• 21. JavaFFI
• 23. JavaScript

22. Java

Overview

Fantom pods can be used as a Java library using two different mechanism:

• JarDist: build task to build Fantom pods into a jar
• Fanc: command line to transpile Fantom pods into Java source

The primary difference is that JarDict packages up the Fantom fcode and emits Java classfiles at runtime. This makes is smaller and is guaranteed to work with all Fantom code. Using fanc to transpile Fantom into Java provides a Java codebase for Fantom that is easier to integrate into tool chains and IDE hints. However fanc has restrictions on the Fantom code being transpiled discussed below.

Java Transpile

Fantom transpiles to Java using the following conventions

• Any type/slot name with "$" is synthetic and should not be used
• Const fields are compiled into a Java getter method
• Mutable fields are compiled into a Java getter and setter method
• Constructors are compiled into static factory methods (never use the Java constructors directly)
• Mixins are compiled to Java interfaces and will make use of Java default methods (note Java interfaces do not support anything than public scope which will expose unwanted details)

Java Transpile Restrictions

The following Fantom language features cannot be used when transpiling to Java code:

• Null safe and elvis expressions can only use effectively final variables from their scope (since they must be compiled into Java closures)
• Switch statements cannot use the same variable name in multiple cases
• Use of ++ and -- is restricted to simple variables and field (it cannot be used with list indexing)
• Chained assignment expressions such as a = b = c may not compile correctly
• Cannot override equals in a mixin

• 21. JavaFFI
• 23. JavaScript