[QCon 2019] Java Futures

Brian Goetz

For other QCon blog posts, see QCon live blog table of contents


  • Java is approaching middle age. Almost 25 years old
  • Keep promises to users
  • Prime directive is compatibility
  • Backward compatibility matters ex: generics. Don’t need to recompile old code
  • Patterns ex: single method interfaces for lambdas rather than having to rewrite libraries
  • Languages features are forever. Interacts with others; even future ones
  • Waited 10 years for generics until had right story/timing. Knew copying C++ was the wrong choice
  • No language is ever finished
  • Languages are never good enough because hardware changes, new problems, developer expectations change
  • mid-2019 edition because things change so fast


  • Used to release based on a feature rather than a data
  • Often didn’t feel worthwhile to do small feature because got stuck behind big ones
  • Now doing about two years of six month release schedule.
  • Release management overhead went down to almost zero
  • Same rate of innovation; just changed rate of release
  • Java 13 already in rampdown. Released in September.
  • Already working on Java 14

Preview feature

  • Risk of things happening too quickly since features are forever
  • Preview means feature is done but not finalized
  • Not experimental/beta.
  • Think of as provisional feature.
  • Expected outcome is that will be promoted to real feature in next version or two
  • Full IDE/Tooling support for preview features
  • Need to turn them on so not accidentally using in production -enable-preview flag

Current initiatives

  • Amber – right sizing language ceremony. Includes local variable type inference and future changes like pattern matching
  • Valhalla – adapt form modern hardware, value types, generic specialization
  • Loom – Fibers
  • Panama – Native code and data

Local Variable type inference

  • In Java 10. Future if on Java 8. Infinite past if you are Brian :).
  • var instead of type for local variable
  • Not syntactic sugar. Can expose “hidden” type (ex: capture types, intersection types and anonymous class types). So see more generics.
  • One of most commonly requested features
  • But also significant/vocal angst – can write bad code; giving into fashion
  • Not controversial once release. Fear of change in advance?
  • Will take time for good practices to emerge
  • Style guide https://openjdk.java.net/projects/amber/LVTIstyle.html

Switch enhancements

  • Preview feature in Java 12 and 13
  • Significant fraction of switch statements want to be expression. SO have to assign in each case.
  • Break is annoying. Irritating and error prone
  • Looked at how switch statements needs to evolve for pattern matching and then made more generally useful.
  • Boilerplate bad because a place for bugs to hide
  • Two changes – can use switch as expression or statement. Streamlined syntax label -> consequence
  • Will be in Java 14 unless feedback said made some horrible error

Multi-line String literals

  • Preview feature in Java 13
  • Require quotes and concatenation. Error prone for JSON, SQL and HTML
  • Manual mangling introduces errors
  • Was going to be a preview feature in Java 12. WIthdrawn because had better idea of how to do it.
  • Triple quotes to start/end
  • Dots for indentation that is just for IDE so don’t get extra whitespace when reformat in IDE.

Pattern matching

  • Intend to deliver in phases; hopefully starting in Java 14
  • Phase 1
    • Replaces if statement for instanceof and then the cast for the same
    • “Not only does the language make you cast explicitly, but it gives you the change to get it wrong”
    • if (a instanceof Integer intvalue) – checks type and gives new variable can use as type safely
    • Simplifies equals method implementation
  • Phase 2
    • Use in switch
    • case Integer i – matches if Integer, bind to i and allow use of variable inside the case.


  • Lots of boilerplate – constructor, accessors, Object methods
  • A lot of code to read only to find out didn’t need to read any of it.
  • IDE generates boilerplate, but it doesn’t help you read it.
  • record Point(int x, int y) {}
  • Like enum, give up some extra features to get functionality.
  • Get sensible defaults in record
  • Declaratively states “I am simply a carrier for my data”
  • Programmer making a commitment
  • [does this discourage adding instance methods?]
  • Product type of algebraic data type

Sealed types

  • Sum type of algebraic data types
  • Shape = Circle + Rect
  • Compiler will know only those types
  • sealed interface Shape {}
  • record Circle(Point center, int radius) implements Shape {}
  • record Rect(Point x,….
  • Can use in if statement and pattern matching to get fields of record as local variables
  • Can use in switch statement. Compiler will complain if don’t address all possible implementations of sealed types
  • ex: case Circle(Var center, var r) -> PI * r * r;

Project Valhalla

  • Aims to reboot the layout of data in memory
  • Hardware change a lot in past 25 years.
  • Cost of memory fetch vs arithmetic has increased heavily in last 25 years
  • An array of objects will use data all over so many cache misses
  • Big change because goes down so far.
  • We want to be able to specify the class should be inlined
  • Giving up immutability and representation polymorphism.
  • Getting hardware friendly data layout
  • Code for the class works like an int

My Impressions

Great talk! It covered both things I had seen and things I hadn’t. Great perspective. Excited for some of the coming features.

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