Summary
-------

Enhance the Java programming language with _[sealed classes and interfaces](https://cr.openjdk.java.net/~briangoetz/amber/datum.html)_.  Sealed classes and interfaces restrict which other classes or interfaces may extend or implement them. This is a [preview language feature](https://openjdk.java.net/jeps/12) in JDK 15.

Goals
-----

 - Allow the author of a class or interface to control which code is responsible for implementing it.
 - Provide a more declarative way than access modifiers to restrict the use of a superclass.
 - Support future directions in [pattern matching](https://cr.openjdk.java.net/~briangoetz/amber/pattern-match.html) by underpinning the _exhaustive_ analysis of patterns.

Non-Goals
---------

 - It is not a goal to provide new forms of access control such as "friends".
 - It is not a goal to change `final` in any way.

Motivation
----------

In Java, a class hierarchy enables the reuse of code via inheritance: The methods of a superclass can be inherited (and thus reused) by many subclasses. However, the purpose of a class hierarchy is not always to reuse code. Sometimes, its purpose is to model the various possibilities that exist in a domain, such as the kinds of shapes supported by a graphics library or the kinds of loans supported by a financial application. When the class hierarchy is used in this way, restricting the set of subclasses can streamline the modeling.

For example, in a graphics library, the author of a class `Shape` may intend that only particular classes can extend `Shape`, since much of the library's work involves handling each kind of shape in the appropriate way. The author is interested in the clarity of code that handles _known_ subclasses of `Shape`, and not interested in writing code to defend against _unknown_ subclasses of `Shape`. Allowing arbitrary classes to extend `Shape`, and thus inherit its code for reuse, is not a goal in this case. Unfortunately, Java assumes that code reuse is always a goal: If `Shape` can be extended at all, then it can be extended by any number of classes. It would be helpful to relax this assumption so that an author can declare a class hierarchy that is not open for extension by arbitrary classes. Code reuse would still be possible within such a closed class hierarchy, but not beyond.  

Java developers are familiar with the idea of restricting the set of subclasses because it often crops up in API design. The language provides limited tools in this area: either make a class `final`, so it has zero subclasses, or make a class or its constructor package-private, so it can only have subclasses in the same package. An example of a package-private superclass [appears in the JDK](http://hg.openjdk.java.net/jdk/jdk/file/tip/src/java.base/share/classes/java/lang/):

```
package java.lang;

abstract class AbstractStringBuilder {...}
public final class StringBuffer  extends AbstractStringBuilder {...}
public final class StringBuilder extends AbstractStringBuilder {...}
```

The package-private approach is useful when the goal is code reuse, such as the subclasses of `AbstractStringBuilder` sharing its code for `append`. However, the approach is useless when the goal is modeling alternatives, since user code cannot access the key abstraction -- the superclass -- in order to `switch` over it. It is not possible to allow users to access the superclass without also allowing them to extend it. (Even within a graphics library that declares `Shape` and its subclasses, it would be unfortunate if only one package could access `Shape`.)  

In summary, it should be possible for a superclass to be widely _accessible_ (since it represents an important abstraction for users) but not widely _extensible_ (since its subclasses should be restricted to those known to the author). Such a superclass should be able to express that it is co-developed with a given set of subclasses, both to document intent for the reader and to allow enforcement by the Java compiler. At the same time, the superclass should not unduly constrain its subclasses by, e.g., forcing them to be `final` or preventing them from defining their own state.


Description
-----------

A _sealed_ class or interface can be extended or implemented only by those classes and interfaces permitted to do so.

A class is sealed by applying the `sealed` modifier to its declaration. Then, after any `extends` and `implements` clauses, the `permits` clause specifies the classes that are permitted to extend the sealed class. For example, the following declaration of `Shape` specifies three permitted subclasses:

```
package com.example.geometry;

public abstract sealed class Shape
    permits Circle, Rectangle, Square {...}
```

The classes specified by `permits` must be located near the superclass: either in the same module (if the superclass is in a named module) or in the same package (if the superclass is in the unnamed module). For example, in the following declaration of `Shape`, its permitted subclasses are all located in different packages of the same named module:

```
package com.example.geometry;

public abstract sealed class Shape 
    permits com.example.polar.Circle,
            com.example.quad.Rectangle,
            com.example.quad.simple.Square {...}
```

When the permitted subclasses are small in size and number, it may be convenient to declare them in the same source file as the sealed class. When they are declared in this way, the `sealed` class may omit the `permits` clause, and the Java compiler will infer the permitted subclasses from the declarations in the source file (which may be auxiliary or nested classes).  For example, if the following code is found in `Shape.java`, then the sealed class `Shape` is inferred to have three permitted subclasses:

```
package com.example.geometry;

abstract sealed class Shape {...}
... class Circle    extends Shape {...}
... class Rectangle extends Shape {...}
... class Square    extends Shape {...}    
```

The purpose of sealing a class is to let client code reason clearly and conclusively about _all_ permitted subclasses. The traditional way to reason about subclasses is with an `if`-`else` chain of `instanceof` tests, but analyzing such chains is difficult for the compiler, so it cannot determine that the tests cover _all_ permitted subclasses. For example, the following method would cause a compile-time error because the compiler does not share the developer's conviction that every subclass of `Shape` is tested and leads to a `return` statement:

```
int getCenter(Shape shape) {
    if (shape instanceof Circle) {
        return ... ((Circle)shape).center() ...
    } else if (shape instanceof Rectangle) {
        return ... ((Rectangle)shape).length() ...
    } else if (shape instanceof Square) {
        return ... ((Square)shape).side() ...
    }
}
```

Appending a catch-all `else` clause would be contrary to the developer's conviction that the tests are already exhaustive. In addition, the compiler has no ability to save the developer if their conviction turns out to be wrong. Suppose the code above is accidentally edited to omit, say, the `instanceof Rectangle` test; no compile-time error will occur. (The omission might be easy to spot with three permitted subclasses, but not with 10 or 20. Even with just three, the code is frustrating to write and tedious to read.)

The ability to reason clearly and conclusively about permitted subclasses will be realized in a future release that supports [pattern matching](https://cr.openjdk.java.net/~briangoetz/amber/pattern-match.html). Instead of inspecting an instance of a sealed class with `if`-`else`, client code will be able to switch over the instance using _type test patterns_ ([JEP 375](http://openjdk.java.net/jeps/375)). This allows the compiler to check that the patterns are _exhaustive_. For example, given the following code, the compiler will infer that every permitted subclass of `Shape` is covered, so no `default` clause (or other total pattern) is needed; moreover, the compiler will give an error if any of the three cases are missing:

```
int getCenter(Shape shape) {
    return switch (shape) {
        case Circle c    -> ... c.center() ...
        case Rectangle r -> ... r.length() ...
        case Square s    -> ... s.side() ...
    };
} 
```

A sealed class imposes three constraints on its permitted subclasses (the classes specified by its `permits` clause):

1. The sealed class and its permitted subclasses must belong to the same module, and, if declared in an unnamed module, the same package.

2. Every permitted subclass must directly extend the sealed class.

3. Every permitted subclass must choose a modifier to describe how it continues the sealing initiated by its superclass:

  - A permitted subclass may be declared `final` to prevent its part of the class hierarchy from being extended further.
  - A permitted subclass may be declared `sealed` to allow its part of the hierarchy to be extended further than envisaged by its sealed superclass, but in a restricted fashion.
  - A permitted subclass may be declared `non-sealed` so that its part of the hierarchy reverts to being open for extension by unknown subclasses. (A sealed class cannot prevent its permitted subclasses from doing this.)

As an example of the third constraint, `Circle` may be `final` while `Rectangle` is `sealed` and `Square` is `non-sealed`:

```
package com.example.geometry;

public abstract sealed class Shape
    permits Circle, Rectangle, Square {...}

public final class Circle extends Shape {...}

public sealed class Rectangle extends Shape 
    permits TransparentRectangle, FilledRectangle {...}
public final class TransparentRectangle extends Rectangle {...}
public final class FilledRectangle extends Rectangle {...}

public non-sealed class Square extends Shape {...}    
```

One and only one of the modifiers `final`, `sealed`, and `non-sealed` must be used by each permitted subclass. It is not possible for a class to be both `sealed` (implying subclasses) and `final` (implying no subclasses),  or both `non-sealed` (implying subclasses) and `final` (implying no subclasses), or both `sealed` (implying restricted subclasses) and `non-sealed` (implying unrestricted subclasses).

(The `final` modifier can be considered as a strong form of sealing, where extension/implementation is prohibited completely. That is, `final` is conceptually equal to `sealed` + a `permits` clause which specifies nothing; note that such a `permits` clause cannot be written in Java.)

*Abstract classes.* A class which is `sealed` or `non-sealed` may be `abstract`, and have `abstract` members. A `sealed` class may permit subclasses which are `abstract` (providing they are then `sealed` or `non-sealed`, rather than `final`).

*Class accessibility.* Because `extends` and `permits` clauses make use of class names, a permitted subclass and its sealed superclass must be accessible to each other. However, permitted subclasses need not have the same accessibility as each other, or as the sealed class. In particular, a subclass may be less accessible than the sealed class; this means that, in a future release when pattern matching is supported by switches, some users will not be able to exhaustively `switch` over the subclasses unless a `default` clause (or other total pattern) is used. Java compilers will be encouraged to detect when a user's `switch` is not as exhaustive as the user imagined it would be, and customize the error message to recommend a `default` clause.

### Sealed interfaces

Similar to the story for classes, an interface is sealed by applying the `sealed` modifier to the interface. After any `extends` clause to specify superinterfaces, the implementing classes and subinterfaces are specified with a `permits` clause. For example:

```
package com.example.expression;

public sealed interface Expr
    permits ConstantExpr, PlusExpr, TimesExpr, NegExpr {...}

public final class ConstantExpr implements Expr {...}
public final class PlusExpr     implements Expr {...}
public final class TimesExpr    implements Expr {...}
public final class NegExpr      implements Expr {...}
```

### Sealed classes and Records

Sealed classes work well with records ([JEP 384](https://openjdk.java.net/jeps/384)), another preview feature of Java 15. Records are implicitly `final`, so a sealed hierarchy with records is slightly more concise than the example above:

```
package com.example.expression;

public sealed interface Expr
    permits ConstantExpr, PlusExpr, TimesExpr, NegExpr {...}

public record ConstantExpr(int i)       implements Expr {...}
public record PlusExpr(Expr a, Expr b)  implements Expr {...}
public record TimesExpr(Expr a, Expr b) implements Expr {...}
public record NegExpr(Expr e)           implements Expr {...}
```

The combination of sealed classes and records is sometimes referred to as [_algebraic data types_](https://en.wikipedia.org/wiki/Algebraic_data_type): Records allow us to express _product types_, and sealed classes allow us to express _sum types_.

### Sealed classes in the JDK

An example of how sealed classes might be used in the JDK is in the `java.lang.constant` package that models [descriptors for JVM entities](https://docs.oracle.com/en/java/javase/14/docs/api/java.base/java/lang/constant/package-summary.html):

```
package java.lang.constant;

public sealed interface ConstantDesc
    permits String, Integer, Float, Long, Double,
            ClassDesc, MethodTypeDesc, DynamicConstantDesc {...}

// ClassDesc is designed for subclassing by JDK classes only
public sealed interface ClassDesc extends ConstantDesc
    permits PrimitiveClassDescImpl, ReferenceClassDescImpl {...}
final class PrimitiveClassDescImpl implements ClassDesc {...}
final class ReferenceClassDescImpl implements ClassDesc {...} 

// MethodTypeDesc is designed for subclassing by JDK classes only
public sealed interface MethodTypeDesc extends ConstantDesc
    permits MethodTypeDescImpl {...}
final class MethodTypeDescImpl implements MethodTypeDesc {...}

// DynamicConstantDesc is designed for subclassing by user code
public non-sealed abstract class DynamicConstantDesc implements ConstantDesc {...}
```

### Java Grammar

```
NormalClassDeclaration:
  {ClassModifier} class TypeIdentifier [TypeParameters]
    [Superclass] [Superinterfaces] [PermittedSubclasses] ClassBody

ClassModifier:
  (one of)
  Annotation public protected private
  abstract static sealed final non-sealed strictfp

PermittedSubclasses:
  permits ClassTypeList

ClassTypeList:
  ClassType {, ClassType}
```

### JVM support for sealed classes

The Java Virtual Machine recognizes `sealed` classes and interfaces at runtime, and prevents extension by unauthorized subclasses and subinterfaces.

Although `sealed` is a class modifier, there is no `ACC_SEALED` flag in the `ClassFile` structure. Instead, the `class` file of a sealed class has a `PermittedSubclasses` attribute which implicitly indicates the `sealed` modifier and explicitly specifies the permitted subclasses:

```
PermittedSubclasses_attribute {
    u2 attribute_name_index;
    u4 attribute_length;
    u2 number_of_classes;
    u2 classes[number_of_classes];
}
```

The list of permitted subclasses is mandatory—even when the permitted subclasses are inferred by the compiler, those inferred subclasses are explicitly included in the `PermittedSubclasses` attribute.

The `class` file of a permitted subclass carries no new attributes.

When the JVM attempts to defines a class whose superclass or superinterface has a `PermittedSubclasses` attribute, the class being defined must be named by the attribute. Otherwise, an `IncompatibleClassChangeError` is thrown.

### Reflection API

The following `public` methods will be added to `java.lang.Class`:

 - `java.lang.constant.ClassDesc[] getPermittedSubclasses()` 
 - `boolean isSealed()` 

The method `getPermittedSubclasses()` returns an array containing `java.lang.constant.ClassDesc` objects representing all the permitted subclasses of the class if it is sealed, and returns an empty array if the class is not sealed.

The method `isSealed` returns true if the given class or interface is sealed. (Compare with `isEnum`.)

Alternatives
------------

Some languages have direct support for _[algebraic data types](https://en.wikipedia.org/wiki/Algebraic_data_type)_ (ADTs), such as Haskell's `data` feature.  It would be possible to express ADTs more directly and in a manner familiar to Java developers through a variant of the `enum` feature, where a sum of products could be defined in a single declaration.  However, this would not support all the desired use cases, such as those where sums range over classes in more than one compilation unit, or sums that range over classes that are not products.

The `permits` clause allows a sealed class, such as the `Shape` class shown earlier, to be accessible-for-invocation by code in any module, but accessible-for-implementation by code in only the same module as the sealed class (or same package if in the unnamed module). This makes the type system more expressive than the access control system. With access control alone, if `Shape` is accessible-for-invocation by code in any module (because its package is exported), then `Shape` is also accessible-for-implementation in any module; and if `Shape` is not accessible-for-implementation in any other module, then `Shape` is also not accessible-for-invocation in any other module.

Dependencies
-----------

Sealed classes do not depend on records (JEP 384) or pattern matching (JEP 375), but they work well with both.