1、Java 8 允许我们通过 default 关键字对接口中定义的抽象方法提供一个默认的实现。

如：

default String init();

2、Lambda 表达式

Collections.sort(names, (String a, String b) -> {
    return b.compareTo(a);
});

例子代码：

/*filter过滤(T-> boolean)*/

List newlist = list.stream().filter(user -> user.getAge() > 20) .collect(Collectors.toList());

/*distinct 去重*/

List newlist = list.stream().distinct().collect(Collectors.toList());

/*sorted排序*/

List newlist = list.stream() .sorted(Comparator.comparingInt(User::getAge)).collect(Collectors.toList());

/*limit返回前n个元素*/

List newlist = list.stream() .sorted(Comparator.comparingInt(User::getAge)).limit(2).collect(Collectors.toList());

/*skip去除前n个元素*/

List newlist = list.stream() .sorted(Comparator.comparingInt(User::getAge)).skip(2).collect(Collectors.toList());

/*map(T->R)*/

List newlist = list.stream() .map(User::getName).distinct().collect(Collectors.toList());

/*allMatch（T->boolean）

检测是否全部满足参数行为*/

boolean flag = list.stream().allMatch(user -> user.getAge() >= 10);

/*anyMatch（T->boolean）

检测是否有任意元素满足给定的条件*/

boolean flag = list.stream() .anyMatch(user -> user.getAge() == 10);

/*noneMatchT->boolean）

流中是否有元素匹配给定的 T -> boolean条件*/

boolean flag = list.stream() .noneMatch(user -> user.getName().contains("张三李四"));

/*findFirst( ):找到第一个元素*/

Optional optionalUser = list.stream() .sorted(Comparator.comparingInt(User::getAge)) .findFirst();

/*count()计算总数*/

long count = list.stream().count();

/*最大值最小值*/

Optional max = list.stream() .collect( Collectors.maxBy( Comparator.comparing(User::getAge) ) );

Optional min = list.stream() .collect( Collectors.minBy( Comparator.comparing(User::getAge) ) );

/*求和_平均值*/

int totalAge = list.stream() .collect(Collectors.summingInt(User::getAge));

double avgAge = list.stream() .collect(Collectors.averagingInt(User::getAge));

/*一次性得到元素的个数、总和、最大值、最小值*/

IntSummaryStatistics statistics = list.stream() .collect(Collectors.summarizingInt(User::getAge));

/*拼接*/

String names = list.stream() .map(User::getName) .collect(Collectors.joining(", "));

/*分组*/

Map> map1 = users().stream() .collect(Collectors.groupingBy(User::getName));

Map>> map2 = users().stream() .collect(Collectors.groupingBy(User::getName, Collectors.groupingBy(User::getAge)));

/*分组合计*/

Map num = users().stream() .collect(Collectors.groupingBy(User::getName, Collectors.counting()));

Map num2 = users().stream() .filter(user -> user.getAge() >= 18) .collect(Collectors.groupingBy(User::getName, Collectors.counting()));