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。
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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)); } } |
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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。
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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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原文链接：https://blog.csdn.net/linsongbin1/article/details/83933703
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