static private void validate(Team team) {
        if (team == null) {
            throw new IllegalArgumentException("team is null");
        }
        if (team.getName() == null || team.getName().isEmpty()) {
            throw new ValidationException("no name");
        }
        if (team.getCoach() == null || team.getCoach().isEmpty()) {
            throw new ValidationException("no coach");
        }
        
        if (team.getStatus() == null) {
            throw new ValidationException("no status");
        }
    }