How to check an alternative of a token in ANTLR's visitor mode?

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I want to build a simple rule engine using ANTLR's visitor mode, but when it comes with alternative tokens, I get confused how to get which alternative is. Can anyone help me with that?(sorry of my poor English)

grammar RuleExpression;
expression: '(' Operator arg* ')';
arg: STRING|INTEGER|BOOLEAN|expression;
STRING: '"' .+ '"';
INTEGER: [0-9]+;
BOOLEAN: 'true'|'false';
Operator: [a-zA-Z]+[0-9]*;
WS : [ \t\r\n]+ -> skip;

How do know an arg is a STRING or INTEGER or BOOLEAN or expression?

antlr compilation java
1 Answers

[...] Is there some method which I can use switch statement?

You can inspect the context's start token type:

@Override
public Object visitArg(RuleExpressionParser.ArgContext ctx) {
  switch (ctx.start.getType()) {
    case RuleExpressionLexer.STRING:
      return ...;
    case RuleExpressionLexer.INTEGER:
      return ...;
    case RuleExpressionLexer.BOOLEAN:
      return ...;
    default:
      // It was an `expression`
      return super.visitExpression(ctx.expression());
  }
}

Or you could label the arg alternatives:

arg
 : STRING     #StringArg
 | INTEGER    #IntegerArg
 | BOOLEAN    #BooleanArg
 | expression #ExpressionArg
 ;

which will cause the visitor to have the following methods:

@Override 
public T visitStringArg(RuleExpressionParser.StringArgContext ctx) ...

@Override 
public T visitIntegerArg(RuleExpressionParser.IntegerArgContext ctx) ...

@Override 
public T visitBooleanArg(RuleExpressionParser.BooleanArgContext ctx) ...

@Override 
public T visitExpressionArg(RuleExpressionParser.ExpressionArgContext ctx) ...

instead of the single T visitArg(RuleExpressionParser.ArgContext ctx) method.

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