概述
對(duì)list進(jìn)行分組是日常開(kāi)發(fā)中，經(jīng)常遇到的，在jdk 8中對(duì)list按照某個(gè)屬性分組的代碼，超級(jí)簡(jiǎn)單。
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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,"優(yōu)惠券1"); coupon coupon2 = new coupon(2,200,"優(yōu)惠券2"); coupon coupon3 = new coupon(3,300,"優(yōu)惠券3"); coupon coupon4 = new coupon(3,400,"優(yōu)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; }} |
上面的例子是對(duì)list按照couponid分組，couponid一樣的，歸為一組。打印結(jié)果如下：
{
1:[
{
"couponid":1,
"name":"優(yōu)惠券1",
"price":100
}
],
2:[
{
"couponid":2,
"name":"優(yōu)惠券2",
"price":200
}
],
3:[
{
"couponid":3,
"name":"優(yōu)惠券3",
"price":300
},
{
"couponid":3,
"name":"優(yōu)惠券4",
"price":400
}
]
}
如果分組后，分組內(nèi)并不想是對(duì)象，而是對(duì)象的屬性，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,"優(yōu)惠券1"); coupon coupon2 = new coupon(2,200,"優(yōu)惠券2"); coupon coupon3 = new coupon(3,300,"優(yōu)惠券3"); coupon coupon4 = new coupon(3,400,"優(yōu)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)); }} |
這樣分組內(nèi)就是name屬性了。打印結(jié)果如下：
{
1:[
"優(yōu)惠券1"
],
2:[
"優(yōu)惠券2"
],
3:[
"優(yōu)惠券3",
"優(yōu)惠券4"
]
}
總結(jié)
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原文鏈接：https://blog.csdn.net/linsongbin1/article/details/83933703
