java 8 部分用法

// 遍历
    public static void main(String[] args) {
        List strList = Arrays.asList("a", "b", "c", "d");
        // s:随便定义
        strList.stream().forEach(s -> {
            System.out.println(s);
        });
    }

// 平方
    public static void main(String[] args) {
        List list = Arrays.asList(1, 2, 6, 9);
        // s:随便定义
        List newList =  list.stream().map(s -> s * s).collect(Collectors.toList());

        // 遍历输出
        newList.stream().forEach(n -> {
            System.out.println(n);
        });
    }

// 转换为大写
    public static void main(String[] args) {
        List strList = Arrays.asList("a", "b", "c", "d");
        // s:随便定义
        strList.stream().map(String::toUpperCase).collect(Collectors.toList()).forEach(s -> {
            System.out.println(s);
        });
    }

// 过滤
    public static void main(String[] args) {
        List strList = Arrays.asList("ab", "bcd", "cde", "de");
        // s:随便定义, 过滤长度不等于2的元素
        strList.stream()
                .filter(s -> s.length() != 2)                   // 过滤
                .collect(Collectors.toList())                   // 转成新的list
                .forEach(s -> {                                 // 遍历
                    System.out.println(s);
        });
    }

// 过滤:得到其中不为空的String
    public static void main(String[] args) {
        List strList = Arrays.asList("ab", "bcd", "cde", "de", "");
        // s:随便定义, 过滤非空的元素
        strList.stream()
                .filter(s -> !s.isEmpty())                      // 过滤
                .collect(Collectors.toList())                   // 转成新的list
                .forEach(s -> {                                 // 遍历
                    System.out.println(s);
        });
    }

// 返回特定结果的list
    public static void main(String[] args) {
        List strList = Arrays.asList("a", "b", "c", "d", "e", "f");
        // s:随便定义,
        List newList = strList.stream()
                .skip(2)                            // 扔掉前面2个元素
                .limit(3)                           // 返回前3个元素
                .collect(Collectors.toList());      // 得到一个新的list
        newList.stream().forEach(s -> {
            System.out.println(s);
        });
    }

// 排序
    public static void main(String[] args) {
        List strList = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9);
        // ln1, ln2:随便定义,
        List newList = strList.stream()
                .sorted((ln1, ln2) -> (ln1-ln2))    // 排序
                .collect(Collectors.toList());      // 返回新的list
        // newList就是排序之后的list
    }

Long id = userList.stream().max(Comparator.comparing(user::getId)).get().getId();
Long id = userList.stream().min(Comparator.comparing(user::getId)).get().getId();

/ 最大值，最小值
    public static void main(String[] args) {
        List strList = Arrays.asList("a","b","d","s","n","yu","luo","shuai","spot", "xhuai");
        // s:随便定义,
        int max = strList.stream()
                .mapToInt(s -> s.length())  // 判断条件:长度
                .max()                      // 获取判断条件的最大值
                .getAsInt();                // 转成int长度
        System.out.println(max);

        // 返回一个新的list, 新的list中元素的长度是strList中元素长度最长的
        List maxList = strList.stream().filter(s -> s.length() == max).collect(Collectors.toList());
        System.out.println(maxList.size());

        // 集合中最小长度
        List strList2 = Arrays.asList("a","b","d","s","nd","yu","luo","shuai");
        int min = strList2.stream()
                .mapToInt(s -> s.length())
                .min().getAsInt();

        System.out.println(min);
    }

// List集合查重
    public static void main(String[] args) {
        List strList = Arrays.asList("a","b","d","s","n","a","s","c","d", "b");

        // 返回一个去掉重复元素之后的新的list
        List noRepeatList = strList.stream()
                .distinct()
                .collect(Collectors.toList());
        System.out.println(noRepeatList.size());
    }

// 匹配
    public static void main(String[] args) {
        /**
         * allMatch: Stream 中全部元素符合传入的predicate, 返回true
         * anyMatch: Stream 中只要有一个元素符合传入的predicate, 返回true
         * noneMatch: Stream 中没有一个元素符合传入的predicate, 返回true
         */
        List strList = Arrays.asList("a", "b", "d", "s", "n", "a", "s", "c", "d", "b");

        // 判断集合中有没有c元素, 返回值boolean
        boolean isExits = strList.stream().anyMatch(s -> s.equals("c"));
        System.out.println(isExits);

        List strList2 = Arrays.asList("a", "b", "d", "s", "n", "a", "s", "c", "d", "");

        // 判断集合是否全不为空
        boolean isFull = strList2.stream().allMatch(s -> s.isEmpty());
        System.out.println(isFull);

        boolean isNotEmpty = strList2.stream().noneMatch(s -> s.isEmpty());
        System.out.println(isNotEmpty);
    }