Java SE 8 Lambdas and Streams - Java: How to Program Early Objects - page 755

Unique employee last names:
Blue
Brown
Green
Indigo
Red

Employee names in order by last name then first name:
Jason Blue
Wendy Brown
Ashley Green
James Indigo
Luke Indigo
Matthew Indigo
Jason Red

Fig. 17.13 | Mapping Employee objects to last names and whole names. (Part 2 of 2.)

17.6.5 Grouping Employee s By Department
Figure 17.14 uses Stream method collect (line 95) to group Employees by department.
Method collect 's argument is a Collector that specifies how to summarize the data into
a useful form. In this case, we use the Collector returned by Collectors static method
groupingBy , which receives a Function that classifies the objects in the stream—the values
returned by this function are used as the keys in a Map . The corresponding values, by de-
fault, are List s containing the stream elements in a given category. When method col-
lect is used with this Collector , the result is a Map<String, List<Employee>> in which
each String key is a department and each List<Employee> contains the Employee s in that
department. We assign this Map to variable groupedByDepartment , which is used in lines
96-103 to display the Employees grouped by department. Map method forEach performs
an operation on each of the Map 's key-value pairs. The argument to the method is an ob-
ject that implements functional interface BiConsumer . This interface's accept method has
two parameters. For Map s, the first parameter represents the key and the second represents
the corresponding value.

91 // group Employees by department
92 System.out.printf( "%nEmployees by department:%n" );
93
94
95
96 groupedByDepartment.forEach(
97
98
99
100
101
102
103 );
104

Map<String, List<Employee>> groupedByDepartment =
list.stream()
.collect(Collectors.groupingBy(Employee::getDepartment));

(department, employeesInDepartment) ->
{
System.out.println(department);
employeesInDepartment.forEach(
employee -> System.out.printf( " %s%n" , employee));
}

Fig. 17.14 | Grouping Employee s by department. (Part 1 of 2.)