英文:
Stream api collectors grouping by zero counted fileds
问题
我有一个对象，有一个字段 type1 用于创建第一组，还有两个字段用于创建第二组。但第二组始终会得到这三个值中的一个 (X、Y、Z)。此时我编写了如下的代码：
Map<String, Map<String, Long>> result = objList.stream()
.collect(Collectors.groupingBy(Obj::type1,
Collectors.groupingBy(Obj::useTwoFieldToGetSecondGroup, Collectors.counting())))
这段代码的结果如下：
{
"A":{
"Y":5,
"Z":3
},
"B":{
"Y":2,
"Z":1
},
"C":{
"X":2
},
"D":{
"X":6,
"Z":2
}
}
但我知道第二类型始终有三种类型的值，我希望看到类似以下的结果：
{
"A":{
"X":0,
"Y":5,
"Z":3
},
"B":{
"X":0,
"Y":2,
"Z":1
},
"C":{
"X":2,
"Y":0,
"Z":0
},
"D":{
"X":6,
"Y":0,
"Z":2
}
}
是否有方法可以轻松地获得这样的结果？
public class Obj {
private String type1; // 第一个映射的键
private String field1; // 通过这两个字段计算出的第二个键
private String field2;
public String useTwoFieldToGetSecondGroup(){
// 使用 field1 和 field2 来获取 X、Y 或 Z
}
}
并且 useTwoFieldToGetSecondGroup 方法如下：
```java
private String useTwoFieldToGetSecondGroup() {
if (field1 && field2) {
return "X";
} else if (!field1 && field2) {
return "Y";
} else {
return "Z";
}
}
英文:
I have an object has a filed type1 used to create first group and i have two fields are used to create second group. But second group always get one of theese three values (X,Y,Z). At this point i wrote a code like this :
Map<String, Map<String, Long>> result = objList.stream()
.collect(Collectors.groupingBy(Obj::type1,
Collectors.groupingBy(Obj::useTwoFieldToGetSecondGroup, Collectors.counting())))
As result of this code i get this :
{
"A":{
"Y":5,
"Z":3
},
"B":{
"Y":2,
"Z":1
},
"C":{
"X":2
},
"D":{
"X":6,
"Z":2
}
}
But i know second type has always three types of values and i want to see sth like this :
{
"A":{
"X":0,
"Y":5,
"Z":3
},
"B":{
"X":0,
"Y":2,
"Z":1
},
"C":{
"X":2,
"Y":0,
"Z":0
},
"D":{
"X":6,
"Y":0,
"Z":2
}
}
are there any vay to get this result easily?
public classObj {
private String type1; //key of first map
private String field1; //second key calculated with this two fields
private String field2;
public String useTwoFieldToGetSecondGroup(){
//use field1 and field2 to get X or Y or Z
}
}
and useTwoFieldToGetSecondGroup method like this:
private String useTwoFieldToGetSecondGroup() {
if(field1 && field2){
return "X";
} else if(!field1 && field2){
return "Y";
} else {
return "Z";
}
}
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