Johny Kumar : How Use Flat Map with Stream in Java

Map is Function that takes an argument type T and Map to another R type Object which is stream of T type Object for example if we pass name to map which is String type to then map return stream of String Stream<String> map work with no problem if initial passing object not collection type .

So if we pass collection suppose List to map then map function convert it Stream of List Stream<List<T>> after this map function will not able to map a List to R type output Object

Using flatMap Stream method

You can fix above problem using flatMap .

flatMap Function takes on argument type T which is Stream<List<E>> then map to new created stream of type R which is Stream<E>

flatMap extract List<E> from Stream<List<E>> then prepare new stream for each object type E present in List<E> to type Stream<E>

FlatMap Java Application

FlatMapApplication.java

package in.jk.java8.flatmap;

import java.util.ArrayList;

import java.util.List;

import in.jk.java8.SoftwareProject;

public class FlatMapApplication {

public static void main(String[] args) {

System.out.println("Stream API diffrence beetwen map and flatMap ");

List<Employee> empList = Employee.getEmployeeList();

System.out.println("Find Name with map ");

empList.stream()

.map(employee -> employee.getName())

.forEach(System.out::println);

System.out.println(" \n Problem with map if maping properties retrun collection \n ");

empList.stream()

.map(employee -> employee.getSoftwareProject())

.map(List::stream)

.forEach(System.out::println);

System.out.println(" \n Solution with map if maping properties retrun collection \n ");

empList.stream()

.map(employee -> employee.getSoftwareProject())

.flatMap(List::stream)

.forEach(System.out::println);

}

Employee .java

class Employee {

private int empId;

private String name;

private String company;

private int salary;

private List<SoftwareProject> softwareProject;

@Override

public String toString() {

return "Employee [empId=" + empId + ", name=" + name + ", company=" + company + ", salary=" + salary

+ ", softwareProject=" + softwareProject + "]";

}

public static List<Employee> getEmployeeList() {

SoftwareProject softwareProject = new SoftwareProject();

softwareProject.setProjectId(1);

softwareProject.setProjectName("E-Filing");

softwareProject.setDomain("Jurisdictions and Law");

softwareProject.setWrritenIn("Java");

SoftwareProject project2 = new SoftwareProject();

project2.setProjectId(2);

project2.setProjectName("SLX India");

project2.setDomain("Employeement");

project2.setWrritenIn("Java");

SoftwareProject softwareProject3 = new SoftwareProject();

softwareProject3.setProjectId(3);

softwareProject3.setProjectName("Equaze");

softwareProject3.setDomain("Education and Management");

softwareProject3.setWrritenIn("Java");

SoftwareProject softwareProject4 = new SoftwareProject();

softwareProject4.setProjectId(4);

softwareProject4.setProjectName("Grains Procurement System");

softwareProject4.setDomain("Food ");

softwareProject4.setWrritenIn("Java");

List<SoftwareProject> projectList = new ArrayList<>();

projectList.add(softwareProject);

projectList.add(project2);

projectList.add(softwareProject3);

projectList.add(softwareProject4);

List<Employee> empList = new ArrayList<Employee>();

Employee employee = new Employee();

employee.setEmpId(1);

employee.setName("J K");

employee.setCompany("Google");

employee.setSalary(1000);

employee.setSoftwareProject(projectList);

Employee employee1 = new Employee();

employee1.setEmpId(2);

employee1.setName("J K");

employee1.setCompany("Google");

employee1.setSalary(2000);

employee1.setSoftwareProject(projectList);

Employee employee2 = new Employee();

employee2.setEmpId(3);

employee2.setName("J K");

employee2.setCompany("Google");

employee2.setSalary(3000);

employee2.setSoftwareProject(projectList);

Employee employee6 = new Employee();

employee6.setEmpId(6);

employee6.setName("JK");

employee6.setCompany("APAR");

employee6.setSalary(7000);

employee6.setSoftwareProject(projectList);

Employee employee7 = new Employee();

employee7.setEmpId(7);

employee7.setName("JK");

employee7.setCompany("JET BRAINS");

employee7.setSalary(8000);

employee7.setSoftwareProject(projectList);

empList.add(employee);

empList.add(employee1);

empList.add(employee2);

empList.add(employee6);

empList.add(employee7);

return empList;

}

SoftwareProject .java

package in.jk.java8;

public class SoftwareProject {

private int projectId;

private String ProjectName;

private String wrritenIn;

private String domain;

public int getProjectId() {

return projectId;

}

public void setProjectId(int projectId) {

this.projectId = projectId;

}

public String getProjectName() {

return ProjectName;

}

public void setProjectName(String projectName) {

ProjectName = projectName;

}

public String getWrritenIn() {

return wrritenIn;

}

public void setWrritenIn(String wrritenIn) {

this.wrritenIn = wrritenIn;

}

public String getDomain() {

return domain;

}

public void setDomain(String domain) {

this.domain = domain;

}

@Override

public String toString() {

return "SofwareProject [projectId=" + projectId + ", ProjectName=" + ProjectName + ", wrritenIn=" + wrritenIn

+ ", domain=" + domain + "]";

}

Output in console ...

Stream API diffrence beetwen map and flatMap

Find Name with map

J K

Problem with map if maping properties retrun collection

java.util.stream.ReferencePipeline$Head@776ec8df

java.util.stream.ReferencePipeline$Head@4eec7777

java.util.stream.ReferencePipeline$Head@3b07d329

java.util.stream.ReferencePipeline$Head@41629346

java.util.stream.ReferencePipeline$Head@404b9385

Solution with map if maping properties retrun collection

SofwareProject [projectId=1, ProjectName=E-Filing, wrritenIn=Java, domain=Jurisdictions and Law]

SofwareProject [projectId=2, ProjectName=SLX India, wrritenIn=Java, domain=Employeement]

SofwareProject [projectId=3, ProjectName=Equaze, wrritenIn=Java, domain=Education and Management]