Java8使用Lambda表达式(流式)快速实现List转map 、分组、过滤等操作

我不是女神ヾ 2023-10-04 23:12 21阅读 0赞

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。

## 1 数据准备 ##

### 1.1 定义1个Fruit对象 ###

package com.wkf.workrecord.work;
    
    import org.junit.Test;
    
    import java.math.BigDecimal;
    import java.util.ArrayList;
    import java.util.List;
    
    /**
     * @author wuKeFan
     * @date 2023-03-09 15:07:04
     */
    public class PublicTest {
    
        @Test
        public void test() {
            List<Fruit> appleList = new ArrayList<>();//存放apple对象集合
            Fruit apple1 =  new Fruit(1,"苹果1",new BigDecimal("3.25"),10);
            Fruit apple2 = new Fruit(1,"苹果2",new BigDecimal("1.35"),20);
            Fruit banana =  new Fruit(2,"香蕉",new BigDecimal("2.89"),30);
            Fruit lychee =  new Fruit(3,"荔枝",new BigDecimal("9.99"),40);
            appleList.add(apple1);
            appleList.add(banana);
            appleList.add(apple2);
            appleList.add(lychee);
        }
    
    }

### 1.2 添加一些测试数据 ###

List<Fruit> fruitList = new ArrayList<>();//存放水果对象集合
            Fruit apple1 =  new Fruit(1,"苹果1",new BigDecimal("3.25"),10);
            Fruit apple2 = new Fruit(1,"苹果2",new BigDecimal("1.35"),20);
            Fruit banana =  new Fruit(2,"香蕉",new BigDecimal("2.89"),30);
            Fruit lychee =  new Fruit(3,"荔枝",new BigDecimal("9.99"),40);
            fruitList.add(apple1);
            fruitList.add(banana);
            fruitList.add(apple2);
            fruitList.add(lychee);

## 2 正文开始 ##

### 2.1 分组 ###

Map<Integer, List<Fruit>> groupBy = fruitList.stream().collect(Collectors.groupingBy(Fruit::getId));
    System.err.println("groupBy:"+groupBy);

输出:

groupBy:{1=[Fruit(id=1, name=苹果1, money=3.25, num=10), Fruit(id=1, name=苹果2, money=1.35, num=20)], 2=[Fruit(id=2, name=香蕉, money=2.89, num=30)], 3=[Fruit(id=3, name=荔枝, money=9.99, num=40)]}

### 2.2 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, Fruit> appleMap = fruitList.stream().collect(Collectors.toMap(Fruit::getId, a -> a,(k1,k2)->k1));
    System.out.println(appleMap);

输出:

{1=Fruit(id=1, name=苹果1, money=3.25, num=10), 2=Fruit(id=2, name=香蕉, money=2.89, num=30), 3=Fruit(id=3, name=荔枝, money=9.99, num=40)}

### 2.3 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<Fruit> filterList = fruitList.stream().filter(a -> a.getName().equals("香蕉")).collect(Collectors.toList());
    System.err.println("filterList:"+filterList);

输出:

filterList:[Fruit(id=2, name=香蕉, money=2.89, num=30)]

### 2.4 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

### 2.5 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);

### 2.6 去重 ###

import static java.util.Comparator.comparingLong;
    import static java.util.stream.Collectors.collectingAndThen;
    import static java.util.stream.Collectors.toCollection;
    
    List<Fruit> unique = fruitList.stream().collect(
        collectingAndThen(
        toCollection(() -> new TreeSet<>(comparingLong(Fruit::getId))), ArrayList::new)
    );

## 3 下表展示 Collectors 类的静态工厂方法 ##

![531719bbe0264ebca851802de1265c20.png][]

[531719bbe0264ebca851802de1265c20.png]: https://img-blog.csdnimg.cn/img_convert/531719bbe0264ebca851802de1265c20.png