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Java 8 Collectors: groupingBy Example

By Arvind Rai, November 29, 2014
groupingBy is a static method of java.util.stream.Collectors in java 8. groupingBy does the grouping of elements on the basis of any given key and returns a Collector. Find the method syntax.
<T,K> Collector<T,?,Map<K,List<T>>> groupingBy(Function<? super T,? extends K> classifier) 
For the example, we will create a Student class.
Student.java
package com.concretepage.util.stream;
public class Student {
    private String name;
    private int age;
    private String className;
    public Student(String name,String className,int age){
        this.name=name;
        this.age=age;
        this.className = className;
    }
    public String getName() {
        return name;
    }
    public int getAge() {
        return age;
    }
    public String getClassName() {
        return className;
    }
} 
Now if we want to group students on the basis of className, we will do as below.
Map<String, List<Student>> stdByClass = list.stream()
                                    .collect(Collectors.groupingBy(Student::getClassName));
We have a list of Student class. Grouping is done on the basis of student class name. List is converted into stream of student object. Then call collect method of stream. groupingBy of Collectors class checks each element of stream and gets class name and then group it as list. Finally we get a map where key is the one by which grouping is done. Find the complete example.
CollectorsGroupingBy.java
package com.concretepage.util.stream;
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
public class CollectorsGroupingBy {
    public static void main(String[] args) {
        Student s1 = new Student("Ram", "A", 20);
        Student s2 = new Student("Shyam", "B", 22);
        Student s3 = new Student("Mohan", "A", 22);
        Student s4 = new Student("Mahesh", "C", 20);
        Student s5 = new Student("Krishna", "B", 21);
        List<Student> list = Arrays.asList(s1,s2,s3,s4,s5);
        //Group Student on the basis of ClassName
        System.out.println("----Group Student on the basis of ClassName----");
        Map<String, List<Student>> stdByClass = list.stream()
                    .collect(Collectors.groupingBy(Student::getClassName));
        
        stdByClass.forEach((k,v)->System.out.println("Key:"+k+"  "+ 
                ((List<Student>)v).stream().map(m->m.getName()).collect(Collectors.joining(","))));
        
        //Group Student on the basis of age
        System.out.println("----Group Student on the basis of age----");
        Map<Integer, List<Student>> stdByAge = list.stream()
                    .collect(Collectors.groupingBy(Student::getAge));
        
        stdByAge.forEach((k,v)->System.out.println("Key:"+k+"  "+ 
                ((List<Student>)v).stream().map(m->m.getName()).collect(Collectors.joining(","))));
    }
} 
Find the output.
----Group Student on the basis of ClassName----
Key:A  Ram,Mohan
Key:B  Shyam,Krishna
Key:C  Mahesh
----Group Student on the basis of age----
Key:20  Ram,Mahesh
Key:21  Krishna
Key:22  Shyam,Mohan 
POSTED BY
ARVIND RAI
ARVIND RAI
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