Java8新特性stream流

一、包装数据类型

@Test
public void main22() {
    List<Integer> list = new ArrayList<Integer>(){{
        add(7); add(5); add(1); add(2);
        add(8); add(4); add(3); add(6);
        add(3); add(6); add(3); add(6);
    }};
    // 过滤
    List<Integer> filterList = list.stream().filter(a -> a < 5).collect(Collectors.toList());
    // 排序（正序）
    List<Integer> sortList1 = list.stream().sorted().collect(Collectors.toList());
    List<Integer> sortList2 = list.stream().sorted(Comparator.comparing(a -> a, Comparator.naturalOrder())).collect(Collectors.toList());
    // 排序（倒序）
    List<Integer> sortList3 = list.stream().sorted(Comparator.comparing(a -> a, Comparator.reverseOrder())).collect(Collectors.toList());
    // 最大数
    Integer max = list.stream().max(Comparator.comparing(a -> a, Comparator.naturalOrder())).get();
    // 最小数
    Integer min = list.stream().min(Comparator.comparing(a -> a, Comparator.naturalOrder())).get();
    // 去重
    List<Integer> distinctList = list.stream().distinct().collect(Collectors.toList());
    // 对每个元素进行操作
    List<Integer> mapList = list.stream().map(a -> a * a).collect(Collectors.toList());

    System.out.println(max);
    System.out.println(min);
    mapList.forEach(a -> System.out.print(a+" "));
}

二、Map类型

import org.apache.commons.lang.StringUtils;

@Test
public void test() {
    HashMap<String, Integer> map = new HashMap<String, Integer>(){{
        put("A", 1); put("B", 2); put("C", 3); put("D", 4);
        put("E", 1); put("F", 1); put("", 1); put("", 1);
    }};
    // key集合转list
    List<String> keyList1 = map.keySet().stream().filter(a -> StringUtils.isNotBlank(a)).collect(Collectors.toList());
    List<String> keyList2 = map.entrySet().stream().map(entry -> entry.getKey()).filter(a -> StringUtils.isNotBlank(a)).collect(Collectors.toList());
    // value集合转list
    List<Integer> valueList = map.entrySet().stream().map(entry -> entry.getValue()).collect(Collectors.toList());
    // 处理value，同理可处理key
    Map<String, Integer> newMap = map.entrySet().stream().collect(Collectors.toMap(Map.Entry::getKey, a -> a.getValue() * a.getValue()));
    // key value换位，key冲突时，新value替换旧value
    Map<Integer, String> reMap1 = map.entrySet().stream().collect(Collectors.toMap(Map.Entry::getValue,
        Map.Entry::getKey,
        (String val1, String val2) -> val2
    ));
    // key value换位，key冲突时，加入value列表中
    Map<Integer, List<String>> reMap2 = map.entrySet().stream().collect(Collectors.toMap(Map.Entry::getValue, 
        a -> new ArrayList<String>(){{ add(a.getKey()); }},
        (List<String> v1, List<String> v2) -> {
            v1.addAll(v2);
            return v1;
        }
    ));

    keyList2.forEach(a -> System.out.printf(a + " "));
    System.out.println();
    reMap2.entrySet().forEach(entry -> System.out.println("key: " + entry.getKey() + ", val: " + entry.getValue()));
}

三、对象List

@Test
public void test() {
    List<User> list = new ArrayList<User>() {{
        add(new User(1, "半月", 18));
        add(new User(2, "无霜", 22));
        add(new User(3, "半月无霜", 24));
        add(new User(3, "半霜", 28));
        add(new User(4, "半霜", 28));
    }};
    // 转Map<id, user>，如遇到key冲突，可参考第二段map解决办法
    Map<Integer, User> map1 = list.stream().collect(Collectors.toMap(User::getId, Function.identity()));
    Map<Integer, User> map2 = list.stream().collect(Collectors.toMap(User::getId, a -> a));
    // 查找
    User user = list.stream().filter(a -> "半月".equals(a.getName())).findFirst().get();
    // 过滤计数
    long count = list.stream().filter(a -> a.getAge() >= 22).count();
    // 排序（通过年龄排正序）
    List<User> orderList = list.stream().sorted(Comparator.comparing(User::getAge, Comparator.naturalOrder())).collect(Collectors.toList());
    // 排序（通过年龄排倒序）
    List<User> reorderList = list.stream().sorted(Comparator.comparing(User::getAge, Comparator.reverseOrder())).collect(Collectors.toList());
    // 对象去重（通过ID去重）
    List<User> unionUser = list.stream().collect(
        Collectors.collectingAndThen(Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(o -> o.getId()))), ArrayList::new));
    // 属性集合（此处为名字集合）
    List<String> nameList = list.stream().map(User::getName).collect(Collectors.toList());    
    // 截取，skip：跳过前n个，limit：取n个
    List<User> limitList = list.stream().skip(1).limit(2).collect(Collectors.toList());
    // 判断是否存在（半月是否存在）
    boolean isHave1 = list.stream().anyMatch(a -> "半月".equals(a.getName()));
    // 判断所有是否满足（是否都大于16岁）
    boolean fullAge = list.stream().allMatch(a -> a.getAge() > 16);
    // 判断是否不存在（里面没有ID为10的用户）
    boolean isHave2 = list.stream().noneMatch(a -> "10".equals(a.getId()));

    limitList.forEach(System.out::println);
    System.out.println(isHave2? "没有ID为10的用户": "有ID为10的用户");
}

@Data
@AllArgsConstructor
class User{
    private Integer id;
    private String name;
    private Integer age;
}

四、分组

@Test
public void test() throws InterruptedException {
    List<User> list = new ArrayList<User>() {{
        add(new User("半月1", 18, 90));
        add(new User("半月2", 18, 80));
        add(new User("半月3", 18, 70));
        add(new User("半月4", 18, 60));
        add(new User("半月无霜1", 20, 90));
        add(new User("半月无霜2", 20, 80));
        add(new User("半月无霜3", 20, 70));
        add(new User("半月无霜4", 20, 60));
    }};
    // 分组，通过分数对用户进行分组
    Map<Integer, List<User>> groupByScore = list.stream().collect(Collectors.groupingBy(
        User::getScore,
        Collectors.mapping(Function.identity(), Collectors.toList())
    ));
    // 分组计算，通过年龄分组计算最大的分数值
    Map<Integer, Optional<Integer>> maxScoreByAge = list.stream().collect(Collectors.groupingBy(
        User::getAge,
        Collectors.mapping(User::getScore, Collectors.maxBy(Integer::compareTo))
    ));
    // 按照条件分组，及格和不及格的两批人的最高分
    Map<Boolean, Optional<Integer>> maxScoreByPassMark = list.stream().collect(Collectors.partitioningBy(
        user -> user.getScore() >= 60,
        Collectors.mapping(User::getScore, Collectors.maxBy(Integer::compareTo))
    ));

    maxScoreByPassMark.keySet().forEach(a -> System.out.println("是否及格：" + a + "：" + maxScoreByPassMark.get(a)));
}

@Data
@AllArgsConstructor
class User{
    private String name;
    private Integer age;
    private Integer score;
}