antlr4 - ANTLR 4 : How to know the existence of subpart in rule -

i have code :

vardeclaration     :   type id ('=' expression)? ';'      ; 

so, not ('=' expression) exist. but, sometimes, want process part, don't know exist or not in context. i'm using antlr 4 (and using listener), how can know this.

thanks :)

in listener (exitvardeclaration) or visitor (visitvardeclaration) check whether ctx.expression() == null. if null, ('=' expression) didn't exist. if non-null, did exist.