ANTLR4 Parser confusion - parsing

Question

im trying to user antlr4 for a project and I got an error im not sure I know how to fix. It seems antlr4 is confuse with two parser rules.
Here is my lexer/parser :
grammar PARSER;
#header {package VSOP.Parser;}
program : code+ ; //statement+ ;
code : classHeader | methodHeader | ;
statement : assign | ifStatement | whileStatement;
classHeader : 'class' TYPE_IDENTIFIER ('extends' TYPE_IDENTIFIER)? '{' classBody '}';
classBody : methodHeader* | field*;
methodHeader : OBJECT_IDENTIFIER '(' (((formal ',')+ (formal)) | (formal)?) ')' ':' varType '{' methodBody '}' ;
methodBody : statement* ;
formal : OBJECT_IDENTIFIER ':' varType ;
field : OBJECT_IDENTIFIER ':' varType ('<-' varValue)? ';' ;
assign : OBJECT_IDENTIFIER ':' varType ('<-' varValue)? ;
whileStatement : 'while' condition* 'do' statement* ;
ifStatement : ifStat elseStat? ; //ifStat elseIfStat* elseStat? ;
ifStat : 'if' condition 'then' statement* ;
//elseIfStat : 'else if' condition 'then' '{' statement* '}' ;
elseStat : 'else' statement* ;
condition : comparaiser CONDITIONAL_OPERATOR comparaiser ;
comparaiser : OBJECT_IDENTIFIER | integer | STRING ;
integer : INTEGER_HEX | INTEGER_DEC | INTEGER_BIN ;
varType : 'bool' | 'int32' | 'string' | 'unit' | TYPE_IDENTIFIER ;
varValue : ('true' | 'false' | STRING | integer) ;
// KEYWORD : 'and' | 'class' | 'do' | 'else' | 'extends' | 'false' | 'if' | 'in' | 'isnull' | 'let' | 'new' | 'not' | 'then' | 'true' | 'unit' | 'while' ;
ARITHMETIC_OPERATOR : '+' | '-' | '*' | '/' | '^' ;
CONDITIONAL_OPERATOR : '=' | '<' | '<=';
MULTILINE_OPEN_COMMENT : '(*' ;
MULTILINE_CLOSE_COMMENT : '*)' ;
MULTILINE_COMMENT : '(*' .*? '*)' ;
INTEGER_BIN : '0'[bB][0-9a-zA-Z]* ;
INTEGER_HEX : '0'[xX][0-9a-zA-Z]* ;
INTEGER_DEC : [0-9][0-9a-zA-Z]* ;
OBJECT_IDENTIFIER : [a-z][a-zA-Z0-9_]* ;
TYPE_IDENTIFIER : [A-Z][a-zA-Z0-9_]* ;
STRING : '"' ( '\\"' | . )*? ('"' | EOF) ;
SINGLE_LINE_COMMENT : '//'~[\r\n]* ;
WS : [ \r\n\t]+ -> skip;
Using the code below, i get the errors
line 5:15 mismatched input '(' expecting ':'
line 5:31 mismatched input ',' expecting {'<-', ';'}
line 5:50 mismatched input ',' expecting {'<-', ';'}
line 5:69 mismatched input ')' expecting {'<-', ';'}
The problem is antlr4 confuse methodHeader and field. If I but the var nbOfEngine below the function, I get the function right, but the variable wrong.If i try them separatly, it work as well. I tried changing their order in the parser, without success.
class Plane extends Transport {
nbOfEngine: int32 ;
startEngine(gazLevel: int32, readyToStart:bool, foodOnBoard: bool) : bool {
}
}
Any idea how to fix this ?
Thanks !

Answer 1

You define classBody to either be a sequence of field definitions or a sequence of method definitions. You don't allow for it to be a sequence of both.
If you change it to (methodHeader | field)* instead, you'll get a sequence that can contain either.

Answer 2

I found the issue in the parser. The problem come from classBody.
classBody : methodHeader* | field*;
Instead Ive written:
classHeader : 'class' TYPE_IDENTIFIER ('extends' TYPE_IDENTIFIER)? '{' classBody* '}';
classBody : methodHeader | field;