List<Fruit> fruitList = Arrays.asList(
                new Fruit("apple", 6,0),
                new Fruit("apple", 6,1),
                new Fruit("banana", 7,1),
                new Fruit("banana", 7,1),
                new Fruit("banana", 7,1),
                new Fruit("grape",8,2));

1.filter

从集合中筛选给定条件的子集

fruitList.stream().filter(t->t.getType()==1).collect(Collectors.toList());

2.map

从集合中取值组成集合

List<String> collect = fruitList.stream().map(Fruit::getName).distinct().collect(Collectors.toList());

//结果
[apple, banana, grape]

3.toMap

userEntity根据id分组

Map<Long, UserEntity> userMap = userEntities.stream().collect(Collectors.toMap(UserEntity::getUserId, Function.identity()));

4.reduce

//给初值，在累加求和
List<Integer> ints = Arrays.asList(1,2,3,4,5,6,7,8,9,10);
        System.out.println("ints sum is:" + ints.stream().reduce(1, (sum, item) -> sum + item));
//结果
56

5.groupingBy

把fruitList先按name分组，并按名称排序，在此基础上继续对type分组

Map<String, Map<Integer, List<Fruit>>> collect = fruitList.stream().collect(Collectors.groupingBy(Fruit::getName, TreeMap::new, Collectors.groupingBy(Fruit::getType)));
        collect.forEach((name,value)->{
            value.forEach((type,value2)->{
                System.out.println(name+"  "+type+"   "+value2);
            });
        });

//运行结果
apple  0   [Fruit{name='apple', price=6.0, type=0}]
apple  1   [Fruit{name='apple', price=6.0, type=1}]
banana  1   [Fruit{name='banana', price=7.0, type=1}, Fruit{name='banana', price=7.0, type=1}, Fruit{name='banana', price=7.0, type=1}]
grape  2   [Fruit{name='grape', price=8.0, type=2}]

groupBy,toMap,join

//groupBy分组后，取自己需要的值
List<Person> list = new ArrayList();
        list.add(new Person(1, "haha",10));
        list.add(new Person(2, "rere",10));
        list.add(new Person(4, "rere",13));
        list.add(new Person(3, "fefe",12));
      
        HashMap<Integer, Map<Integer, String>> collect = list.stream().collect
                (Collectors.groupingBy(Person::getYear, HashMap::new, Collectors.toMap(Person::getId, Person::getName)));
        Map<Integer, List<Person>> collect2 = list.stream().collect(Collectors.groupingBy(Person::getYear));
//        List<Person> collect = list.stream().sorted((o1,o2)->o2.getYear()-o1.getYear()).limit(20).collect(Collectors.toList());
        String collect1 = list.stream().map(Person::getName).collect(Collectors.joining(","));
        System.out.println(collect);
        System.out.println(collect2);
        System.out.println(collect1);

//运行结果
{10={1=haha, 2=rere}, 12={3=fefe}, 13={4=rere}}
{10=[Person{id=1, name='haha', year=10}, Person{id=2, name='rere', year=10}], 12=[Person{id=3, name='fefe', year=12}], 13=[Person{id=4, name='rere', year=13}]}
haha,rere,rere,fefe

对groupby中的内容拼接

Map<String, List<Fruit>> collect = fruitList.stream().collect(Collectors.groupingBy(t -> t.getName() + t.getPrice()));

//结果
{grape8.0=[Fruit{name='grape', price=8.0, type=2}], apple6.0=[Fruit{name='apple', price=6.0, type=0}, Fruit{name='apple', price=6.0, type=1}], banana7.0=[Fruit{name='banana', price=7.0, type=1}, Fruit{name='banana', price=7.0, type=1}, Fruit{name='banana', price=7.0, type=1}]}

List<Item> items = Arrays.asList(
    new Item("apple",10),
    new Item("banana",20),
    new Item("orange",10),
    new Item("watermelon",10),
    new Item("papaya",20),
    new Item("apple",10),
    new Item("banana",10),
    new Item("apple",20)
);

Map<String,Long> counting = items.stream().collect(Collectors.groupingBy(Item::getName,Collectors.counting()));
System.out.println(counting);

Map<String,Integer> sum = items.stream().collect(Collectors.groupingBy(Item::getName,Collectors.summingInt(Item::getQty)));
System.out.println(sum);

//输出
{
  papaya=1，banana=2，apple=3，orange=1，watermelon=1
}
{
  papaya=20，banana=30，apple=40，orange=10，watermelon=10
}

6.Collectors.mapping

在group分组后，取出每组对象中需要的值到list中

Map<String, List<Double>> collect1 = fruitList.stream().collect(Collectors.groupingBy(Fruit::getName, TreeMap::new, Collectors.mapping(Fruit::getPrice, Collectors.toList())));

//运行结果
{apple=[6.0, 6.0], banana=[7.0, 7.0, 7.0], grape=[8.0]}

7.排序

//上架的销量前20的货品
        List<GoodsEntity> collect = this.list(wrapper.eq("status",1)).stream().sorted(Comparator.comparing(GoodsEntity::getSalesVolume).reversed()).limit(20).collect(Collectors.toList());

//dataForms根据日期从小到大排序
Collections.sort(dataForms, (o1, o2) -> {
            Date dt1 = DateUtils.stringToDate(o1.getDate(),"yyyy-MM-dd");
            Date dt2 = DateUtils.stringToDate(o2.getDate(),"yyyy-MM-dd");
            if(dt1.getTime()<dt2.getTime()){
                return -1;
            }else {
                return 1;
            }
        });