概述
對List進行分組是日常開發中，經常遇到的，在JDK 8中對List按照某個屬性分組的代碼，超級簡單。
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,"優惠券1");
Coupon coupon2 = new Coupon(2,200,"優惠券2");
Coupon coupon3 = new Coupon(3,300,"優惠券3");
Coupon coupon4 = new Coupon(3,400,"優惠券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;
}
}
上面的例子是對List按照couponId分組，couponId一樣的，歸為一組。打印結果如下：
{
1:[
{
"couponId":1,
"name":"優惠券1",
"price":100
}
],
2:[
{
"couponId":2,
"name":"優惠券2",
"price":200
}
],
3:[
{
"couponId":3,
"name":"優惠券3",
"price":300
},
{
"couponId":3,
"name":"優惠券4",
"price":400
}
]
}
如果分組后，分組內并不想是對象，而是對象的屬性，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,"優惠券1");
Coupon coupon2 = new Coupon(2,200,"優惠券2");
Coupon coupon3 = new Coupon(3,300,"優惠券3");
Coupon coupon4 = new Coupon(3,400,"優惠券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));
}
}
這樣分組內就是name屬性了。打印結果如下：
{
1:[
"優惠券1"
],
2:[
"優惠券2"
],
3:[
"優惠券3",
"優惠券4"
]
}
總結
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