java8 快速實現List轉map 、分組、過濾等操作
阿新 • • 發佈:2018-11-28
利用java8新特性,可以用簡潔高效的程式碼來實現一些資料處理。
定義1個Apple物件:
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; } }
List<Apple> appleList = new ArrayList<>();//存放apple物件集合 Apple apple1 = new Apple(1,"蘋果1",new BigDecimal("3.25"),10); Apple apple12 = new Apple(1,"蘋果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、分組
List裡面的物件元素,以某個屬性來分組,例如,以id分組,將id相同的放在一起:
//List 以ID分組 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='蘋果1', money=3.25, num=10}, Apple{id=1, name='蘋果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轉Map
id為key,apple物件為value,可以這麼做:
/**
* List -> Map
* 需要注意的是:
* toMap 如果集合物件有重複的key,會報錯Duplicate key ....
* apple1,apple12的id都為1。
* 可以用 (k1,k2)->k1 來設定,如果有重複的key,則保留key1,捨棄key2
*/
Map<Integer, Apple> appleMap = appleList.stream().collect(Collectors.toMap(Apple::getId, a -> a,(k1,k2)->k1));
{1=Apple{id=1, name='蘋果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、過濾Filter
從集合中過濾出來符合條件的元素:
//過濾出符合條件的資料
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
將集合中的資料按照某個屬性求和:
//計算 總金額
BigDecimal totalMoney = appleList.stream().map(Apple::getMoney).reduce(BigDecimal.ZERO, BigDecimal::add);
System.err.println("totalMoney:"+totalMoney); //totalMoney:17.48
5.查詢流中最大 最小值
Collectors.maxBy 和 Collectors.minBy 來計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;
// 根據id去重
List<Person> unique = appleList.stream().collect(
collectingAndThen(
toCollection(() -> new TreeSet<>(comparingLong(Apple::getId))), ArrayList::new)
);
下表展示 Collectors 類的靜態工廠方法。