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JDT Core/Plan/4.8/JEP286

Planning page for JDT Implementation of Local Variable Type Inference (JEP 286) for Photon (4.8)

Main tracking bug: JEP286 Bug

Compiler changes

Changes regarding the compiler: Compiler Bug

Grammar changes

Since var is not a keyword, but a reserved type name AND we need to be backwards compatible anyway, it should be handled in type resolution instead of grammar. Local variable type inference should be denoted by a bit AST node and type binding.

Type checking

  • Determine the type of the var declaration
  • Determine upwards/downwards type projection for type
  • Fallback to "Object" type in case of error

Error reports

  • Errors where local variable type cannot be inferred:
    • Cannot use 'var' on variable without initializer
    • Lambda expression needs an explicit target-type
    • Variable initializer is 'null'
    • Inferred type is 'void'
    • When method reference needs an explicit target-type.
    • array initializer needs an explicit target-type
  • Error when defining a type called 'var':

This should be a configurable 'warning'/'info' for Java 9 and earlier.

Code generation

Shouldn't be much work since the stack frame would be filled in with the bound type anyway.

Java Search support

Changes regarding the Search: Search Bug

DOM support

Changes regarding the DOM: DOM Bug

Content Assist support

UI Changes

Code hover


  • Allow JDT UI to target Java 10 in Compiler Preferences - bug 529432
  • Clean up option to prefer 'var' over specific types (to control the irritants generated by the compiler) - [based on need - low prio].

Quick assists

  • Convert 'var' into the resolved type - bug 531039.
  • Convert explicit typed local variable into 'var', like changing "String coolIde = "Eclipse";" into "var coolIde = "Eclipse";" It should also include: Convert diamond operator to var, like fixing "List<String> names = new LinkedList<>()" into "var names = new LinkedList<String>()" - bug 531040.

Code templates

  • New templates with 'var' in for loops and introduce new variable - [can be added if useful - need examples - low prio].

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