英文:
Stream collect groupingBy output customization
问题
我有一个JAVA类，如下所示：
public class Hello {
public String field1;
public String field2;
public String field3;
}
现在我有一个Hello对象的列表，我想按field1对列表元素进行分组。我的问题是，我只想得到一组field3作为分组元素，而不是所有Hello对象的字段。
例如，作为输出，我想要得到一个映射：
field1Value1 -> [field3Value1, field3Value2, field3Value3]
field1Value2 -> [field3Value4, field3Value5, field3Value6]
我尝试使用流来实现它，当然：
HelloList.stream.collect(Collectors.groupingBy(Hello::field1, Collectors.toSet()));
就像上面这样，我会得到一个由field1映射的Hello对象的集合，不幸的是，这不是我想要的。
英文:
I have a JAVA class described as bellow:
public class Hello {
public String field1;
public String field2;
public String field3;
}
Now I have a list of Hello objects, I want to group the list element by field1. My issue, I want to have only a set of field3 as grouped elements, not all the Hello object fields.
For example, as output, I want to get a map:
field1Value1 -> [field3Value1, field3Value2, field3Value3]
field1Value2 -> [field3Value4, field3Value5, field3Value6]
I have tried to do it using steams of course:
HelloList.stream.collect(Collectors.groupingBy(Hello::field1, Collectors.toSet()));
Like above I will get a set of Hello objects mapped by field1, unfortunately that's not what I want.
答案1
得分: 3
我认为你正在寻找 Collectors.mapping：
helloList.stream()
.collect(groupingBy(Hello::field1, mapping(Hello::field3, toSet())));
总的来说，最好随时准备好Collectors的Javadoc，因为有许多有用的组合操作（比如这个mapping和它的逆操作collectingAndThen），当你有类似的问题时，浏览列表以找到合适的工具会非常有帮助。
英文:
I think you're looking for Collectors.mapping:
helloList.stream()
.collect(groupingBy(Hello::field1, mapping(Hello::field3, toSet())));
In general, it's a good idea to have the Javadoc for Collectors handy, because there are a number of useful composition operations (such as this mapping and its inverse collectingAndThen), and there are enough that when you have a question like this it's useful to look over the list to find an appropriate tool.
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