/**
     * A visitor for types.  A visitor is used to implement operations
     * (or relations) on types.  Most common operations on types are
     * binary relations and this interface is designed for binary
     * relations, that is, operations on the form
     * Type × S → R.
     * <!-- In plain text: Type x S -> R -->
     *
     * @param <R> the return type of the operation implemented by this
     * visitor; use Void if no return type is needed.
     * @param <S> the type of the second argument (the first being the
     * type itself) of the operation implemented by this visitor; use
     * Void if a second argument is not needed.
     */
    public interface Visitor<R,S> {
        R visitClassType(ClassType t, S s);
        R visitWildcardType(WildcardType t, S s);
        R visitArrayType(ArrayType t, S s);
        R visitMethodType(MethodType t, S s);
        R visitPackageType(PackageType t, S s);
        R visitTypeVar(TypeVar t, S s);
        R visitCapturedType(CapturedType t, S s);
        R visitForAll(ForAll t, S s);
        R visitUndeterminedVar(UndeterminedVar t, S s);
        R visitErrorType(ErrorType t, S s);
        R visitType(Type t, S s);
    }

/**
     * A default visitor for types.  All visitor methods except
     * visitType are implemented by delegating to visitType.  Concrete
     * subclasses must provide an implementation of visitType and can
     * override other methods as needed.
     *
     * @param <R> the return type of the operation implemented by this
     * visitor; use Void if no return type is needed.
     * @param <S> the type of the second argument (the first being the
     * type itself) of the operation implemented by this visitor; use
     * Void if a second argument is not needed.
     */
    public static abstract class DefaultTypeVisitor<R,S> implements Type.Visitor<R,S> {
        final public R visit(Type t, S s)               { return t.accept(this, s); }
        public R visitClassType(ClassType t, S s)       { return visitType(t, s); }
        public R visitWildcardType(WildcardType t, S s) { return visitType(t, s); }
        public R visitArrayType(ArrayType t, S s)       { return visitType(t, s); }
        public R visitMethodType(MethodType t, S s)     { return visitType(t, s); }
        public R visitPackageType(PackageType t, S s)   { return visitType(t, s); }
        public R visitTypeVar(TypeVar t, S s)           { return visitType(t, s); }
        public R visitCapturedType(CapturedType t, S s) { return visitType(t, s); }
        public R visitForAll(ForAll t, S s)             { return visitType(t, s); }
        public R visitUndeterminedVar(UndeterminedVar t, S s)         { return visitType(t, s); }
        public R visitErrorType(ErrorType t, S s)       { return visitType(t, s); }
    }

/**
     * A <em>simple</em> visitor for types.  This visitor is simple as
     * captured wildcards, for-all types (generic methods), and
     * undetermined（未确定的） type variables (part of inference) are hidden.
     * Captured wildcards are hidden by treating them as type
     * variables and the rest are hidden by visiting their qtypes.
     *
     * @param <R> the return type of the operation implemented by this
     * visitor; use Void if no return type is needed.
     * @param <S> the type of the second argument (the first being the
     * type itself) of the operation implemented by this visitor; use
     * Void if a second argument is not needed.
     */
    public static abstract class SimpleTypeVisitor<R,S> extends DefaultTypeVisitor<R,S> {
        @Override
        public R visitCapturedType(CapturedType t, S s) {
            return visitTypeVar(t, s);
        }
        @Override
        public R visitForAll(ForAll t, S s) {
            return visit(t.qtype, s);
        }
        @Override
        public R visitUndeterminedVar(UndeterminedVar t, S s) {
            return visit(t.qtype, s);
        }
    }

/**
     * A visitor for symbols.  A visitor is used to implement operations
     * (or relations) on symbols.  Most common operations on types are
     * binary relations and this interface is designed for binary
     * relations, that is, operations on the form
     * Symbol × P → R.
     * <!-- In plain text: Type x P -> R -->
     *
     * @param <R> the return type of the operation implemented by this
     * visitor; use Void if no return type is needed.
     * @param <P> the type of the second argument (the first being the
     * symbol itself) of the operation implemented by this visitor; use
     * Void if a second argument is not needed.
     */
    public interface Visitor<R,P> {
        R visitClassSymbol(ClassSymbol s, P arg);
        R visitMethodSymbol(MethodSymbol s, P arg);
        R visitPackageSymbol(PackageSymbol s, P arg);
        R visitOperatorSymbol(OperatorSymbol s, P arg);
        R visitVarSymbol(VarSymbol s, P arg);
        R visitTypeSymbol(TypeSymbol s, P arg);
        R visitSymbol(Symbol s, P arg);
    }　　

 /**
     * A default visitor for symbols.  All visitor methods except
     * visitSymbol are implemented by delegating to visitSymbol.  Concrete
     * subclasses must provide an implementation of visitSymbol and can
     * override other methods as needed.
     *
     * @param <R> the return type of the operation implemented by this
     * visitor; use Void if no return type is needed.
     * @param <S> the type of the second argument (the first being the
     * symbol itself) of the operation implemented by this visitor; use
     * Void if a second argument is not needed.
     */
    public static abstract class DefaultSymbolVisitor<R,S> implements Symbol.Visitor<R,S> {
        final public R visit(Symbol s, S arg)                   { return s.accept(this, arg); }
        public R visitClassSymbol(ClassSymbol s, S arg)         { return visitSymbol(s, arg); }
        public R visitMethodSymbol(MethodSymbol s, S arg)       { return visitSymbol(s, arg); }
        public R visitOperatorSymbol(OperatorSymbol s, S arg)   { return visitSymbol(s, arg); }
        public R visitPackageSymbol(PackageSymbol s, S arg)     { return visitSymbol(s, arg); }
        public R visitTypeSymbol(TypeSymbol s, S arg)           { return visitSymbol(s, arg); }
        public R visitVarSymbol(VarSymbol s, S arg)             { return visitSymbol(s, arg); }
    }

/**
 * A combined type/symbol visitor for generating non-trivial(有意义的） localized string
 * representation of types and symbols.
 *
 */
public abstract class Printer implements Type.Visitor<String, Locale>, Symbol.Visitor<String, Locale> 
    ...
}