JAVA JDK8 List分组的实现和用法
JAVA JDK8 List分组的实现和用法 时间:2018-12-06 今天小编就为大家分享一篇关于JAVA JDK8 List分组的实现和用法，小编觉得内容挺不错的，现在分享给大家，具有很好的参考价值，需要的朋友一起跟随小编来看看吧 概述
对List进行分组是日常开发中，经常遇到的，在JDK 8中对List按照某个属性分组的代码，超级简单。
package test; import com.alibaba.fastjson.JSON; import com.alibaba.fastjson.serializer.SerializerFeature; import java.util.ArrayList; import java.util.List; import java.util.Map; import java.util.stream.Collectors; public class ListGroupTest { public static void main(String[] args) { List couponList = new ArrayList<>(); Coupon coupon1 = new Coupon(1,100,“优惠券1”); Coupon coupon2 = new Coupon(2,200,“优惠券2”); Coupon coupon3 = new Coupon(3,300,“优惠券3”); Coupon coupon4 = new Coupon(3,400,“优惠券4”); couponList.add(coupon1); couponList.add(coupon2); couponList.add(coupon3); couponList.add(coupon4); Map<Integer, List> resultList = couponList.stream().collect(Collectors.groupingBy(Coupon::getCouponId)); System.out.println(JSON.toJSONString(resultList, SerializerFeature.PrettyFormat)); } } package test; public class Coupon { private Integer couponId; private Integer price; private String name; public Coupon(Integer couponId, Integer price, String name) { this.couponId = couponId; this.price = price; this.name = name; } public Integer getCouponId() { return couponId; } public void setCouponId(Integer couponId) { this.couponId = couponId; } public Integer getPrice() { return price; } public void setPrice(Integer price) { this.price = price; } public String getName() { return name; } public void setName(String name) { this.name = name; } } 上面的例子是对List按照couponId分组，couponId一样的，归为一组。打印结果如下：
{ 1:[ { “couponId”:1, “name”:“优惠券1”, “price”:100 } ], 2:[ { “couponId”:2, “name”:“优惠券2”, “price”:200 } ], 3:[ { “couponId”:3, “name”:“优惠券3”, “price”:300 }, { “couponId”:3, “name”:“优惠券4”, “price”:400 } ] }
如果分组后，分组内并不想是对象，而是对象的属性，也可以做到的。
package test; import com.alibaba.fastjson.JSON; import com.alibaba.fastjson.serializer.SerializerFeature; import java.util.ArrayList; import java.util.List; import java.util.Map; import java.util.stream.Collectors; public class ListGroupTest2 { public static void main(String[] args) { List couponList = new ArrayList<>(); Coupon coupon1 = new Coupon(1,100,“优惠券1”); Coupon coupon2 = new Coupon(2,200,“优惠券2”); Coupon coupon3 = new Coupon(3,300,“优惠券3”); Coupon coupon4 = new Coupon(3,400,“优惠券4”); couponList.add(coupon1); couponList.add(coupon2); couponList.add(coupon3); couponList.add(coupon4); Map<Integer, List> resultList = couponList.stream().collect(Collectors.groupingBy(Coupon::getCouponId,Collectors.mapping(Coupon::getName,Collectors.toList()))); System.out.println(JSON.toJSONString(resultList, SerializerFeature.PrettyFormat)); } } 这样分组内就是name属性了。打印结果如下：
{ 1:[ “优惠券1” ], 2:[ “优惠券2” ], 3:[ “优惠券3”, “优惠券4” ] }
总结
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