
JAVA8的Stream操作
分组之后继续操作
示例1-转List
java
import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
/**
* 公众号:干货食堂
*/
public class Test {
public static void main(String[] args) {
List<Cpv> init = init();
List<Map<String, Object>> hit_val_param = init.stream()
.collect(Collectors.groupingBy(
Cpv::getProp,
Collectors.mapping(Cpv::getVal, Collectors.toList())
))
.entrySet().stream()
.map(e -> {
Map<String, Object> map = new LinkedHashMap<>();
map.put("prop", e.getKey());
map.put("valList", e.getValue());
return map;
})
.collect(Collectors.toList());
System.out.println(hit_val_param);
}
/**
* 初始化数据
*/
public static List<Cpv> init() {
List<Cpv> userList = new ArrayList<Cpv>();
userList.add(new Cpv("颜色","红"));
userList.add(new Cpv("颜色","蓝"));
userList.add(new Cpv("颜色","绿"));
userList.add(new Cpv("功能","防水"));
userList.add(new Cpv("材质","木材"));
return userList;
}
@Data
@AllArgsConstructor
@NoArgsConstructor
public static class Cpv {
String prop;
String val;
}
}
结果:
json
[
{
"prop": "功能",
"valList": ["防水"]
},
{
"prop": "颜色",
"valList": ["红", "蓝", "绿"]
},
{
"prop": "材质",
"valList": ["木材"]
}
]
示例2-转Map
java
import com.alibaba.fastjson.JSON;
import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
/**
* 公众号:干货食堂
*/
public class Test {
public static void main(String[] args) {
//需求,有一个sku集合,需要用java8转成Map<String,Integer> 的形式,
//逻辑是,一个itemId对应多个skuId, Map的key是 itemId,val逻辑是如果itemId下的sku集合中的flag字段有一个为1那么就是1
List<Sku> skuList = init();
// 方案1:使用分组+anyMatch(推荐,可读性好)
Map<String, Integer> resultMap = skuList.stream()
.collect(Collectors.groupingBy(
Sku::getItemId,
Collectors.collectingAndThen(
Collectors.toList(),
list -> list.stream().anyMatch(sku -> sku.getFlag() == 1) ? 1 : 0
)
));
System.out.println(JSON.toJSON(resultMap));
}
/**
* 初始化数据
*/
public static List<Sku> init() {
List<Sku> userList = new ArrayList<Sku>();
userList.add(new Sku("1","100",0));
userList.add(new Sku("1","101",1));
userList.add(new Sku("1","102",0));
userList.add(new Sku("2","103",0));
userList.add(new Sku("2","104",0));
return userList;
}
@Data
@AllArgsConstructor
@NoArgsConstructor
public static class Sku {
String itemId;
String skuId;
Integer flag;
}
}
结果:
json
{
"1": 1,
"2": 0
}
优雅的获取多层对象嵌套值
对于层次很深的对象结构,每次都需要判空处理,很麻烦,我们可以借助java8的Optional来简化操作
java
import com.alibaba.fastjson.JSON;
import lombok.Builder;
import lombok.Data;
import java.util.Optional;
/**
* 公众号:干货食堂
*/
public class Test {
public static void main(String[] args) {
Person person = init();
String name = Optional.ofNullable(person.getProvince())
.map(Province::getCity)
.map(City::getXian)
.orElse(null);
System.out.println(JSON.toJSON(name));
}
/**
* 初始化数据
*/
public static Person init() {
City city = City.builder().xian("公众号:干货食堂").build();
Province province = Province.builder().city(city).build();
return Person.builder().province(province).build();
}
@Data @Builder
public static class Person {
private Province province;
}
@Data @Builder
public static class Province {
private City city;
}
@Data @Builder
public static class City {
private String xian;
}
}