java 中的Collection<? extends Object>c是什么意思

<?extends Object>表示Collection集合中可以存放任意的Object子类
详细情况请看sun 网站

示列代码：

public BusinessAuthenticationToken(Object principal, String phone, Integer merchantId,
                                   boolean cxqyStaff, boolean normalMerchantAuth,
                                   Collection<? extends GrantedAuthority> authorities) {
    super(authorities);
    if (authorities == null || authorities.size() == 0) {
        setAuthenticated(false);
    } else {
        super.setAuthenticated(true);
    }
    this.principal = principal;
    this.phone = phone;
    this.cxqyStaff = cxqyStaff;
    this.merchantId = merchantId;
    this.normalMerchantAuth = normalMerchantAuth;
}