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java8快速实现List转map 、分组、过滤等操作

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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。

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)
    );

下表展示 Collectors 类的静态工厂方法。

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