Java8使用stream流，快速实现List转map 、分组、过滤等操作

时间：2025-03-20 10:47:27

文章目录

    • 0.准备资源
    • 1. 分组
    • 2. List转Map
    • 3.过滤Filter
    • 4.求和
    • 5.集合中的元素去重
    • 6.集合中的元素转新集合
    • 7.集合去空

0.准备资源

public class Apple {
    private Integer id;
    private String name;
    private BigDecimal money;
    private Integer num;
//省略 get/set/满参
}
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：

Map<Integer, List> groupBy =().collect((Apple::getId));
(“groupBy:”+groupBy);

打印结果如下:

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)->k2));
        System.err.println("appleMap:"+appleMap);
//appleMap:{1=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}}

3.过滤Filter

从集合中过滤出来符合条件的元素：

List<Apple> filterList = appleList.stream().filter(a -> a.getName().equals("香蕉")).collect(Collectors.toList());
System.err.println("filterList:"+filterList);
//filterList:[Apple{id=2, name='香蕉', money=2.89, num=30}]
List<String> divisions = strings.stream().filter(o -> o.contains(name)).collect(Collectors.toList());

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

//求集合元素和
Stream<Integer> stream = Arrays.stream(new Integer[]{1, 2, 3, 4, 5, 6, 7, 8});
Integer result = stream.reduce(0, Integer::sum);
System.out.println(result);              //36


stream = Arrays.stream(new Integer[]{1, 2, 3, 4, 5, 6, 7});
//求和
stream.reduce((i, j) -> i + j).ifPresent(System.out::println);    //28

//求最大值
stream.reduce(Integer::max).ifPresent(System.out::println);    //7


//求最小值
stream.reduce(Integer::min).ifPresent(System.out::println);   //1

//做逻辑 >最大值 ，<最小值
stream = Arrays.stream(new Integer[]{9, 2, 3, 8, 5, 5, 7});
stream.reduce((i, j) -> i > j ? j : i).ifPresent(System.out::println);  //2

//求逻辑求乘机
stream = Arrays.stream(new Integer[]{1, 2, 3, 4, 5, 6, 7});
int result2 = stream.filter(i -> i % 2 == 0).reduce(1, (i, j) -> i * j);
Optional.of(result2).ifPresent(System.out::println);   //48

5.集合中的元素去重

List<String> reportDeptNames = deptEntitys.stream().map(t -> t.getReportDeptName()).distinct().collect(Collectors.toList());

6.集合中的元素转新集合

List<String> specificReasons = exceptionManageList.stream().map(ExceptionManageEntity::getSpecificReason).collect(Collectors.toList());

7.集合去空

deptEntitys.removeAll(Collections.singleton(null));
或者
List<String> reportDeptNames = deptEntitys.stream().map(DeptEntity::getReportDeptName).filter(x -> x != null).collect(Collectors.toList());

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