圖片.png

利用java8新特性，可以用簡(jiǎn)潔高效的代碼來實(shí)現(xiàn)一些數(shù)據(jù)處理。
定義1個(gè)Apple對(duì)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;
    }
}

添加一些測(cè)試數(shù)據(jù)：

List<Apple> appleList = new ArrayList<>();//存放apple對(duì)象集合
 
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里面的對(duì)象元素，以某個(gè)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(zhuǎn)Map

id為key，apple對(duì)象為value，可以這么做：

/**
 * List -> Map
 * 需要注意的是：
 * toMap 如果集合對(duì)象有重復(fù)的key，會(huì)報(bào)錯(cuò)Duplicate key ....
 *  apple1,apple12的id都為1。
 *  可以用 (k1,k2)->k1 來設(shè)置，如果有重復(fù)的key,則保留key1,舍棄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='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：

//過濾出符合條件的數(shù)據(jù)
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

將集合中的數(shù)據(jù)按照某個(gè)屬性求和:

//計(jì)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、查找流中最大 最小值

Collectors.maxBy 和 Collectors.minBy 來計(jì)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);

ptional<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(jù)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(tài)工廠方法。

圖片.png

圖片.png

引用 https://mp.weixin.qq.com/s/20K3pluRBhcAZTV8vN-sAA