Towards member patterns

Brian Goetz
January 2024

Time to check in on where things are in the bigger picture of patterns as class members. Note: while this document may have illustrative examples, you should not take that as a definitive statement of syntax.

We’ve already dipped our toes in the water with record patterns. A record pattern looks like:

case R(p1, p2, ... pn):

where R is a record type and are nested patterns that are matched to its components. Because records are defined by their state description, we can automatically derive record patterns “for free”, just as we derive record constructors, accessors, etc.

There are many other classes that would benefit from being deconstructible with patterns. To that end, we will generalize record patterns to deconstruction patterns, where any class can declare an explicit deconstruction pattern and participate in pattern matching like records do.

Deconstruction patterns are not the end of the user-declared pattern story. Just as some classes prefer to expose static factories rather than constructors, they will be able to expose corresponding static patterns. And there is also a role for “instance patterns” and “pattern objects” as well.

Looking only at record and deconstruction patterns, it might be tempting to think that patterns are “just” methods with multiple return. But this would be extrapolating from a special case. Pattern matching is intrinsically conditional; the extraction of values from a target is conditioned on whether the target matches the pattern. For the patterns we’ve seen so far — type patterns and record patterns — matching can be determined entirely by types. But more sophisticated patterns can also depend on other aspects of object state. For example, a pattern corresponding to the static factory Optional::of requires not only that the match candidate be of type Optional, but that the match candidate is an Optional that actually holds a value. Similarly, a pattern corresponding a regular expression requires the match candidate to not only be a String, but to match the regular expression.

The key intuition around patterns

A key capability of objects is aggregation; the combination of component values into a higher-level composite that incorporates those components. Java facilitates a variety of idioms for aggregation, including constructors, factories, builders, etc. The dual of aggregation is destructuring or decomposition, which takes an aggregate and attempts to recover its “ingredients”. However, Java’s support for destructuring has historically been far more ad-hoc, largely limited to “write some getters”. Pattern matching seeks to put destructuring on the same firm foundation as aggregation.

Deconstruction patterns (such as record patterns) are the dual of construction. If we construct an object:

Object o = new Point(x, y);

we can deconstruct it with a deconstruction pattern:

if (o instanceof Point(var x, var y)) { ... }

Intuitively, this pattern match asks “could this object have come from invoking the constructor new Point(x, y) for some x and y, and if so, tell me what they are.”

While not all patterns exist in direct correspondence to another constructor or method, this intuition that a pattern reconstructs the ingredients to an aggregation operation is central to the design; we’ll explore the limitations of this intuition in greater detail later.

Use cases for declared patterns

Before turning to how patterns fit into the object model, let’s look at some of the potential use cases for patterns in APIs.

Recovering construction arguments

Deconstruction patterns are the dual of constructors; where a constructor takes N arguments and aggregates them into an object, a deconstruction pattern takes an aggregate and decomposes it into its components. Constructors are unusual in that they are instance behavior (they have an implicit this argument), but are not inherited; deconstruction patterns are the same. For deconstruction patterns (but not for all instance patterns), the match candidate is always the receiver. Tentatively, we’ve decided that deconstruction patterns are always unconditional; that a deconstruction pattern for class Foo should match any instance of Foo. At the use site, deconstruction patterns use the same syntax as record patterns:

case Point(int x, int y):

Just as constructors can be overloaded, so can deconstruction patterns. However, the reasons we might overload deconstruction patterns are slightly different than for constructors, and so it may well be the case that we end up with fewer overloads of deconstruction patterns than we do of constructors. Constructors often form telescoping sets, both for reasons of syntactic convenience at the use site (fewer arguments to specify) and to avoid brittleness (clients can let the class implementation pick the defaults rather than hard-coding them.) This motivation is less pronounced for deconstruction patterns (unwanted bindings can be ignored with _), so it is quite possible that authors will choose to have one deconstruction pattern overload per telescoping constructor set, rather than one per constructor.

There is no requirement for deconstruction patterns to expose the exact same API as constructors, but we expect this will be common, at least for classes for which the construction process is effectively an aggregation operation on the constructor arguments.

Recovering static factory arguments

Not all classes want to expose their constructors; sometimes classes prefer to expose static factories instead. In this case, the class should be able to expose corresponding static patterns as well.

For a class like Optional, which exposes factories Optional::of and Optional::empty, the object state incorporates not only the factory arguments, but which factory was chosen. Accordingly, it makes sense to deconstruct the object in the same way:

switch (optional) {
    case Optional.of(var payload): ...
    case Optional.empty(): ...

Such patterns are necessarily conditional, asking the Pattern Question: “could this Optional have come from the Optional::of factory, and if so, with what argument?” Static patterns, like static methods, lack a receiver, so this is not defined in the body of a static pattern. However, we will need a way to denote the match candidate, so its state can be examined by the pattern body.

Another feature of static methods is that they can be used to put a factory for a class C in another class, whether one in the same maintenance domain (such as the Collections) or in some other package. This feature is shared by static patterns.

Conversions and queries

Another application for static patterns is the dual of static methods for conversions. For a static method like Integer::toString, which converts an int to its String representation, a corresponding static pattern Integer::toString can ask the Pattern Question: “could this String have come from converting an integer to String, and if so, what integer”.

Some groups of query methods in existing APIs are patterns in disguise. The class java.lang.Class has a pair of instance methods, Class::isArray and Class::getComponentType, that work together to determine if the Class describes an array type, and if so, provide its component type. This question is much better framed as a single pattern:

case Class.arrayClass(var componentType):

The two existing methods are made more complicated by their relationship to each other; Class::getComponentType has a precondition (the Class must describe an array type) and therefore has to specify and implement what to do if the precondition fails, and the relationship between the methods is captured only in documentation. By combining them into a single pattern, it become impossible to misuse (because of the inherent conditionality of patterns) and easier to understand (because it can all be documented in one place.)

This hypothetical Class::arrayClass pattern also has a sensible dual as a factory method:

static<T> Class<T[]> arrayClass(Class<T> componentType)

which produces the array Class for the array type whose component type is provided. An API need not provide both directions of a conversion, but if it does, the two generally strengthen each other. This method/pattern pair could be either static or instance members, depending on API design choice.

Another form of “conversion” method / pattern pair, even though both types are the same, is “power of two”. A powerOfTwo method takes an exponent and returns the resulting power of two; a powerOfTwo pattern asks if its match candidate is a power of two, and if so, binds the base-two logarithm.

Numeric conversions

As Project Valhalla gives us the ability to declare new numeric types, we will want to be able to convert these new types to other numeric types. For unconditional conversions (such as widening half-float to float), an ordinary method will suffice:

float widen(HalfFloat f);

But the reverse is unlikely to be unconditional; narrowing conversions can fail if the value cannot be represented in the narrower type. This is better represented as a pattern which asks the Pattern Question: “could this float have come from widening a HalfFloat, and if so, tell me what HalfFloat that is.” A widening conversion (or boxing conversion) is best represented by a pair of members, an ordinary method for the unconditional direction, and a pattern for the conditional direction.

Conditional extraction

Some operations, such as matching a string to a regular expression with capture groups, are pattern matches in disguise. We should be able to take a regular expression R and match against it with instanceof or switch, binding capture groups (using varargs patterns) if it matches.

Member patterns in the object model

We currently have three kinds of executable class members: constructors, static methods, and instance methods. (Actually constructors are not members, but we will leave this pedantic detail aside for now.) As the above examples show, each of these can be amenable to a dual member which asks the Pattern Question about it.

Patterns are dual to constructors and methods in two ways: structurally and semantically. Structurally, patterns invert the relationship between inputs and outputs: a method takes N arguments as input and produces a single result, and the corresponding pattern takes a candidate result (the “match candidate”) and conditionally produces N bindings. Semantically, patterns ask the Pattern Question: could this result have originated by some invocation of the dual operation.

Patterns as inverse methods and constructors

One way to frame patterns in the object model is as inverse constructors and inverse methods. For purposes of this document, I will use an illustrative syntax that directly evokes this duality (but remember, we’re not discussing syntax now):

class Point {
    final int x, y;

    // Constructor
    Point(int x, int y) { ... }

    // Deconstruction pattern
    inverse Point(int x, int y) { ... }

class Optional<T> {
    // Static factories
    static<T> Optional<T> of(T t) { ... }
    static<T> Optional<T> empty() { ... }

    // Static patterns
    static<T> inverse Optional<T> of(T t) { ... }
    static<T> inverse Optional<T> empty() { ... }

Point has a constructor and an inverse constructor (deconstruction pattern) for the external representation (int x, int y); in an inverse constructor, the binding list appears where the parameter list does in the constructor. Optional<T> has static factories and corresponding patterns for empty and of. As with inverse constructors, the binding list of a pattern appears in the position that the parameters appear in a method declaration; additionally, the match candidate type appears in the position that the return value appears in a method declaration. In both cases, the declaration site and use site of the pattern uses the same syntax.

In the body of an inverse constructor or method, we need to be able to talk about the match candidate. In this model, the match candidate has a type determined by the declaration (for an inverse constructor, the class; for an inverse method, the type specified in the “return position” of the inverse method declaration), and there is a predefined context variable (e.g., that) that refers to the match candidate. For inverse constructors, the receiver (this) is aliased to the match candidate (that), but not necessarily so for inverse methods.

Do all methods potentially have inverses?

We’ve seen examples of constructors, static methods, and instance methods that have sensible inverses, but not all methods do. For example, methods that operate primarily by side effects (such as mutative methods like setters or List::add) are not suitable candidates for inverses. Similarly, pure functions that “co-mingle” their arguments (such as arithmetic operators) are also not suitable candidates for inverses, because the ingredients to the operation typically can’t be recovered from the result (i.e., 4 could be the result of plus(2, 2) or plus(1, 3)).

Intuitively, the methods that are invertible are the ones that are aggregative. The constructor of a (well-behaved) record is aggregative, since all the information passed to the constructor is preserved in the result. Factories like Optional::of are similarly aggregative, as are non-lossy conversions such as widening or boxing conversions.

Ideally, an aggregation operation and its corresponding inverse form an embedding projection pair between the aggregate and a component space. Intuitively, an embedding-projection pair is an algebraic structure defined by a pair of functions between two sets such that composing in one direction (embed-then-project) is an identity, and composing in the other direction (project-then-embed) is a well-behaved approximation.


Conversion methods are a frequent candidate for inversion. We already have

static String toString(int i) { ... }

to which the obvious inverse is

static inverse String toString(int i) { ... }

and we can inspect a string to see if it is the string representation of an integer with

if (s instanceof Integer.toString(int i)) { ... }

This composes nicely with deconstruction patterns; if we have a Box<String> and want to ask whether the contained string is really the string representation of an integer, we can ask:

case Box(Integer.toString(int i)):

which conveniently looks just like the composition of constructors or factories used to create such an instance (new Box(Integer.toString(3))).

When it comes to user-definable numeric conversions, the most likely strategy involves combining related operators in a single witness object. For example, numeric conversion might be modeled as:

interface NumericConversion<FROM, TO> {
    TO convert(FROM from);
    inverse TO convert(FROM from);

which reflects the fact that conversion is total in one direction (widening, boxing) and conditional in the other (narrowing, unboxing.)

Regular expression matching

Regular expressions are a form of ad-hoc pattern; a given string might match a given regex, or not, and if it does, it might product multiple bindings (the capture groups.) It would be nice to be able to express regular expression matches as ordinary pattern matches.

Conveniently, we already have an object representation of regular expressions — java.util.Pattern. Which is an ideal place to put an instance pattern:

// varargs pattern
public inverse String match(String... groups) {
    Matcher m = matcher(that);    // *that* is the match candidate
    if (m.matches())              // receiver for matcher() is the Pattern
        __yield IntStream.range(1, m.groupCount())

And now, we want to express “does string s match any of these regular expressions”:

static final Pattern As = Pattern.compile("([aA]*)");
static final Pattern Bs = Pattern.compile("([bB]*)");
static final Pattern Cs = Pattern.compile("([cC]*)");


switch (aString) {
    case As.match(String as) -> ...
    case Bs.match(String bs) -> ...
    case Cs.match(String cs) -> ...

Essentially, j.u.r.Pattern becomes a pattern object, where the state of the object is used to determine whether or not it matches any given input. (There is nothing stopping a class from having multiple patterns, just as it can have multiple methods.)

Pattern resolution

When we invoke a method, sometimes we are able to refer to the method with an unqualified name (e.g., m(3)), and sometimes the method must be qualified with a type name, package name, or a receiver object. The same is true for declared patterns.

Constructors for classes that are in the same package, or have been imported, can be referred to with an unqualified name; constructors can also be qualified with a package name. The same is true for deconstruction patterns:

case Foo(int x, int y):         // unqualified
case x, int y): // qualified by package

Static methods that are declared in the current class or an enclosing class, or are statically imported, can be referred to with an unqualified name; static methods can also be qualified with a type name. The same is true for static patterns:

case powerOfTwo(int exp):  // unqualified
case Optional.of(var e):   // qualified by class

Instance methods invoked on the current object can be referred to with an unqualified name; instance methods can also be qualified by a receiver object. The same is true for instance patterns:

case match(String s):    // unqualified
case As.match(String s): // qualified by receiver

In a qualified pattern x.y, x might be a package name, a class name, or a (effectively final) receiver variable; we use the same rules for choosing how to interpret a qualifier for patterns as we do for method invocations.

Benefits of explicit duality

Declaring method-pattern pairs whose structure and name are the same yields many benefits. It means that we take things apart using the same abstractions used to put them together, which makes code more readable and less error-prone.

Referring to a inverse pair of operations by a single name is simpler than having separate names for each direction; not only don’t we need to come up with a name for the other direction, we also don’t need to teach clients that “these two names are inverses”, because the inverses have the same name already. What we know about the method Integer::toString immediately carries over to its inverse.

Further, thinking about a method-pattern pair provides a normalizing force to actually ensuring the two are inverses; if we just had two related methods xToY and yToX, they might diverge subtly because the connection between the two members is not very strong.

Finally, this gives the language permission to treat the pair of members as a thing in some cases, such as the use of ctor-dtor pairs in “withers” or serialization.

The explicit duality takes a little time to get used to. We have many years of experience of naming a method for its directionality, so people’s first reaction is often “the pattern should be called Integer.fromString, not Integer.toString”. So people will initially bristle at giving both directions the same name, especially when one implies a directionality such as toString. (In these cases, we can fall back on a convention that says that we should name it for the total direction.)

Pattern lambdas, pattern objects, pattern references

Interfaces with a single abstract method (SAM) are called functional interfaces and we support a conversion (historically called SAM conversion) from lambdas to functional interfaces. Interfaces with a single abstract pattern can benefit from a similar conversion (call this “SAP” conversion.)

In the early days of Streams, people complained about processing a stream using instanceof and cast:

Stream<Object> objects = ...
Stream<String> strings = objects.filter(x -> x instanceof String)
                                .map(x -> (String) x);

This locution is disappointing both for its verbosity (saying the same thing in two different ways) and its efficiency (doing the same work basically twice.) Later, it became possible to slightly simplify this using mapMulti:

objects.mapMulti((x, sink) -> { if (x instanceof String s) sink.accept(s); })

But, ultimately this stream pipeline is a pattern match; we want to match the elements to the pattern String s, and get a stream of the matched string bindings. We are now in a position to expose this more directly. Suppose we had the following SAP interface:

interface Match<T, U> {
    inverse U match(T t);

then Stream could expose a match method:

<U> Stream<T> match(Match<T, U> pattern);

We can SAP-convert a lambda whose yielded bindings are compatible with the sole abstract pattern in the SAP interface::

Match<Object, String> m = o -> { if (o instanceof String s) __yield(s); };
... stream.match(m) ...

And we can do the same with pattern references to existing patterns that are compatible with the sole pattern in a SAP interface. As a special case, we can also support a conversion from type patterns to a compatible SAP type with an instanceof pattern reference (analogous to a new method reference):


where String::instanceof means the same as the previous lambda example. This means that APIs like Stream can abstract over conditional behavior as well as unconditional.