Featherweight Java

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Featherweight Java

• COMP431 (Compilers) by Dr Alex Potanin

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Featherweight Java

• class A extends Object
• A() super()
• class B extends Object
• B() super()
• class Pair extends Object
• Object fst Object snd
• Pair(Object fst, Object snd)
• super() this.fst fst this.snd snd
• Pair setfst(Object newfst)
• return new Pair(newfst, this.snd)

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Featherweight Java

• Inside every large language is a small language
  struggling to get out (Tony Hoare)
• Featherweight Java (FJ) is a minimal core
  calculus for modelling Javas type system
• It only has five expressions
• Object creation
• Method invocation
• Field access
• Casting
• Variable
• FJ is only a little larger than lambda calculus!

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Featherweight Java

• FJ has no assignment (trivially fixable see
  Types and Programming Languages by Benjamin
  Pierce)
• FJs type system is proved to be sound
• FJ used as a starting point for modelling new
  features to be introduced to Java (e.g.
  Featherweight Generic Ownership, Featherweight
  Immutable Java, etc.)
• Featherweight Generic Java extends FJ with
  Generic Types (as in Java 5)
• Uses the typing environment Gto keep track of
  variable types

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FJ Program

• FJ program is a collection of class definitions
  followed by an expression to be evaluated (think
  of the latter as the main method in Java)
• class Foo extends Object
• class Bar extends Object
• new Pair(new Foo(), new Bar()).fst
• The above evaluates to new Foo() which is an
  example of normal form in FJ.

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FJ Syntax
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FJ Class
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FJ Method
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Lecture 11

• Starts here

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FJ Expressions (1 of 2)
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FJ Expressions (2 of 2)
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FJ Subtyping
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FJ Field Lookup
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FJ Method Type and Body Lookups
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FJ Computation Rules
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FJ Congruence Rules
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Featherweight Generic Java

• class A extends Object
• A() super()
• class B extends Object
• B() super()
• class PairltX extends Object, Y extends Objectgt
• extends Object
• X fst Y snd
• Pair(X fst, Y snd)
• super() this.fst fst this.snd snd
• ltZ extends Objectgt PairltZ, Ygt setfst(Z newfst)
  
• return new PairltZ, Ygt(newfst, this.snd)