Pattern match Optional in Java 21

I'm going to describe a trick to get pattern patching on Optional in Java 21, but one you'll probably never actually use.

Using Optional

As of Java 21, Pattern matching in Java allows us to check a value against a type like an instanceof with a new variable being declared of the correct type. Pattern matching can handle simple types and the deconstruction of records. But pattern matching of arbitrary classes like Optional is not yet supported. (Work to support pattern match methods is ongoing).

In normal code, the best way to use Optional is with one of the functional methods:

  var addressOpt = findAddress(personId);
  var addressStr = addressOpt
        .map(address -> address.format())
        .orElse("No address available");

This works well in most cases. But sometimes you want to use the Optional with a return statement. This results in code using get() like this:

  var addressOpt = findAddress(personId);
  if (addressOpt.isPresent()) {
    // early return if address found
    return addressOpt.get().format();
  }
  // lots of other code to handle case when address not found

One way to improve this is to write a simple method:

  /**
   * Converts an optional to an iterable for use in the for-each statement.
   *
   * @param <lT> the type of optional element
   * @param optional the optional
   * @return an iterable representation of the optional
   */
  public static <lT> Iterable<lT> inOptional(Optional<lT> optional) {
    return optional.isPresent() ? List.of(optional.get()): List.of();
  }

Which allows the following neat form:

  for (var address : inOptional(findAddress(personId))) {
    // early return if address found
    return address.format();
  }
  // lots of other code to handle case when address not found

This is a great approach providing that you don't need an else branch.

Using Optional with Pattern matching

With Java 21 and pattern matching we have a new way to do this!

  if (findAddress(personId).orElse(null) instanceof Address address) {
    // early return if address found
    return address.format();
  } else {
    // lots of other code to handle case when address not found
  }

This makes use of the fact that instanceof rejects null. Any expression can be used that creates an Optional - just pop .orElse(null) on the end.

Note that this does not work with Java 17 in most cases, because unconditional patterns were not permitted. (In most cases, the expression will return a value of the type being checked for, and the compiler rejects it as being "always true". Of course this was a simplification in ealier Java versions, as the null really needed to be checked for.)

Is this a useful trick?

In reality, this is of marginal use. Where I think it could be of is a long if-else chain, for example:

  if (obj instanceof Integer value) {
    // use value
  } else if (obj instanceof Long value) {
    // use value
  } else if (obj instanceof Double value) {
    // use value
  } else if (lookupConverter(obj).orElse(null) instanceof Converter conv) {
    // use converter
  } else {
    // even more options
  }

(It is valuable here as it avoids calling lookupConverter(obj) until necessary.)

Anyway, it is a fun trick even if you never use it!

One day soon I imagine we will use something like this:

  if (lookupConverter(obj) instanceof Optional.of(Converter conv)) {
    // use converter
  } else {
    // even more options
  }

which I think we'd all agree is the better approach.

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