Explore Java 17 and beyond

Explore Java 17 Records, Sealed Classes and Pattern Matching José
Paumard @JosePaumard Java Developer Advocate Java Platform Group

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5/13/2022 Copyright © 2021, Oracle and/or its affiliates | 3
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Internal/Restricted/Highly Restricted 4 Records

https://github.com/JosePaumard/ 2022_DevNexus-lab

Short answer: a class! Less short answer: a class - That extends the Record class - That is final What is a Record?

And you can write: I Need to Write a Record! public record User(String name, int age) { } User sarah = new User("Sarah", 23); System.out.println(sarah); $ User[name=Sarah, age=23]

1) A constructor 2) Accessors: name() and age() 3) The following methods: toString(), equals(), hashCode() What Do you Get with a Record? public record User(String name, int age) { }

You cannot extend a record A record cannot extend anything You cannot create any instance field in a record You can implement interfaces You can add any instance method You can add any static field or method Restrictions on Records

name and age are the components of the record Components are both final instance fields And accessors Components of a Record public record User(String name, int age) { }

More on Components public record User(String name, int age) { } User user = new User("Sarah", 23); Class<User> userClass = User.class; boolean isRecord = userClass.isRecord(); Component[] components = userClass.getRecordComponents();

More on Components public record User(String name, int age) { } User user = new User("Sarah", 23); RecordComponent nameComponent = components[0]; Class<?> declaring = nameComponent.getDeclaringRecord(); String name = (String) nameComponent.getAccessor()invoke(user);

More on Components public record User(@UserName String name, int age) { } @Target(ElementType.RECORD_COMPONENT) @interface UserName {}

The state of a record is entirely defined by its components - You cannot add more state by extension, or by extending anything - You cannot add any instance field Record State

The constructor of a record that takes its components is called the canonical constructor Record Building public record User(String name, int age) { public record User(String name, int age) { this.name = name; this.age = age; } }

Why would you write your own canonical constructor? 1) Defensive copy Record Building public record State(List<City> cities) { public State(List<City> cities) { this.cities = List.copyOf(cities); } }

In that case, you need to write your own accessor too! Record Building public record State(List<City> cities) { public State(List<City> cities) { this.cities = List.copyOf(cities); } public List<City> cities() { return List.copyOf(cities); } }

Why would you write your own canonical constructor? 2) Validation Record Building public record Range(int begin, int end) { public Range(int begin, int end) { if (begin > end) { throw new IllegalArgumentException("..."); } this.begin = begin; this.end = end; } }

Why would you write your own canonical constructor? 2) Validation, with the Compact Constructor Record Building public record Range(int begin, int end) { public Range { if (begin > end) { throw new IllegalArgumentException("..."); } } }

A record constructor must call the canonical constructor Record Building public record Range(int begin, int end) { public Range(int end) { this.begin = 0; this.end = end; } }

A record constructor must call the canonical constructor Record Building public record Range(int begin, int end) { public Range(int end) { this(0, end); } }

You cannot build a record without calling its canonical constructor Deserialization calls the canonical constructor of a record

Deserialization calls the canonical constructor of a record Records are a perfect choice for Data Transport Objects Record Serialization

Internal/Restricted/Highly Restricted 24 Sealed Types

Short answer: type (interface, class, abstract class)! Less short answer: a type - That declares its subtypes - That lives in the same package or module as them What is a Sealed Type?

A sealed type must declare its extensions Syntax public sealed interface Shape permits Square, Rectangle, WeirdShape { }

An extension of a sealed type must be: - final - or sealed - or non-sealed Syntax

An extension of a sealed type must be: - final Syntax public record Square(int edge) implements Shape { }

An extension of a sealed type must be: - sealed Syntax public sealed class Rectangle permits ExtendedRectangle { }

An extension of a sealed type must be: - non-sealed Syntax public non-sealed abstract class WeirdShape { }

Internal/Restricted/Highly Restricted 31 Pattern Matching

?Record and Array Pattern Matching? Record Sealed Classes Switch Expression
Constant Dynamic Inner Classes private in VM Nestmates Pattern Matching for instanceof 11 14 16 17 Switch on Types 19

In reference to the Amber Chronicles by Roger Zelazny Project Amber

A pattern is a combination of a match predicate that determines if the pattern matches a target, along with a set of pattern variables that are conditionally extracted if the pattern matches the target. Java 16 : Pattern Match for IntanceOf Pattern Matching for Java Gavin Bierman and Brian Goetz, September 2018 https://github.com/openjdk/amber-docs/blob/master/site/design-notes/pattern-matching-for-java.md

String name = the type of the pattern String = the name of the pattern name = the binding variable o = target operand Type Pattern if (o instanceof String name) { // some code }

Type Pattern if (o instanceof String name) { Sytem.out.println(name.toUpperCase()); } if (o instanceof String name && name.length() > 0) { Sytem.out.println(name.toUpperCase()); }

Type Pattern if (o instanceof String name) { Sytem.out.println(name.toUpperCase()); } if (!(o instanceof String name)) { // you cant use name here! } Sytem.out.println(name.toUpperCase());

Internal/Restricted/Highly Restricted 38 Pattern Matching for Switch

Pattern Matching for Switch interface Shape {} record Square(int edge) implements Shape {} record Circle(int radius) implements Shape {}

Pattern Matching for Switch double area(Shape shape) { return switch (shape) { case Square square -> square.edge()*square.edge(); case Circle circle -> Math.PI*circle.radius()* circle.radius(); default -> 0; } }

Switch on a Sealed Type sealed interface Shape permits Square, Circle {} record Square(int edge) implements Shape {} record Circle(int radius) implements Shape {}

Switch on a Sealed Type double area(Shape shape) { return switch (shape) { case Square square -> square.edge()*square.edge(); case Circle circle -> Math.PI*circle.radius()* circle.radius(); // default -> 0; // Not needed! } }

The compiler can: - check if a type is not present and raise an error - raise a warning if an uneeded default is present Switch on a Sealed Type

Guarded Pattern (prev. 17 and 18) double area(Shape shape) { return switch (shape) { case Square square && square.edge() == 0 -> 0; case Square square -> square.edge()*square.edge(); case Circle circle -> Math.PI*circle.radius()* circle.radius(); } }

Guarded Pattern (prev. 19) double area(Shape shape) { return switch (shape) { case Square square when square.edge() == 0 -> 0; case Square square -> square.edge()*square.edge(); case Circle circle -> Math.PI*circle.radius()* circle.radius(); } }

(19 prev) Switch and Record Pattern Matching double area(Shape shape) { return switch (shape) { case Square(int edge) when edge == 0 -> 0; case Square(int edge) -> edge*edge; case Circle(int radius) -> Math.PI*radius*radius; } }

Internal/Restricted/Highly Restricted 47 Record Pattern Matching (prev Java 19)

Record Pattern Matching record User(String name, int age) {} if (o instanceof User user)) { String name = user.name(); int age = user.age(); }

name and age are initialized by calling the record accessors So defensive copy is possible Record Pattern Matching record User(String name, int age) {} if (o instanceof User(String name, int age))) { // use name and age }

Record Pattern Matching record User(String name, int age) {} if (o instanceof User(String name, int age) user)) { // use name, age and user }

Internal/Restricted/Highly Restricted 51 Array Pattern Matching

Array Pattern Matching if (o instanceof String[] array && array.length() >= 2) { // do something with array[0] and array[1] }

Array Pattern Matching if (o instanceof String[] {s1, s2}) { // do something with s1 and s2 }

Nesting Patterns + Type Inference if (o instanceof Circle[] {Circle(var r1), Circle(var r2)}) { // do something with r1 and r2 } record Circle(int radius) {}

Nesting Patterns + Type Inference if (o instanceof Circle[] { Circle(Point(int x1, int y1), _), Circle(Point(int x2, int y2), _)}) { // do something with x1, y1, x2, y2 } record Circle(Point center, int radius) {} record Point(int x, int y) {}

Nesting Patterns + Type Inference List<Point> points = ...; for (Point point: points) { int x = point.x(); int y = point.x(); // do something with x and y } record Point(int x, int y) {}

Nesting Patterns + Type Inference List<Point> points = ...; for (Point(int x, int y): points) { // do something with x and y } record Point(int x, int y) {}

Internal/Restricted/Highly Restricted 58 The Future of Pattern Matching

Nested Patterns, needed binding variables, type inference Syntaxic Sugars if (shape instanceof Circle(var center, _) && center instanceof Point(int x, int y)) { // center and radius are binding variables } if (shape instanceof Circle(Point(int x, int y), _)) { // center and radius are binding variables }

The deconstruction uses the canonical constructor of a record What about: - factory methods? Deconstruction

Deconstruction Using Factory Methods interface Shape { static Circle circle(double radius) { return new Circle(radius); } static Square square(double edge) { return new Square(edge); } } record Circle(double radius) {} record Square(double edge) {}

Then this code could become possible: Deconstruction Using Factory Methods double area = switch(shape) { case Shape.circle(double radius) -> Math.PI*radius*radius; case Shape.square(double edge) -> edge*edge; }

With factory methods Deconstruction Using Factory Methods if (opt instanceof Optional.of(var max)) { // max is a binding variable }

With factory methods Deconstruction Using Factory Methods if (s instanceof String.format("%s is %d years old", String name, int age)) { // name and age are binding variables }

Map deconstruction This is an extended form of Pattern Matching where you check the value of a binding variable Deconstruction Using Factory Methods if (map instanceof Map.withMapping("name", var name) && map instanceof Map.withMapping("email", var email)) { // name and email are binding variables }

Pattern combination More Examples if (map instanceof Map.withMapping("name", var name) __AND Map.withMapping("email", var email)) { // name and email are binding variables } __AND = pattern combination

More Examples { "firstName": "John", "lastName": "Smith", "age": 25, "address" : { "street": "21 2nd Street", "city": "New York", "state": "NY", "postalCode": "10021" } } if (json instanceof stringKey("firstName", var firstName) __AND stringKey("lastName", var lastName) __AND intKey("age", var age) __AND objectKey("address", stringKey("street", var street) __AND stringKey("city", var city) __AND stringKey("state", var state) )) { // firstName, lastName, age, // street, city, state, ... // are binding variables }

If Java Embraces « Map Literals » Map<String, String> map = { "firstName": "John", "lastName": "Smith", "age": "25" } if (map instanceof { "firstName": var firstName, "lastName": var lastName, "age": Integer.toString(var age) }) { // firstName, lastName, age // are binding variables // age is a deconstructed int }

The deconstruction uses the canonical constructor of a record What about: - factory methods? - classes that are not records? Deconstruction

What About Your POJOs? public class Point { private int x, y; public Point(int x, int y) { this.x = x; this.y = y; } public deconstructor(int x, int y) { x = this.x; y = this.y; } } This is the output of the deconstructor It allows both: - overloading - defensive copy

Match can check if an expression matches a pattern Just as if checks for a boolean expression Introducing Match

Pattern with Match record Point(int x, int y) {} record Circle(Point center, int radius) implements Shape {} Circle circle = ...; match Circle(var center, var radius) = circle; // center and radius are binding variables

Pattern with Match record Point(int x, int y) {} record Circle(Point center, int radius) implements Shape {} Circle circle = ...; match Circle(var center, var radius) = circle else throw new IllegalStateException("Not a circle");

Pattern with Match record Point(int x, int y) {} record Circle(Point center, int radius) implements Shape {} Circle circle = ...; match Circle(var center, var radius) = circle else { center = new Point(0, 0); // this is called radius = 0; // an anonymous matcher }

You can use match with more than one pattern… … or use nested patterns Pattern with Match Shape shape = ...; match Rectangle(var p1, var p2) = shape, Point(var x0, var y0) = p1, Point(var x1, var y2) = p2;

• Constant Patterns: checks the operand with a constant value • Type Patterns: checks if the operand has the right type, casts it, and creates a binding variable Patterns at a Glance

• Patterns + Deconstruction: checks the operand type, casts it, bind the component to binding variables • Patterns + Method: uses a factory method or a deconstructor • Patterns + Var: infers the right type, and creates the binding variable • Pattern + _: infers the right type, but does not create the binding variable Patterns at a Glance

Where are we? • Pattern Matching for instanceof (16) • Pattern Matching for Switch (prev 17, 18, 19) • Record Pattern Matching (19) • Array Pattern Matching • Match • Literals Patterns at a Glance

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