java8快速实现List转map 、分组、过滤等操作
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。
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。
以上就是本文的全部内容,希望对大家的学习有所帮助,也希望大家多多支持我们。
赞 (0)