🐤 👨🏿‍✈️ 🏝️ Scala程序员眼中的Java 15 🥒 🍏 👨

在“ Scala开发人员”课程的前夕，我们邀请您参加开放式网络研讨会“ Scala中的Functional Programming”。

现在，让我们分享一个传统的，有用的翻译。

. , Java. ( ) Java 15, — Java 17 LTS ( ).

Java , JVM . , , ( var) switch-. Scala — , - .

, Java 15 ( ) Scala. , JVM . , , Java ( -).

, () . , , , Java.

, private final
,
-
equals, hashCode toString

class Person {
    private final String name;
    private final int age;

    Person(String name, int age) { 
        this.name = name;
        this.age = get;
    }

    String name() { return name; }
    int age() { return age; }

    public boolean equals(Object o) { 
        if (!(o instanceof Person)) return false;
        Person other = (Person) o;
        return other.name == name && other.age = age;
    }

    public int hashCode() {
        return Objects.hash(name, age);
    }

    public String toString() { 
        return String.format("Person[name=%s, age=%d]", name, age);
    }
}

, , IDE (. Lombok). , . !

record Person(String name, int age) { }

21 ! . , :

var john = new Person("john", 76);

Scala — case- ( ). :

case class Person(name: String, age: Int)

case-?

equals, hashCode toString ( ).
: private final+ - Java, val Scala.
.
/case- .
interface, case- — trait ( Java- interface Scala).
java.lang.Record, case- scala.Product.

: private public. , ; , , . Scala case- private public, .
, . Scala case- , ( : case- case-).
(), , case- ( ).
, . , :

record Person(String name, int age) {
  Person {
    if (age < 0)
      throw new IllegalArgumentException("Too young");
  }
}

Scala .

9 . (Loïc Descotte) (Jarek Ratajski), case- Scala: copy. ( - ), . Copy — Scala, , !

, Scala case class: , , case-; . Java — «», , ( enum ).

, Java 15: . , , , . , .

, Animal :

public sealed interface Animal permits Cat, Dog, Elephant {...}

public class Cat implements Animal { ... }
public class Dog implements Animal { ... }
public class Elephant implements Animal { ... }

permits, , . , , , Java .

Scala (sealed), . permits . , , . , Java, Scala, , , ( ):

sealed trait Animal
class Cat extends Animal { ... }
class Dog extends Animal { ... }
class Elephant extends Animal { ... }

(, Scala public , , Java package-private).

Java Scala case-. , . case- product, sum.

? Java :

final, ;
sealed, permits;
non-sealed, .

, . :

public sealed interface Animal permits Cat, Dog, Elephant
public final class Cat implements Animal { ... }
public sealed class Dog permits Chihuahua, Pug implements Animal {}
public non-sealed class Elephant implements Animal { ... }

, non-sealed, , , .

Scala ; :

final
sealed ( );
, .

( ) non-sealed:

sealed trait Animal
final class Cat extends Animal { ... }
sealed class Dog extends Animal { ... }
class Elephant extends Animal { ... }

instanceof

, , , , « », instanceof:

if (myValue instanceof String s) {
    // s is in scope and is a String
} else {
    // s is not in scope
}

var s = (String) myValue if, , if (, ).

- flow-typing TypeScript, ( s) .

Scala :

myValue match {
  case s: String => // s is in scope and is a String
  case _ =>         // s is not in scope
}

, .

Java , , :

if (myValue instanceof String s && s.length() > 42) { ... }

Scala :

myValue match {
  case s: String if s.length() > 42 => ...
}

, Java , — :

var isLongString = myValue instanceof String s && s.length() > 42

Scala, , ( ), :

val isLongString = myValue match {
  case s: String if s.length() > 42 => true
  case _ => false
}

, Scala — . , case-, .

Java? , . sealed , . Scala match- sealed.

, , , . Java \n. ! «» .

""", :

var response = """
   <html>
     <body>Internal server error</body>
   </html>"""

""" , . . :

<html>
  <body>Internal server error</body>
</html>

, — , ! : . , .

Scala , , :

val response = """<html>
     <body>Internal server error</body>
   </html>"""

Scala — . :

val response = """<html>
    |  <body>Internal server error</body>
    |</html>""".stripMargin)

( |, stripMargin.)

, Scala, , Java .

Scala . Java , String::formatted. Scala s:

val message = "Internal server error"
val response = s"""<html>
    |  <body>$message</body>
    |</html>""".stripMargin)

Scala, Java — . «» . Scala Java . , Java.

, Java, — , Java-. , «» .

, , . , switch-, Java 12, switch (, , ). Scala , .

— , , . — Scala class, Java .

, Scala , Java, — , . , Scala , , , , ( ) .

Java 16 ( ), Scala , ! , Scala Scala Times.

.

Scala程序员眼中的Java 15

instanceof

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