This example shows how to group employees by their departments. The intention of this example, to show the use of the <b>Collectors.groupingBy</b> functionality.

Java Collectors groupby.java

/*
    This example shows how to group employees by their departments.
    The intention of this example, to show the use of the <b>Collectors.groupingBy</b>
    functionality.

 */
import java.util.List;
import java.util.stream.Collectors;

class Dept {
    private int deptId;
    private String deptName;

    public Dept(int deptId, String deptName) {
        this.deptId = deptId;
        this.deptName = deptName;
    }

    public int getDeptId() {
        return deptId;
    }

    public String getDeptName() {
        return deptName;
    }
}

class Emp {
    private Dept dept;
    private String empName;

    public Emp(Dept dept, String empName) {
        this.dept = dept;
        this.empName = empName;
    }

    public Dept getDept() {
        return dept;
    }

    public String getEmpName() {
        return empName;
    }
}

var acc = new Dept(1,"Account");
var digital = new Dept(2,"Digital");
var marketing = new Dept(3,"Marketing");

var employees = List.of(
        new Emp(acc, "Acc-1"),
        new Emp(acc, "Acc-2"),
        new Emp(digital, "Digital-1"),
        new Emp(marketing, "m-1"),
        new Emp(marketing, "m-2"),
        new Emp(marketing, "m-3")
);

// Group employees by department
 employees.stream()
    .collect(Collectors.groupingBy(Emp::getDept))
         .forEach((d,l) -> System.out.println(d.getDeptName()+" => "+
                 l.stream()
                 .map(Emp::getEmpName)
                 .collect(Collectors.joining(", "))
         )
 );