Member Avatar for stevanity

I wanna know what type of parser is used in the javac compiler. whether a top-down or bottom-up parser is used..... ?

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Member Avatar for zeroliken

I think it's a top-down parser
It starts at the top of the program which is line 1 right?

Member Avatar for stevanity

Ermm. do u know whats the difference between a top down and bottom up parser?? its an internal thing.. evry compiler starts from the line 1. Its about how the tokens are validated...

Member Avatar for leegao

To be fair, top-down isn't a very descriptive name for the category of grammars.

If we're using http://java.sun.com/docs/books/jls/second_edition/html/syntax.doc.html without any modification, then this grammar is probably not suitable for LL(1).

The most obvious problem comes from Selector -> . x | . y | . z However, a LL(2) parser generator or one that does smart refactoring on the grammar (not possible with the action code however) will be able create a correct parser for this production rule.

More subtle issues includes:

1. The famous dangle else problem.
in the following production

Statement -> if ParExpression Statement [else Statement]

if we get rid of the extended syntax, (using the rules of left-factoring, in that A -> alpha [E] beta === A -> E' beta; E' -> epsilon | E), then we have
Statement -> if ParExpression Statement ELSE'
ELSE' -> epsilon | else Statement

since ELSE' is nullable, in order to fill the jump table for ELSE'[else], we must look at Follow(ELSE') = FOLLOW(Statement) which includes FIRST(ELSE') = {else}. Hence, we have a duplicity for the transition from ELSE' under the lookahead terminal of "else". In LL(1), this is an ambiguous production.

2. Expression3 -> Type Expression3
| Primary {Selector} {PostfixOp}

Since FIRST(Primary) U FIRST(Type) = {int,long,float,double,boolean}, Expression3 cannot predictively determine what action to take on encountering any of the primitive type tokens.

A few more ambiguities such as trying to determine how any of the assignment operators ought to proceed on the [Expression1Rest] rule, given that it is nullable and a few even more subtle ones.

Edited by leegao because: n/a
Member Avatar for hfx642

Well... My error lists have always been in order of ascending line numbers.

Edited by hfx642 because: n/a
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