[2021 kcdc] Algebraic in Java: Pattern Matching

This post is my live blog from KCDC. For more, see the 2021 KCDC live blog TOC

Speaker: Chandra Guntur

Twitter: @cguntur

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Background

  • This is part two of a talk. Part 1 covered algebraic types (ex: enums). Don’t need to have seen it for this talk
  • Pattern matching came froM SNOBOL (string oriented and symbolic language) in 1960s
  • This presentation is in https://github.com/c-guntur/algebraic-in-java

Switch case pattern matching

  • case can be type of object. ex: case Integer i -> “int”
  • Must put “case null” in order to avoid a null pointer
  • Guarded expressions: case Integer i && i < 50
  • Can use same pattern variable name across case statements
  • Flow scoping

If statement pattern matching

Deconstructing is from JDK 18 preview – https://openjdk.java.net/jeps/405 (preview features can change)

  • if (o instanceof Point p)
  • if (o instanceof Point(int x, int y) – deconstruct value. Implicitly type safe because null won’t reach branch (null instanceof X is always false)
  • if (r instanceOf Rect(Point(var x, var y), var lr) – nested deconstruction
  • If have varargs, can match by coordinates/number of elements. Ex Point() matches no params, Point(var a) matches one, Point(var a, …) matches one or more
  • For array, if (o instanceof String[] { String s1, … } – one or more elements in array

Pattern matching isn’t new

  • Always had instanceof
  • Visitor pattern
  • Regular expressions

My take

I’m excited Java is going to be adding a lot of this. The format of presenting from inside IntelliJ is interesting. The presentation was short though. Only half an hour. I liked the answer to my question about scope – pretend there is an else block!

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