JavaCC syntactic lookahead to the rescue

02 Mar 2006

There's been a grammar bug in PMD for a while; it wouldn't parse non-static initializers that started with a reference to a field, i.e.:

class Foo extends Bar {
   // Bar declares x, and Foo uses it
   x = 5; 

This is fairly bizarre code, but it's certainly valid Java, and PMD was erroring out on it. Here's the fix:

Before: ! { LOOKAHEAD(3) Initializer()
After:  ! { LOOKAHEAD(["static"] "{" ) Initializer()

Instead of blindly looking ahead three tokens, JavaCC's syntactic lookahead lets us look for several specific tokens and choose the parser action based on those. Fun stuff!

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