@Override
    public void visitLiteralExpression(
      @NotNull PsiLiteralExpression expression) {
      super.visitLiteralExpression(expression);
      final PsiType type = expression.getType();
      if (!TypeUtils.isJavaLangString(type)) {
        return;
      }
      final String value = (String)expression.getValue();
      if (value == null || value.length() != 1) {
        return;
      }
      if (!isArgumentOfIndexOf(expression)) {
        return;
      }
      registerError(expression, expression);
    }
@Override
    public void visitLiteralExpression(
      @NotNull PsiLiteralExpression expression)