JEP 456: Unnamed Variables & Patterns

OwnerAngelos Bimpoudis
TypeFeature
ScopeSE
StatusIntegrated
Release22
Componentspecification / language
Discussionamber dash dev at openjdk dot org
EffortS
DurationS
Reviewed byBrian Goetz
Endorsed byBrian Goetz
Created2023/07/10 16:17
Updated2023/11/27 19:45
Issue8311828

Summary

Enhance the Java programming language with unnamed variables and unnamed patterns, which can be used when variable declarations or nested patterns are required but never used. Both are denoted by the underscore character, _.

History

Unnamed variables and unnamed patterns first previewed in JDK 21 via JEP 443, which was titled Unnamed Patterns and Variables. We here propose to finalize this feature without change.

Goals

Non-Goals

Motivation

Developers sometimes declare variables that they do not intend to use, whether as a matter of code style or because the language requires variable declarations in certain contexts. The intent of non-use is known at the time the code is written, but if it is not captured explicitly then later maintainers might accidentally use the variable, thereby violating the intent. If we could make it impossible to accidentally use such variables then code would be more informative, more readable, and less prone to error.

Unused variables

The need to declare a variable that is never used is especially common in code whose side-effect is more important than its result. For example, this code calculates total as the side effect of a loop, without using the loop variable order:

static int count(Iterable<Order> orders) {
    int total = 0;
    for (Order order : orders)    // order is unused
        total++;
    return total;
}

The prominence of the declaration of order is unfortunate, given that order is not used. The declaration can be shortened to var order, but there is no way to avoid giving this variable a name. The name itself can be shortened to, e.g., o, but this syntactic trick does not communicate the intent that the variable never be used. In addition, static analysis tools typically complain about unused variables, even when the developer intends non-use and may not have a way to silence the warnings.

For another example where the side effect of an expression is more important than its result, the following code dequeues data but needs only two out of every three elements:

Queue<Integer> q = ... // x1, y1, z1, x2, y2, z2 ..
while (q.size() >= 3) {
   int x = q.remove();
   int y = q.remove();
   int z = q.remove();            // z is unused
    ... new Point(x, y) ...
}

The third call to remove() has the desired side effect — dequeuing an element — regardless of whether its result is assigned to a variable, so the declaration of z could be elided. However, for maintainability, the author of this code may wish to consistently denote the result of remove() by declaring a variable. They currently have two options, both unpleasant:

Unused variables occur frequently in two other statements that focus on side effects:

Even code without side effects must sometimes declare unused variables. For example:

...stream.collect(Collectors.toMap(String::toUpperCase,
                                   v -> "NODATA"));

This code generates a map which maps each key to the same placeholder value. Since the lambda parameter v is not used, its name is irrelevant.

In all these scenarios, where variables are unused and their names are irrelevant, it would be better if we could simply declare variables with no name. This would free maintainers from having to understand irrelevant names, and would avoid false positives on non-use from static analysis tools.

The kinds of variables that can reasonably be declared with no name are those which have no visibility outside a method: local variables, exception parameters, and lambda parameters, as shown above. These kinds of variables can be renamed or made unnamed without external impact. In contrast, fields — even if they are private — communicate the state of an object across methods, and unnamed state is neither helpful nor maintainable.

Unused pattern variables

Local variables can also be declared by type patterns — such local variables are known as pattern variables — and so type patterns can also declare variables that are unused. Consider the following code, which uses type patterns in the case labels of a switch statement that switches over an instance of a sealed class Ball:

sealed abstract class Ball permits RedBall, BlueBall, GreenBall { }
final  class RedBall   extends Ball { }
final  class BlueBall  extends Ball { }
final  class GreenBall extends Ball { }

Ball ball = ...
switch (ball) {
    case RedBall   red   -> process(ball);
    case BlueBall  blue  -> process(ball);
    case GreenBall green -> stopProcessing();
}

The cases of the switch examine the type of the Ball using type patterns, but the pattern variables red, blue, and green are not used on the right-hand sides of the case clauses. This code would be clearer if we could elide these variable names.

Now suppose that we define a record class Box which can hold any type of Ball, but might also hold the null value:

record Box<T extends Ball>(T content) { }

Box<? extends Ball> box = ...
switch (box) {
    case Box(RedBall   red)     -> processBox(box);
    case Box(BlueBall  blue)    -> processBox(box);
    case Box(GreenBall green)   -> stopProcessing();
    case Box(var       itsNull) -> pickAnotherBox();
}

The nested type patterns still declare pattern variables that are not used. Since this switch is more involved than the previous one, eliding the names of the unused variables in the nested type patterns would even further improve readability.

Unused nested patterns

We can nest records within records, leading to situations in which the shape of a data structure is as important as the data items within it. For example:

record Point(int x, int y) { }
enum Color { RED, GREEN, BLUE }
record ColoredPoint(Point p, Color c) { }

... new ColoredPoint(new Point(3,4), Color.GREEN) ...

if (r instanceof ColoredPoint(Point p, Color c)) {
    ... p.x() ... p.y() ...
}

In this code, one part of the program creates a ColoredPoint instance while another part uses a pattern instanceof to test whether a variable is a ColoredPoint and, if so, extract its two component values.

Record patterns such as ColoredPoint(Point p, Color c) are pleasingly descriptive, but it is common for programs to use only some of the component values for further processing. For example, the code above uses only p in the if block, not c. It is laborious to write out type patterns for all the components of a record class every time we do such pattern matching. Furthermore, it is not visually clear that the entire Color component is irrelevant; this makes the condition in the if block harder to read, too. This is especially evident when record patterns are nested to extract data within components, as in:

if (r instanceof ColoredPoint(Point(int x, int y), Color c)) {
    ... x ... y ...
}

We could use an unnamed pattern variable to reduce the visual cost, e.g. ColoredPoint(Point(int x, int y), Color _), but the presence of the Color type in the type pattern is distracting. We could remove that by using var, e.g. ColoredPoint(Point(int x, int y), var _), but the nested type pattern var _ still has excessive weight. It would better to reduce the visual cost even further by omitting unnecessary components altogether. This would both simplify the task of writing record patterns and improve readability, by removing clutter from the code.

Description

An unnamed variable is declared by using an underscore character, _ (U+005F), to stand in for the name of the local variable in a local variable declaration statement, or an exception parameter in a catch clause, or a lambda parameter in a lambda expression.

An unnamed pattern variable is declared by using an underscore character to stand in for the pattern variable in a type pattern.

The unnamed pattern is denoted by an underscore character and is equivalent to the unnamed type pattern var _. It allows both the type and name of a record component to be elided in pattern matching.

A single underscore character is the lightest reasonable syntax for signifying the absence of a name. It is commonly used in other languages, such as Scala and Python, for this purpose. A single underscore was, originally, a valid identifier in Java 1.0, but we later reclaimed it for unnamed variables and patterns: We started issuing compile-time warnings when underscore was used as an identifier in Java 8 (2014), and we removed such identifiers from the language specification, thereby turning those warnings into errors, in Java 9 (2017, JEP 213).

The ability to use underscore in identifiers of length two or more is unchanged, since underscore remains a Java letter and a Java letter-or-digit. For example, identifiers such as _age and MAX_AGE and __ (two underscores) continue to be legal.

The ability to use underscore as a digit separator is also unchanged. For example, numeric literals such as 123_456_789 and 0b1010_0101 continue to be legal.

Unnamed variables

The following kinds of declarations can introduce either a named variable (denoted by an identifier) or an unnamed variable (denoted by an underscore):

Declaring an unnamed variable does not place a name in scope, so the variable cannot be written or read after it is initialized. An initializer must be provided for an unnamed variable declared in a local variable declaration statement or in the resource specification of a try-with-resources statement.

An unnamed variable never shadows any other variable, since it has no name, so multiple unnamed variables can be declared in the same block.

Here are the examples given above, rewritten to use unnamed variables.

Unnamed pattern variables

An unnamed pattern variable can appear in a type pattern (JLS §14.30.1), including var type patterns, regardless of whether the type pattern appears at the top level or is nested in a record pattern. For example, the Ball example can now be written:

switch (ball) {
    case RedBall _   -> process(ball);          // Unnamed pattern variable
    case BlueBall _  -> process(ball);          // Unnamed pattern variable
    case GreenBall _ -> stopProcessing();       // Unnamed pattern variable
}

and the Box and Ball example:

switch (box) {
    case Box(RedBall _)   -> processBox(box);   // Unnamed pattern variable
    case Box(BlueBall _)  -> processBox(box);   // Unnamed pattern variable
    case Box(GreenBall _) -> stopProcessing();  // Unnamed pattern variable
    case Box(var _)       -> pickAnotherBox();  // Unnamed pattern variable
}

By allowing us to elide names, unnamed pattern variables make run-time data exploration based on type patterns visually clearer, both in switch blocks and with the instanceof operator.

Multiple patterns in case labels

Currently, case labels are restricted to contain at most one pattern. With the introduction of unnamed pattern variables and unnamed patterns, it is more likely that we will have within a single switch block several case clauses with different patterns but the same right-hand side. For example, in the Box and Ball example the first two clauses have the same right-hand side but different patterns:

switch (box) {
    case Box(RedBall _)   -> processBox(box);
    case Box(BlueBall _)  -> processBox(box);
    case Box(GreenBall _) -> stopProcessing();
    case Box(var _)       -> pickAnotherBox();
}

We could simplify matters by allowing the first two patterns to appear in the same case label:

switch (box) {
    case Box(RedBall _), Box(BlueBall _) -> processBox(box);
    case Box(GreenBall _)                -> stopProcessing();
    case Box(var _)                      -> pickAnotherBox();
}

We therefore revise the grammar for switch labels (JLS §14.11.1) to

SwitchLabel:
    case CaseConstant {, CaseConstant}
    case null [, default]
    case CasePattern {, CasePattern } [Guard]
    default

and define the semantics of a case label with multiple patterns as matching a value if the value matches any of the patterns.

If a case label has multiple patterns then it is a compile-time error for any of the patterns to declare any pattern variables.

A case label with multiple case patterns can have a guard. The guard governs the case as a whole, rather than the individual patterns. For example, assuming that there is an int variable x, the first case of the previous example could be further constrained:

case Box(RedBall _), Box(BlueBall _) when x == 42 -> processBox(b);

Guards are properties of case labels, not individual patterns within a case label, so writing more than one guard is prohibited:

case Box(RedBall _) when x == 0, Box(BlueBall _) when x == 42 -> processBox(b);
    // compile-time error

The unnamed pattern

The unnamed pattern is an unconditional pattern that matches anything but declares and initializes nothing. Like the unnamed type pattern var _ , the unnamed pattern can be nested in a record pattern. It cannot, however, be used as a top-level pattern in, e.g., an instanceof expression or a case label.

Consequently, the earlier example can omit the type pattern for the Color component entirely:

if (r instanceof ColoredPoint(Point(int x, int y), _)) { ... x ... y ... }

Likewise, we can extract the Color component value while eliding the record pattern for the Point component:

if (r instanceof ColoredPoint(_, Color c)) { ... c ... }

In deeply nested positions, using the unnamed pattern improves the readability of code that does complex data extraction. For example:

if (r instanceof ColoredPoint(Point(int x, _), _)) { ... x ... }

This code extracts the x coordinate of the nested Point while making it clear that the y and Color component values are not extracted.

Revisiting the Box and Ball example, we can further simplify its final case label by using the unnamed pattern instead of var _:

switch (box) {
    case Box(RedBall _), Box(BlueBall _) -> processBox(box);
    case Box(GreenBall _)                -> stopProcessing();
    case Box(_)                          -> pickAnotherBox();
}

Risks and Assumptions

Alternatives