JDK8：stream

List<User> userList = new ArrayList<>();
        userList.add(new User(1,"zhangsan","123","男"));
        userList.add(new User(2,"lisi","234", "男"));
        userList.add(new User(3,"one","123","女"));
        userList.add(new User(4,"demo","123", "男"));
        
        //1、foreach
        userList.stream().forEach(System.out::println); (可能被替代)
        userList.forEach(System.out::println)
        //2、filter（查询性别为男的数据）
        List<User> list = userList.stream().filter(e -> e.getGender().equals("男")).collect(Collectors.toList());
        //3、findFirst
        Optional<User> first = userList.stream().filter(e -> e.getGender().equals("女")).findFirst();
        if(first.isPresent()){
            User user = first.get();
        }
        //4、toSet
        Set<User> collect = userList.stream().filter(e -> e.getGender().equals("男")).collect(Collectors.toSet());
        collect.forEach(System.out::println);
        //5、去重（根据某个字段去重需要重写equals方法）
        userList.stream().distinct().collect(Collectors.toList());
        //6、查询前2条
        userList.stream().limit(2).collect(Collectors.toList()).forEach(System.out::println);
        //7、查询从第2条以后的数据（不包括第2条）
        userList.stream().skip(2).collect(Collectors.toList()).forEach(System.out::println);
        /**
         * 8、
         * anyMatch表示，判断的条件里，任意一个元素成功，返回true
         * allMatch表示，判断条件里的元素，所有的都是，返回true
         * noneMatch跟allMatch相反，判断条件里的元素，所有的都不是，返回true
         */
        System.out.println(userList.stream().anyMatch(e -> e.getGender().equals("男")));
        System.out.println(userList.stream().allMatch(e -> e.getGender().equals("男")));
        System.out.println(userList.stream().noneMatch(e -> e.getGender().equals("男")));
        //10、list 转 map
        Map<String, String> map = userList.stream().collect(Collectors.toMap(User::getUsername, User::getPassword));
        //11、针对重复key的  覆盖之前的value
        Map<String, String> map1 = userList.stream().collect(Collectors.toMap(User::getUsername, User::getPassword,(k,v) -> v));
        //12、排序（正序）
        List<User> sortList = userList.stream().sorted(Comparator.comparing(User::getUsername)).collect(Collectors.toList());
        // 倒序
        List<User> reverseList = userList.stream().sorted(Comparator.comparing(User::getUsername).reversed()).collect(Collectors.toList());
        reverseList.forEach(System.out::println);
        //非 stream 排序
        userList.sort(Comparator.comparing(User::getUsername));
        //13 、实体类集合中取出其中一个字段的集合
        List<Integer> roleIdList = roleGroupList.stream().map(SysRoleGroup::getRoleId).collect(Collectors.toList());
        //14、分组
        Map<Long, List<ShoppingCart>> collect = list.stream().collect(Collectors.groupingBy(ShoppingCart::getCategoryId));