Learn Java 8 streams with an example

Learn Java 8 streams with an example - map() method

March 18, 2021

The map() function is a method in the Stream class that represents a functional programming concept. In simple words, the map() is utilized to transform one object into another by applying a function.

Example 1: Stream map() function with the operation of converting uppercase to lowercase.

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

/*Stream map() function with operation of converting 
   uppercase to lowercase.
 */
public class DriverClass {
    public static void main(String[] args) {
        List<String> list = Arrays.
            asList("SAW", "NUN", "JOKER", "REVENANT", "CAST AWAY");
        List<String> result = list.stream().
                        map(String::toLowerCase).
                                       collect(Collectors.toList());
        System.out.println(result);
    }
}

Example 2: Java program to convert List of String to List of Integers

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

/*Java program to convert a Stream of Strings 
    to a Stream of Integers.
 */
public class DriverClass {
    public static void main(String[] args) {

        List<String> list = Arrays.
                    asList("1", "3", "6", "22", "12345");
        List<Integer> result = list.stream().
                                  map(Integer::valueOf).
                                   collect(Collectors.toList());
        System.out.println(result);
    }
}

Example 3: Java program to convert List of Employee to List of Integers(Employee Id)

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

/*Java Stream map example 
 */
public class DriverClass {
    public static void main(String[] args) {

        Emp emp1 = new Emp(1, "sibin", 23232, "developer");
        Emp emp2 = new Emp(2, "sabin", 123232, "developer");
        Emp emp3 = new Emp(3, "safin", 443232, "developer");
        List<Emp> list = new ArrayList<>();
        list.add(emp1);
        list.add(emp2);
        list.add(emp3);
        List<Integer> employeeIds = list.stream().
                                     map(e -> e.getId()).
                                      collect(Collectors.toList());
        System.out.println(employeeIds);
    }
}
class Emp {
    private int id;
    private String name;
    private long salary;
    private String designation;

    public Emp() {
    }

    public Emp(int id, String name, 
                  long salary, String designation) {

        this.id = id;
        this.name = name;
        this.salary = salary;
        this.designation = designation;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public long getSalary() {
        return salary;
    }

    public void setSalary(long salary) {
        this.salary = salary;
    }

    public String getDesignation() {
        return designation;
    }

    public void setDesignation(String designation) {
        this.designation = designation;
    }
}