JSON Using Java 17 Preview Features 21

🤔 What if one wanted to build from scratch a new JSON library using new Java features like:

• sealed interfaces
• type switches

🙂 There it is

😎 (2023-09-28) Update: this is now working out of the box on JDK ≥ 21

TL; DR

Sealed interfaces and type switches allow kinda union type in Java world:

let value: <Array | Boolean | Null | Number | Object | String>

... becomes ...

public sealed interface Value
  permits Array, Boolean, Null, Number, Object, String {...}
...
Value value;

... with same kind of type safety:

• value can be any of the permitted types, but nothing else.
• any new subtype of Value must be handled properly in all switches.

Example 1

One can iterate over all the values of a given array and deal with each one with a switch statement like this:

json.Array array = ...
array.forEach(v -> switch (v) {
  case Array   array -> doingStufWithArray  (array);
  case Boolean  bool -> doingStufWithBoolean(bool);
  case Null      nil -> doingStufWithNull   (nil);
  case Number number -> doingStufWithNumber (number);
  case Object object -> doingStufWithObject (object);
  case String string -> doingStufWithString (string);
});

Example 2

One can iterate over all the member of a given object and deal with each one with a switch statement like this:

json.Object object = ...
object.forEach((k, v) -> switch (v) {
  case Array   array -> doingStufWithNamedArray  (k,array);
  case Boolean  bool -> doingStufWithNamedBoolean(k,bool);
  case Null      nil -> doingStufWithNamedNull   (k,nil);
  case Number number -> doingStufWithNamedNumber (k,number);
  case Object object -> doingStufWithNamedObject (k,object);
  case String string -> doingStufWithNamedString (k,string);
});

Example 3

The Value interface might define the default #accept(Visitor) method as follows:

public sealed interface Value
  permits Array, Boolean, Null, Number, Object, String {
  ...
  default void accept(Visitor visitor) {
    switch (this) {
      case Array   array -> visitor.visit(array);
      case Boolean  bool -> visitor.visit(bool);
      case Null      nil -> visitor.visit(nil);
      case Number number -> visitor.visit(number);
      case Object object -> visitor.visit(object);
      case String string -> visitor.visit(string);
    }
  }
  ...
}

Thanks

Inspired by Francois Teychene's tools-in-action on Algebraic Data Types (ADT) at DevoxxFR 2022.

DISCLAIMER

⚠ Do not use in production ⚠

😉