Main Tutorials

Java 8 Streams map() examples

In Java 8, stream().map() lets you convert an object to something else. Review the following examples :

1. A List of Strings to Uppercase

1.1 Simple Java example to convert a list of Strings to upper case.

TestJava8.java

package com.mkyong.java8;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class TestJava8 {

    public static void main(String[] args) {

        List<String> alpha = Arrays.asList("a", "b", "c", "d");

        //Before Java8
        List<String> alphaUpper = new ArrayList<>();
        for (String s : alpha) {
            alphaUpper.add(s.toUpperCase());
        }

        System.out.println(alpha); //[a, b, c, d]
        System.out.println(alphaUpper); //[A, B, C, D]

        // Java 8
        List<String> collect = alpha.stream().map(String::toUpperCase).collect(Collectors.toList());
        System.out.println(collect); //[A, B, C, D]

        // Extra, streams apply to any data type.
        List<Integer> num = Arrays.asList(1,2,3,4,5);
        List<Integer> collect1 = num.stream().map(n -> n * 2).collect(Collectors.toList());
        System.out.println(collect1); //[2, 4, 6, 8, 10]

    }

}

2. List of objects -> List of String

2.1 Get all the name values from a list of the staff objects.

Staff.java

package com.mkyong.java8;

import java.math.BigDecimal;

public class Staff {

    private String name;
    private int age;
    private BigDecimal salary;
	//...
}
TestJava8.java

package com.mkyong.java8;

import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class TestJava8 {

    public static void main(String[] args) {

        List<Staff> staff = Arrays.asList(
                new Staff("mkyong", 30, new BigDecimal(10000)),
                new Staff("jack", 27, new BigDecimal(20000)),
                new Staff("lawrence", 33, new BigDecimal(30000))
        );

        //Before Java 8
        List<String> result = new ArrayList<>();
        for (Staff x : staff) {
            result.add(x.getName());
        }
        System.out.println(result); //[mkyong, jack, lawrence]

        //Java 8
        List<String> collect = staff.stream().map(x -> x.getName()).collect(Collectors.toList());
        System.out.println(collect); //[mkyong, jack, lawrence]

    }

}

3. List of objects -> List of other objects

3.1 This example shows you how to convert a list of staff objects into a list of StaffPublic objects.

Staff.java

package com.mkyong.java8;

import java.math.BigDecimal;

public class Staff {

    private String name;
    private int age;
    private BigDecimal salary;
	//...
}

StaffPublic.java

package com.mkyong.java8;

public class StaffPublic {

    private String name;
    private int age;
    private String extra;
    //...
}


3.2 Before Java 8.

BeforeJava8.java

package com.mkyong.java8;

import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class BeforeJava8 {

    public static void main(String[] args) {

        List<Staff> staff = Arrays.asList(
                new Staff("mkyong", 30, new BigDecimal(10000)),
                new Staff("jack", 27, new BigDecimal(20000)),
                new Staff("lawrence", 33, new BigDecimal(30000))
        );

        List<StaffPublic> result = convertToStaffPublic(staff);
        System.out.println(result);

    }

    private static List<StaffPublic> convertToStaffPublic(List<Staff> staff) {

        List<StaffPublic> result = new ArrayList<>();

        for (Staff temp : staff) {

            StaffPublic obj = new StaffPublic();
            obj.setName(temp.getName());
            obj.setAge(temp.getAge());
            if ("mkyong".equals(temp.getName())) {
                obj.setExtra("this field is for mkyong only!");
            }

            result.add(obj);
        }

        return result;

    }

}

Output


[
	StaffPublic{name='mkyong', age=30, extra='this field is for mkyong only!'}, 
	StaffPublic{name='jack', age=27, extra='null'}, 
	StaffPublic{name='lawrence', age=33, extra='null'}
]

3.3 Java 8 example.

NowJava8.java

package com.mkyong.java8;

package com.hostingcompass.web.java8;

import java.math.BigDecimal;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class NowJava8 {

    public static void main(String[] args) {

        List<Staff> staff = Arrays.asList(
                new Staff("mkyong", 30, new BigDecimal(10000)),
                new Staff("jack", 27, new BigDecimal(20000)),
                new Staff("lawrence", 33, new BigDecimal(30000))
        );

		// convert inside the map() method directly.
        List<StaffPublic> result = staff.stream().map(temp -> {
            StaffPublic obj = new StaffPublic();
            obj.setName(temp.getName());
            obj.setAge(temp.getAge());
            if ("mkyong".equals(temp.getName())) {
                obj.setExtra("this field is for mkyong only!");
            }
            return obj;
        }).collect(Collectors.toList());

        System.out.println(result);

    }
	
}

Output


[
	StaffPublic{name='mkyong', age=30, extra='this field is for mkyong only!'}, 
	StaffPublic{name='jack', age=27, extra='null'}, 
	StaffPublic{name='lawrence', age=33, extra='null'}
]

References

  1. Processing Data with Java SE 8 Streams, Part 1
  2. Java 8 – Filter a Map examples
  3. Java 8 flatMap example
  4. Collectors JavaDoc

About Author

author image
Founder of Mkyong.com, love Java and open source stuff. Follow him on Twitter. If you like my tutorials, consider make a donation to these charities.

Comments

Subscribe
Notify of
18 Comments
Most Voted
Newest Oldest
Inline Feedbacks
View all comments
Max Lambrecht
6 years ago

Instead of map(x -> x.getName()) can be used map(Staff::getName).

Ashish Agrawal
4 years ago
Reply to  Max Lambrecht

Yes

Vinayak A
6 years ago

Hello Sir,

Thanks for sharing such a nice post.

Wayn
6 years ago

Wouldn’t this bit create a new instance of StaffPublic for each element in the List?:

.map(temp -> {
StaffPublic obj = new StaffPublic();
obj.setName(temp.getName());
obj.setAge(temp.getAge());
if (“mkyong”.equals(temp.getName())) {
obj.setExtra(“this field is for mkyong only!”);
}
return obj;
}

havryliuk
5 years ago
Reply to  Wayn

yes, it will, this is what is needed

M Chandry
2 years ago

maybe its just me,

But I cant find on entire internet an easy way to implement a search through a Map, to find one ‘key'(String for example) and then print it out with streams/Lambda….

wondering if you can help?

proudlyjewish
3 years ago

Hi MKyong,

maybe you could help me.
I have to convert an Entity into a DTO.

In my Reader module, I have a method that fetches data from a DB and inserts it into our local DB.

I have to get this referente (of the inserted data) in the ‘processor’ module.
Actually I couldnt do it so maybe I could use map to convert it and get this updated table records in order to publish them in a rabbit queue, but I don´t know how to do it.

Novi
3 years ago

hello sir servlet code

TARIQ
3 years ago

hi , i am trying copy list elements from list<A> to LIst<B> and facing some type error :

my code is :

   List<AttachmentTypeDto> attachmentTypeList=clientDetails.getAttachmentTypeDto();
         List<ClientAttachmentTypeEntity> mappedAttachmentList=attachmentTypeList.stream().map(temp ->{
            ClientAttachmentTypeEntity toAttachmentObject=new ClientAttachmentTypeEntity();
            toAttachmentObject.setAttachmentTypeId(temp.getAttachmentTypeId());
            toAttachmentObject.setClientId(temp.getClientId());

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

it gives me error like this :

The method map(Function<? super AttachmentTypeDto,? extends R>) in the type Stream<AttachmentTypeDto> is not applicable for the arguments ((<no type> temp) -> {})

Akhil
3 years ago
Reply to  TARIQ

you may please add return toAttachmentObject before closing the block under map

AniWare
3 years ago

Thanks mkyong

Himanshu Dave
4 years ago

Does new object Creation affects performance???
// convert inside the map() method directly.
List result = staff.stream().map(temp -> {
StaffPublic obj = new StaffPublic();
obj.setName(temp.getName());
obj.setAge(temp.getAge());
if (“mkyong”.equals(temp.getName())) {
obj.setExtra(“this field is for mkyong only!”);
}
return obj;

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

Laurence O'Toole
4 years ago

public class ExerciseMapExample2 {

public static void main(String ...args) {

List<Staff> staff = Arrays.asList(
new Staff("mkyong", 30, new BigDecimal(10000)),
new Staff("jack", 27, new BigDecimal(20000)),
new Staff("lawrence", 33, new BigDecimal(30000))
);

// convert inside the map() method directly.
List<StaffPublic> result = staff.stream().map(ExerciseMapExample2::process).collect(Collectors.toList());

System.out.println(result);

}

private static StaffPublic process(Staff temp) {
StaffPublic obj = new StaffPublic();
obj.setName(temp.getName());
obj.setAge(temp.getAge());
if ("mkyong".equals(temp.getName())) {
obj.setExtra("this field is for mkyong only!");
}
return obj;
}
}

Jithesh Thomas
4 years ago

very nice blog and simple

Venkatesh
5 years ago

3.3 Java 8 example. ,
NowJava8 Class.

// convert inside the map() method directly.
List result = staff.stream().map(temp -> {
StaffPublic obj = new StaffPublic();
obj.setName(temp.getName());
obj.setAge(temp.getAge());
if (“mkyong”.equals(temp.getName())) {
obj.setExtra(“this field is for mkyong only!”);
}
return obj;
}).collect(Collectors.toList());

System.out.println(result);
the above function will nowhere return the output you have specified. It will only something like the below.
com.sample.process.StaffPublic@3b07d329, com.sample.process.StaffPublic@41629346, com.sample.process.StaffPublic@404b9385

Please putt correct code and respective output for the same.

Ashish Agrawal
4 years ago
Reply to  Venkatesh

Hi Vankatesh,

Please override toString() method in “StaffPublic” class then you will get correct output.

code is perfect, such a nice blog, Thanks MKYong.

saaa
5 years ago

good

Carol
6 years ago

Good article. Thanks