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));
  }
}

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(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(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：