java lambda 表达式中的双冒号的用法说明 ::

@Test
public void convertTest() {
  List<String> collected = new ArrayList<>();
  collected.add("alpha");
  collected.add("beta");
  collected = collected.stream().map(string -> string.toUpperCase()).collect(Collectors.toList());
  System.out.println(collected);
}

@Test
public void convertTest() {
  List<String> collected = new ArrayList<>();
  collected.add("alpha");
  collected.add("beta");
  collected = collected.stream().map(String::toUpperCase).collect(Collectors.toCollection(ArrayList::new));//注意发生的变化
  System.out.println(collected);
}

//注册功能
public void register(User user){
//补齐数据
user.setUid(CommonUtils.uuid());
user.setStatus(false);
user.setActivationCode(CommonUtils.uuid() + CommonUtils.uuid());
try {
userDao.save(user);
} catch (Exception e) {
throw new RuntimeException();
}
//发送邮件
//加载配置文件
Properties properties = new Properties();
try {
properties.load(this.getClass().getClassLoader().getResourceAsStream("email_template.properties"));
} catch (IOException e1) {
throw new RuntimeException();
}
String host = properties.getProperty("host");
String username = properties.getProperty("username");
String password = properties.getProperty("password");

String from = properties.getProperty("from");
String to = user.getEmail();
String subject = properties.getProperty("subject");
//把占位符用后面的参数替换，后面参数可变
String content = MessageFormat.format(properties.getProperty("content"), user.getActivationCode());

//发送邮件3步曲
Session session = MailUtils.createSession(host, username, password);
Mail mail = new Mail(from, to, subject, content);
try {
MailUtils.send(session, mail);
} catch (Exception e) {
throw new RuntimeException();
}
}