Java8通过Stream流实现List转map 、分组、过滤等操作

Java8 Stream List Map 分组 过滤
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;
        this.money = 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、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：

//List 以ID分组 Map<Integer,List<Apple>>
Map<Integer, List<Apple>> groupBy = appleList.stream().collect(Collectors.groupingBy(Apple::getId));
System.err.println("groupBy:"+groupBy);
{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=[Apple{id=3, name='荔枝', money=9.99, num=40}]}

2、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().collect(Collectors.toMap(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}
}

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")).collect(Collectors.toList());
System.err.println("filterList:"+filterList);
[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:

//计算 总金额
BigDecimal totalMoney = appleList.stream().map(Apple::getMoney).reduce(BigDecimal.ZERO, BigDecimal::add);
System.err.println("totalMoney:"+totalMoney);  //totalMoney:17.48

5、查找流中最大 最小值

Collectors.maxBy 和 Collectors.minBy 来计算流中的最大或最小值。

Optional<Dish> maxDish = Dish.menu.stream().
      collect(Collectors.maxBy(Comparator.comparing(Dish::getCalories)));
maxDish.ifPresent(System.out::println);
Optional<Dish> minDish = Dish.menu.stream().
      collect(Collectors.minBy(Comparator.comparing(Dish::getCalories)));
minDish.ifPresent(System.out::println);

6、去重

import static java.util.Comparator.comparingLong;
import static java.util.stream.Collectors.collectingAndThen;
import static java.util.stream.Collectors.toCollection;
// 根据id去重
List<Person> unique = appleList.stream().collect(
    collectingAndThen(
        toCollection(() -> new TreeSet<>(comparingLong(Apple::getId))), ArrayList::new)
);

Collectors 类的静态工厂方法