Java 8 Stream map() examples - Stream Conversions

1. Overview

In this article, we'll be learning about a new map() function in Java 8 that is introduced in Stream API. map() method converts a Stream from one form to another Stream. This takes input X type Stream and converts into Y type output Stream.

This is widely used as part of new JDK 8 api.

2. Java 8 Stream map() Syntax

Below is the standard syntax from java api.

<R> Stream<R> map(Function<? super T,? extends R> mapper)

map() returns a stream consisting of the results of applying the given function to the elements of this stream.

This method takes the Function function interface as an argument and returns a new Stream. This is also an intermediate operation. map() operation is a lazy one and will be executed after invoking the terminal operation such as forEach(), min(), max() and collect() methods.

3. Java Stream map() example to find the multiplication of 5

map() method logic is map(value -> value * 5); here. This will be executed after invocation of forEach() method in the next line.

package com.java.w3schools.blog.java.program.to.java8.map;

import java.util.stream.Stream;

/**
 * Java program to find the multiplications of 5 using stream map() function.
 * 
 * @author JavaProgramTo.com
 *
 */
public class StreamMapExample1 {

 public static void main(String[] args) {

  // Creating int stream using Stream.of() method.
  Stream<Integer> intValues = Stream.of(1, 2, 3, 4, 5, 6, 7);

  // multiplication of 5
  Stream fifthTableStream = intValues.map(value -> value * 5);

  // iterating final stream
  fifthTableStream.forEach(output -> System.out.println(output));

 }

}

Output:

5
10
15
20
25
30
35

4. Java Stream map() example to find the length of each string

package com.java.w3schools.blog.java.program.to.java8.map;

import java.util.stream.Stream;

/**
 * Java program to find each string length using stream map() function.
 * 
 * @author JavaProgramTo.com
 *
 */
public class StreamMapExample2 {

 public static void main(String[] args) {

  // Creating string stream using Stream.of() method.
  Stream<String> intValues = Stream.of("java", "program", "to", "com", "java-w3schools", "blog", "");

  // string length stream
  Stream<Integer> lengthStream = intValues.map(string -> string.length());

  // iterating final stream
  lengthStream.forEach(output -> System.out.println(output));

 }

}

Output:

4
7
2
3
14
4
0

5. Java Stream map() example to convert to UpperCase

package com.java.w3schools.blog.java.program.to.java8.map;

import java.util.stream.Stream;

/**
 * Java program to convert into uppercase using stream map() function.
 * 
 * @author JavaProgramTo.com
 *
 */
public class StreamMapExample3 {

 public static void main(String[] args) {

  // Creating string stream using Stream.of() method.
  Stream<String> intValues = Stream.of("java", "program", "to", "com", "java-w3schools", "blog", "");

  // upper case stream
  Stream<String> uppercaseStream = intValues.map(string -> string.toUpperCase());

  // iterating final stream
  uppercaseStream.forEach(output -> System.out.println(output));

 }

}

Output:

JAVA
PROGRAM
TO
COM
JAVA-W3SCHOOLS
BLOG

6. Java Stream map() example to collect the names into List of String's

User.java

package com.java.w3schools.blog.java.program.to.java8.map;

public class User {

 private int userId;
 private String name;
 private String email;

 public User(int userId, String name, String email) {
  super();
  this.userId = userId;
  this.name = name;
  this.email = email;
 }

 public int getUserId() {
  return userId;
 }

 public void setUserId(int userId) {
  this.userId = userId;
 }

 public String getName() {
  return name;
 }

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

 public String getEmail() {
  return email;
 }

 public void setEmail(String email) {
  this.email = email;
 }
 
 @Override
  public String toString() {
   return "User [userId=" + userId + ", name=" + name + ", email=" + email + "]";
  }
 

}

Main program:

package com.java.w3schools.blog.java.program.to.java8.map;

import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;

/**
 * Java program to collect all user email addresses into List using stream map()
 * function.
 * 
 * @author JavaProgramTo.com
 *
 */
public class StreamMapExample4 {

 public static void main(String[] args) {

  // Creating string stream using Stream.of() method.
  Stream<User> intValues = Stream.of(new User(100, "jhon", "jhon@gmail.com"),
    new User(200, "cena", "cena@gmail.com"), new User(300, "burg", "burg@gmail.com"));

  // list of email addresses
  List<String> emailList = intValues.map(user -> user.getEmail()).collect(Collectors.toList());

  // iterating final stream
  emailList.forEach(output -> System.out.println(output));

 }

}

Output:

jhon@gmail.com
cena@gmail.com
burg@gmail.com

7. Java Stream map() example to collect User objects into List

package com.java.w3schools.blog.java.program.to.java8.map;

import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;

/**
 * Java program to collect User objects into List using stream map() function.
 * 
 * @author JavaProgramTo.com
 *
 */
public class StreamMapExample6 {

 public static void main(String[] args) {

  // Creating string stream using Stream.of() method.
  Stream<User> intValues = Stream.of(new User(500, "nag", "nag@gmail.com"),
    new User(700, "remo", "remo@gmail.com"), new User(900, "beri", "beri@gmail.com"));

  // list of user objects
  List<User> emailList = intValues.map(user -> {

   if (user.getEmail().contains("gmail.com")) {
    return user;
   } else
    return user;

  }).collect(Collectors.toList());

  // iterating final User list
  emailList.forEach(output -> System.out.println(output));

 }

}

Output:

User [userId=500, name=nag, email=nag@gmail.com]
User [userId=700, name=remo, email=remo@gmail.com]
User [userId=900, name=beri, email=beri@gmail.com]

8. Conclusion

In this article, We have seen how to use map() method and it's example programs.

Java 8 flatMap() examples

javJ 8 map() vs flatMap()