How to apply Filtering on groupBy in java streams

Question

How do you group first and then apply filtering using Java streams?

Example: Consider this Employee class: I want to group by Department with a list of an employee having a salary greater than 2000.

public class Employee {     private String department;     private Integer salary;     private String name;      //getter and setter      public Employee(String department, Integer salary, String name) {         this.department = department;         this.salary = salary;         this.name = name;     } }    

This is how I can do this

List<Employee> list   = new ArrayList<>(); list.add(new Employee("A", 5000, "A1")); list.add(new Employee("B", 1000, "B1")); list.add(new Employee("C", 6000, "C1")); list.add(new Employee("C", 7000, "C2"));  Map<String, List<Employee>> collect = list.stream()     .filter(e -> e.getSalary() > 2000)     .collect(Collectors.groupingBy(Employee::getDepartment));   

Output

{A=[Employee [department=A, salary=5000, name=A1]],  C=[Employee [department=C, salary=6000, name=C1], Employee [department=C, salary=7000, name=C2]]} 

As there are no employees in Department B with a salary greater than 2000. So there is no key for Department B: But actually, I want to have that key with empty list –

Expected output

{A=[Employee [department=A, salary=5000, name=A1]],  B=[],  C=[Employee [department=C, salary=6000, name=C1], Employee [department=C, salary=7000, name=C2]]} 

How can we do this?

Answer 1

You can make use of the Collectors.filtering API introduced since Java-9 for this:

Map<String, List<Employee>> output = list.stream()             .collect(Collectors.groupingBy(Employee::getDepartment,                     Collectors.filtering(e -> e.getSalary() > 2000, Collectors.toList()))); 

Important from the API note :

• The filtering() collectors are most useful when used in a multi-level reduction, such as downstream of a groupingBy or partitioningBy.

• A filtering collector differs from a stream's filter() operation.