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'}
]
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?
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.
hello sir servlet code
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) -> {})
you may please add return toAttachmentObject before closing the block under map
Thanks mkyong
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());
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;
}
}
very nice blog and simple
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.
Hi Vankatesh,
Please override toString() method in “StaffPublic” class then you will get correct output.
code is perfect, such a nice blog, Thanks MKYong.
good
Hello Sir,
Thanks for sharing such a nice post.
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;
}
yes, it will, this is what is needed
Good article. Thanks
Instead of map(x -> x.getName()) can be used map(Staff::getName).
Yes