java-23

Java 23

To run each example use: java --enable-preview --source 23 <FileName.java>

JEPs

• 455 - Primitive Types in Patterns, instanceof, and switch (Preview)
• 466 - Class-File API (Second Preview)
• 467 - Markdown Documentation Comments
• 469 - Vector API (Eighth Incubator)
• 473 - Stream Gatherers (Second Preview)
• 474 - ZGC: Generational Mode by Default
• 476 - Module Import Declarations (Preview)
• 477 - Implicitly Declared Classes and Instance Main Methods (Third Preview)

Features

• Primitive Types in Patterns, instance of and switch
  • goal: "enhance pattern matching by allowing primitive type patterns in all pattern contexts"
  • in JDK 21, primitive type patterns are allowed only as nested patterns in record pattern (e.g.: v instanceof JsonNumber(double d))
  • this JEP will:
    • extend pattern matching to allow primitive type patterns in all places (e.g.: v instanceof JsonNumber(int i));
    • enhance instanceof and switch to support primitive type patterns as top level pattern
    • enhance instanceof to check the primitive type, beside the reference type checking, and also verify the primitive support value:
      • allowing to do an implicity cast during matching if, and only if, there is no loss of information (exact conversion)
      • e.g.: if an int is to large for a byte it will not be matched
      • instanceof will be used as safeguard for all conversions
      • in JLS §5.5 we can see the permited conversions
    • enhance the switch to works with literals of all primitive types:
      • until JDK 21, boolean, float, double and long cannot be used as case value
      • switch will be exhaustive if it contains a unconditional type pattern
        • an int is always a long, so a label case long l will match any integer
      • switch and floating-points:
        • if the switch's selector expression is a float then any case constants must be a floating-point literals
        • floating-point literal in case labels is defined in terms of representation equivalence
          • a label case 1.0f is considered the same as case 0.99999999f, will throw a duplicate label error
• Stream Gatherers
  • re-preview without change
• ZGC: Generational Mode by Default
  • switch the default mode of ZGC to use generational mode
  • deprecate the non-generational mode to be removed in the future
    • reduce the maintenance cost of supporting two models
  • default mode will act like using command-line args: -XX:+UseZGC -XX:+ZGenerational
  • non-generational can still be used with: -XX:+UseZGC -XX:-ZGenerational
  • Generational ZSG should be a better solution for most use cases
• Module Import Declarations
  • allow import all packages exported by a module
  • goal is simplify the learning and exploring of APIs without having to know its exactly package
  • when importing a module, it will automatically import all its exported packages and its transitive dependencies
  • syntax:
    • import module [Module Name]
    • import module java.sql
  • ambiguous import:
    • in case of two module exporting the same class name, we will have to import the class directly

    • import module java.base; // java.util.Date
      import module java.sql; // java.sql.Date
      import java.sql.Date;

• Implicitly Declared Classes and Instance Main Methods
  • major change from JDK 21 and 22
  • implicitly import methods from a new class java.io.IO
    • new class created for convenience IO
    • provides methods: print(Object), println(Object) and readln(String)
    • this class uses return from System.console() to print and read from the default input and output streams
  • implicitly class will automatically import all of the public top-level classes and interfaces from module java.base

Links

• JDK 23 - JEP Dashboard
• JDK 32 JEPs