I found the code for grouping the objects by some field name from POJO. Below is the code for that:
public class Temp {
static class Person {
private String name;
private int age;
private long salary;
Person(String name, int age, long salary) {
this.name = name;
this.age = age;
this.salary = salary;
}
@Override
public String toString() {
return String.format("Person{name='%s', age=%d, salary=%d}", name, age, salary);
}
}
public static void main(String[] args) {
Stream<Person> people = Stream.of(new Person("Paul", 24, 20000),
new Person("Mark", 30, 30000),
new Person("Will", 28, 28000),
new Person("William", 28, 28000));
Map<Integer, List<Person>> peopleByAge;
peopleByAge = people
.collect(Collectors.groupingBy(p -> p.age, Collectors.mapping((Person p) -> p, toList())));
System.out.println(peopleByAge);
}
}
And the output is (which is correct):
{24=[Person{name='Paul', age=24, salary=20000}], 28=[Person{name='Will', age=28, salary=28000}, Person{name='William', age=28, salary=28000}], 30=[Person{name='Mark', age=30, salary=30000}]}
But what if I want to group by multiple fields? I can obviously pass some POJO in groupingBy()
method after implementing equals()
method in that POJO but is there any other option like I can group by more than one fields from the given POJO?
E.g. here in my case, I want to group by name and age.
ベストアンサー1
You have a few options here. The simplest is to chain your collectors:
Map<String, Map<Integer, List<Person>>> map = people
.collect(Collectors.groupingBy(Person::getName,
Collectors.groupingBy(Person::getAge));
Then to get a list of 18 year old people called Fred you would use:
map.get("Fred").get(18);
A second option is to define a class that represents the grouping. This can be inside Person. This code uses a record
but it could just as easily be a class (with equals
and hashCode
defined) in versions of Java before JEP 359 was added:
class Person {
record NameAge(String name, int age) { }
public NameAge getNameAge() {
return new NameAge(name, age);
}
}
Then you can use:
Map<NameAge, List<Person>> map = people.collect(Collectors.groupingBy(Person::getNameAge));
and search with
map.get(new NameAge("Fred", 18));
Finally if you don't want to implement your own group record then many of the Java frameworks around have a pair
class designed for this type of thing. For example: apache commons pair If you use one of these libraries then you can make the key to the map a pair of the name and age:
Map<Pair<String, Integer>, List<Person>> map =
people.collect(Collectors.groupingBy(p -> Pair.of(p.getName(), p.getAge())));
and retrieve with:
map.get(Pair.of("Fred", 18));
Personally I don't really see much value in generic tuples now that records are available in the language as records display intent better and require very little code.