Flatmap

• is applicable for anything that is a “container of something”

  • Stream and CompletableFuture

• Combination of mapping then flattening

• The idea for map

  • take all elements of a stream or collection and apply a function to

    Stream.of(1, 2, 3, 4)
    .map(x -> x * 2)
    .collect(Collectors.toList())
    // -> [2, 4, 6, 8]

• Applying map to create a list of lists

  Stream.of(1, 2, 3, 4)
    .map(x -> Stream.of(-x, x, x + 1))
    .collect(Collectors.toList())
  /*
  [java.util.stream.ReferencePipeline$Head@3532ec19,
  java.util.stream.ReferencePipeline$Head@68c4039c,
  java.util.stream.ReferencePipeline$Head@ae45eb6,
  java.util.stream.ReferencePipeline$Head@59f99ea]
  */

  • For every singular element, we’re creating a plural. If you perform a map with a plural

  • Instead use flatmap

    Stream.of(1, 2, 3, 4)
     .flatMap(x -> Stream.of(-x, x, x+1))
     .collect(Collectors.toList())
    // [-1, 1, 2, -2, 2, 3, -3, 3, 4, -4, 4, 5]

• Example

  • We are given only a Stream and our goal is to determine all the salaries of all employees, including Managers and their Employees.

    class Employee {
    private String firstName, lastName;
    private Integer yearlySalary;
    // getters, setters, toString
    }
    class Manager extends Employee {
    private List<Employee> employeeList;
    // getters, setters, toString
    }

  • Instead of using forEach and start digging through those salaries.

    • This, unfortunately, would model our code to the structure of the data and would cause needless complexity

  • Use flatmap

    List.of(manager1, manager2).stream()
    .flatMap(m ->
      Stream.concat(m.getEmployeeList().stream(), Stream.of(m)))
    .distinct()
    .mapToInt(Employee::getYearlySalary)