lambda

// Java 8之前：
List features = Arrays.asList("Lambdas", "Default Method", "Stream API", "Date and Time API");
for (String feature : features) {
System.out.println(feature);
}

// Java 8之后：
List features = Arrays.asList("Lambdas", "Default Method", "Stream API", "Date and Time API");
features.forEach(n -> System.out.println(n));

// 使用Java 8的方法引用更方便，方法引用由::双冒号操作符标示，
// 看起来像C++的作用域解析运算符
features.forEach(System.out::println);

   allfreightList.forEach(freight ->
                {
                    if (!freigthIdSet.contains(freight.getId())) {
                        freight.setDeleted(true);
                        freight.audit(identity);
                        customerVisitFreightService.saveOrUpdate(freight);
                    }
                }
        );

// 将字符串换成大写并用逗号链接起来
List<String> G7 = Arrays.asList("USA", "Japan", "France", "Germany", "Italy", "U.K.","Canada");
String G7Countries = G7.stream().map(x -> x.toUpperCase()).collect(Collectors.joining(", "));
System.out.println(G7Countries);

从 Java 8 引入的一个很有趣的特性是 Optional 类。Optional 类主要解决的问题是臭名昭著的空指针异常（NullPointerException）
本质上，这是一个包含有可选值的包装类，这意味着 Optional 类既可以含有对象也可以为空。

 User user = null;
    User user2 = new User("anna@gmail.com", "1234");
    User result = Optional.ofNullable(user).orElse(user2);

public void whenThrowException_thenOk() {
    User result = Optional.ofNullable(user)
      .orElseThrow( () -> new IllegalArgumentException());
}