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<Coupon> 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<Coupon>> 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<Coupon> 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<String>> 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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