java8--List转为Map、分组、过滤、求和等操作利⽤java8新特性，可以⽤简洁⾼效的代码来实现⼀些数据处理~
list集合中取出某⼀属性的⽅法 :
List<User> list = new ArrayList<User>();
User user1 = new User("第⼀位","⽤户1");
list.add(user1);
User user2 = new User("第⼆位","⽤户2");
list.add(user2);
User user3 = new User("第三位","⽤户3");
list.add(user3);
List<String> tableNames=list.stream().map(User::getMessage).List());
定义1个Apple对象：
public class Apple {
private Integer id;
private String name;
private BigDecimal money;
private Integer num;
public Apple(Integer id, String name, BigDecimal money, Integer num) {
this.id = id;
this.name = name;
< = money;
this.num = num;
}
}
添加⼀些测试数据：
List<Apple> appleList = new ArrayList<>();//存放apple对象集合
Apple apple1 =  new Apple(1,"苹果1",new BigDecimal("3.25"),10);
Apple apple12 = new Apple(1,"苹果2",new BigDecimal("1.35"),20);
Apple apple2 =  new Apple(2,"⾹蕉",new BigDecimal("2.89"),30);
Apple apple3 =  new Apple(3,"荔枝",new BigDecimal("9.99"),40);
appleList.add(apple1);
appleList.add(apple12);
appleList.add(apple2);
appleList.add(apple3);
1. List转Map
id为key，apple对象为value，可以这么做：
/**
* List -> Map
* 需要注意的是：
* toMap 如果集合对象有重复的key，会报错Duplicate key ....
*  apple1,apple12的id都为1。
*  可以⽤ (k1,k2)->k1 来设置，如果有重复的key，则保留key1，舍弃key2
*/
Map<Integer, Apple> appleMap = appleList.stream().Map(Apple::getId, a -> a,(k1,k2)->k1));
打印appleMap：
{1=Apple{id=1, name='苹果1', money=3.25, num=10}, 2=Apple{id=2, name='⾹蕉', money=2.89, num=30}, 3=Apple{id=3, name='荔枝', money=9.99, num=40}}
2. 分组
List⾥⾯的对象元素，以某个属性来分组，例如，以id分组，将id相同的放在⼀起：
苹果apple id注册
//List 以ID分组 Map<Integer,List<Apple>>
Map<Integer, List<Apple>> groupBy = appleList.stream().upingBy(Apple::getId));
{1=[Apple{id=1, name='苹果1', money=3.25, num=10}, Apple{id=1, name='苹果2', money=1.35, num=20}], 2=[Apple{id=2, name='⾹蕉', money=2.89, num=30}], 3
3. 过滤filter
从集合中过滤出来符合条件的元素：
//过滤出符合条件的数据
List<Apple> filterList = appleList.stream().filter(a -> a.getName().equals("⾹蕉")).List());
[Apple{id=2, name='⾹蕉', money=2.89, num=30}]
4. 求和
将集合中的数据按照某个属性求和:
BigDecimal:
//计算总⾦额
BigDecimal totalMoney = appleList.stream().map(Apple::getMoney).reduce(BigDecimal.ZERO, BigDecimal::add);
Integer:
//计算数量
int sum = appleList.stream().mapToInt(Apple::getNum).sum();