[java8] list中stream流操作

日常开发工作中的许多业务场景，都需要将数据库中查到的各种数据，格式化到对应的vo中，为了方便快捷地将完成这类业务，都需要对业务类的list使用流进行各种操作。准备对之前工作中目前能想到的用list stream流操作做一下记录。

List排序

对List中对象的某字段进行正序或倒叙排列

public static void sortTest() {
        List<ClassEntity> classes = Lists.newArrayList();
        classes.add(new ClassEntity().setNames("bb").setIds(22).setUserId(2));
        classes.add(new ClassEntity().setNames("aa").setIds(11).setUserId(1));
        classes.add(new ClassEntity().setNames("dd").setIds(44).setUserId(4));
        classes.add(new ClassEntity().setNames("cc").setIds(33).setUserId(3));
        //  正序
        classes.sort(Comparator.comparing(ClassEntity::getUserId));
        log.info("正序排序后的顺序为：{}", classes);
        // 倒序
        classes.sort(Comparator.comparing(ClassEntity::getUserId).reversed());
        log.info("倒序排序后的顺序为：{}", classes);
}

执行结果为

正序排序后的顺序为：[StreamTest.ClassEntity(userId=1, ids=11, names=aa), StreamTest.ClassEntity(userId=2, ids=22, names=bb), StreamTest.ClassEntity(userId=3, ids=33, names=cc), StreamTest.ClassEntity(userId=4, ids=44, names=dd)]

倒序排序后的顺序为：[StreamTest.ClassEntity(userId=4, ids=44, names=dd), StreamTest.ClassEntity(userId=3, ids=33, names=cc), StreamTest.ClassEntity(userId=2, ids=22, names=bb), StreamTest.ClassEntity(userId=1, ids=11, names=aa)]

List转Map

List转HashMap与guava的MultiMap

List转Map

正常使用

public static void listToMapTest() {
        List<ClassEntity> classes = Lists.newArrayList();
        classes.add(new ClassEntity().setNames("aa").setIds(11).setUserId(1));
        classes.add(new ClassEntity().setNames("bb").setIds(22).setUserId(2));
        classes.add(new ClassEntity().setNames("cc").setIds(33).setUserId(3));

        Map<Integer, String> map = classes.stream()
                .collect(Collectors.toMap(ClassEntity::getUserId, ClassEntity::getNames);

        map.forEach((k, v) -> {
            System.out.println(
                    "k:" + k + ",v:" + v
            );
        });
    }

执行结果为

k:1,v:aa
k:2,v:bb
k:3,v:cc

上面这种正常的用法在正常情况下可以正常执行，但如果数据的key有重复的情况呢。比如这样呢

classes.add(new ClassEntity().setNames("aa").setIds(11).setUserId(1));
classes.add(new ClassEntity().setNames("bb").setIds(22).setUserId(2));
classes.add(new ClassEntity().setNames("cc").setIds(33).setUserId(3));
classes.add(new ClassEntity().setNames("dd").setIds(33).setUserId(3));

此时会报如下异常

Exception in thread "main" java.lang.IllegalStateException: Duplicate key cc

为了防止执行时碰到这种情况抛出异常可以这么操作

Map<Integer, String> map = classes.stream().collect(Collectors.toMap(ClassEntity::getUserId, ClassEntity::getNames, (key1, key2) -> key2));

在Collectors.toMap中加入了这一行 (key1, key2) -> key2) ，这时如果发生key重复的情况，value值默认会替换为最后一次取到的value值，结果也就成了。

k:1,v:aa
k:2,v:bb
k:3,v:dd

List转MultiMap

MultiMap是guava中实现的value为List类型的Map，如果发生上面List转Map时key重复的情况时，可以考虑用这种方法，转为MultiMap。

public static void listToMultiMap() {
        List<ClassEntity> classes = Lists.newArrayList();
        classes.add(new ClassEntity().setNames("aa").setIds(11).setUserId(1));
        classes.add(new ClassEntity().setNames("ff").setIds(44).setUserId(1));
        classes.add(new ClassEntity().setNames("bb").setIds(22).setUserId(2));
        classes.add(new ClassEntity().setNames("cc").setIds(33).setUserId(3));

        ImmutableListMultimap<Integer, ClassEntity> orderItemMap = Multimaps.index(classes, new Function<ClassEntity, Integer>() {
            @Override
            public Integer apply(ClassEntity classEntity) {
                return classEntity.getUserId();
            }
        });
        log.info("multiMap:{}", orderItemMap);
    }

执行结果为：

{
	1 = [StreamTest.ClassEntity(userId = 1, ids = 11, names = aa), StreamTest.ClassEntity(userId = 1, ids = 44, names = ff)],
	2 = [StreamTest.ClassEntity(userId = 2, ids = 22, names = bb)],
	3 = [StreamTest.ClassEntity(userId = 3, ids = 33, names = cc)]
}

两个不同对象的List取相交的map

public static void listToMap() {
        List<User> users = Lists.newArrayList();
        users.add(new User().setName("a").setId(1));
        users.add(new User().setName("b").setId(2));
        users.add(new User().setName("c").setId(3));

        List<ClassEntity> classes = Lists.newArrayList();
        classes.add(new ClassEntity().setNames("aa").setIds(11).setUserId(1));
        classes.add(new ClassEntity().setNames("bb").setIds(22).setUserId(2));
        classes.add(new ClassEntity().setNames("cc").setIds(33).setUserId(3));

        Map<Integer, String> map = users.stream().collect(
                Collectors.toMap(user -> user.getId(), user -> classes.get(users.indexOf(user)).getNames())
        );
        map.forEach((key, value) -> {
            System.out.println(key + "：" + value);
        });
    }

执行结果为：

1：aa
2：bb
3：cc

List转key为id，value为对象

public static void listToMapTest2() {
        List<ClassEntity> classes = Lists.newArrayList();
        classes.add(new ClassEntity().setNames("aa").setIds(11).setUserId(1));
        classes.add(new ClassEntity().setNames("bb").setIds(22).setUserId(2));
        classes.add(new ClassEntity().setNames("cc").setIds(33).setUserId(3));

        Map<Integer, ClassEntity> idAndBeadMap = classes.stream().collect(Collectors.toMap(ClassEntity::getIds, java.util.function.Function.identity()));
        idAndBeadMap.forEach((k, v) -> {
            log.info("key为：{},value为：{}", k, v);
        });
    }

执行结果为：

key为：33,value为：ClassEntity(userId=3, ids=33, names=cc)
key为：22,value为：ClassEntity(userId=2, ids=22, names=bb)
key为：11,value为：ClassEntity(userId=1, ids=11, names=aa)

List取属性用字符分割

public static void joiningTest() {
       List<ClassEntity> classes = Lists.newArrayList();
       classes.add(new ClassEntity().setNames("bb").setIds(22).setUserId(2));
       classes.add(new ClassEntity().setNames("aa").setIds(11).setUserId(1));
       classes.add(new ClassEntity().setNames("cc").setIds(33).setUserId(3));

       String a = classes.stream().map(c -> String.valueOf(c.getUserId())).collect(Collectors.joining(","));
       log.info("加逗号转String为：{}", a);
   }

执行结果为：

加逗号转String为：2,1,3