Ashu Ashu - 2 months ago 17
Java Question

JAVA-8 streams collect advanced usage

package streams;

import java.util.Arrays;
import java.util.List;

class Student{
String name;
int age;
String type;

public Student(){}

public Student(String name, int age, String type) {
this.name = name;
this.age = age;
this.type = type;
}

public String getName() {
return name;
}

public void setName(String name) {
this.name = name;
}

public int getAge() {
return age;
}

public void setAge(int age) {
this.age = age;
}

public String getType() {
return type;
}

public void setType(String type) {
this.type = type;
}

@Override
public String toString() {
return "Student{" +
"name='" + name + '\'' +
", age=" + age +
", type='" + type + '\'' +
'}';
}

public static List<Student> generateData() {

List<Student> st = Arrays.asList(new Student("Ashish", 27, "College"),
new Student("Aman", 24, "School"),
new Student("Rahul", 18, "School"),
new Student("Ajay", 29, "College"),
new Student("Mathur", 25, "College"),
new Student("Modi", 28, "College"),
new Student("Prem", 15, "School"),
new Student("Akash", 17, "School"));
return st;
}
}





//AdvancedStreamingP2 class uses the Student class

package streams;

import java.util.List;
import java.util.Map;
import java.util.Optional;
import java.util.stream.Collectors;

import static java.util.Comparator.comparing;
import static java.util.stream.Collectors.*;

public class AdvancedStreamingP2 {

public static void main(String[] args) {

List<Student> studentList = Student.generateData();
System.out.println("\n---------- Extracting Student Name with Max Age by Type -----------");
Map<String, Optional<Student>> stuMax = studentList.stream().collect(groupingBy(Student::getType, maxBy(comparing(Student::getAge))));
stuMax.forEach((k, v) -> System.out.println("Key : " + k + ", Value :" + v.get()));
}
}


I want to extract the student name, with the max age, grouping student by Type. Is it possible by using any combination in "collect" itself?

I want the output like :

---------- Extracting Student Name with Max Age by Type -----------

Key : School, Value : Aman
Key : College, Value : Ajay

Answer

Yes, you can use Collectors.collectingAndThen. This collector adapts an existing collector to perform an additional finisher operation. In this case, the finisher operation simply returns the name of the Student.

Map<String, String> stuMax = 
    studentList.stream()
               .collect(groupingBy(
                    Student::getType, 
                    collectingAndThen(maxBy(comparing(Student::getAge)), v -> v.get().getName()) 
               ));

Output:

---------- Extracting Student Name with Max Age by Type -----------
Key : School, Value :Aman
Key : College, Value :Ajay

Side-note: instead of using comparing, you can use comparingInt since Person.getAge() returns an int: this avoids unnecessary boxing.

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