1, T2, T3> {
    final T1 v1;
    final T2 v2;
    final T3 v3;

    // Lots of useful stuff here

What would be nice in a tuple is something like a forEach() method that iterates over all attributes:

tuple(1, "a", null).forEach(System.out::println);

The above would simply yield:


Now, what would this forEach() method’s argument type be? It would have to look like this:

class Tuple3<T1, T2, T3> {
    void forEach(Consumer<? super T1 | T2 | T3> c) {}

The consumer would receive an object that is of type T1 or T2 or T3. But a consumer that accepts a common super type of the previous three types is OK as well. For example, if we have:

Tuple2<Integer, Long> tuple = tuple(1, 2L);
tuple.forEach(v -> 

The above would compile, because Number (or, more formally, Number & Comparable<?> is a common super type of Integer and Long, and it contains a doubleValue() method.

As soon as we’re adding e.g. a String to the tuple, the following will no longer compile:

Tuple3<Integer, Long, String> tuple = 
    tuple(1, 2L, "A");

// Doesn't compile
tuple.forEach((Number v) -> 

Unfortunately, this is not possible in Java

Java currently supports union types (see also algebraic data types) only for exception catch blocks, where you can write things like:

interface X {
    default void print() {}
class X1 extends RuntimeException implements X {}
class X2 extends RuntimeException implements X {}

// With the above
try {
catch (X1 | X2 e) {
    // This compiles for the same reasons!

But unfortunately, catch blocks are the only place in Java that allows for using properties of union types.

This is where Daniel’s clever and cunning workaround comes into play. We can write a static method that performs some “pattern-matching” (if you squint) using generics, the other way round:

static <
    T1 extends T, 
    T2 extends T, 
    T3 extends T
void forEach(
    Tuple3<T1, T2, T3> tuple, 
    Consumer<? super T> consumer
) {

The above can now be used typesafely to infer the common super type(s) of T1, T2, and T3:

Tuple2<Integer, Long> t = tuple(1, 2L);
forEach(t, c -> {

yielding, as expected:


It makes sense, because the generic type constraints are simply specified “the other way round”, i.e. when T1 extends T, forcibly, T super T1

If you squint really hard ;-)

This technique is supposedly used by Daniel in Javaslang’s upcoming pattern matching API. We’re looking forward to seeing that in action!

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