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Converting or transforming a List and Array Objects in Java is a common task when programming. In the tutorial, We show how to do the task with lots of Java examples code by 2 approaches:
- Using Traditional Solution with basic Looping
- Using a powerful API – Java 8 Stream Map
Now let’s do details with examples!
Related posts:
– Java 8
– Java 8 Streams
Java Transform a List with Traditional Solution by Looping Examples
Before Java 8, for mapping a List Objects, we use the basic approach with looping technical solution.
Looping Example – Java Transform an Integer List
– Example: How to double value for each element in a List?
package com.ozenero.stream;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
public class StreamMapExamples {
public static void main(String[] args) {
List intLst = Arrays.asList(1, 2, 3, 4);
// How to double value for each element in a List
// n -> n*2
List newLst = new ArrayList();
for(int i : intLst) {
newLst.add(i*2);
}
System.out.println(newLst);
// [2, 4, 6, 8]
}
} Looping Example – Java Transform a String List
– Example: How to uppercase string value for each element in a String List?
package com.ozenero.stream;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
public class StreamMapExamples {
public static void main(String[] args) {
List strLst = Arrays.asList("one", "two", "three", "four");
// How to upper-case for each element in a List
// one -> ONE
List newLst = new ArrayList();
for(String str : strLst) {
newLst.add(str.toUpperCase());
}
System.out.println(newLst);
// [ONE, TWO, THREE, FOUR]
}
}
Looping Example – Transform a Custom Object List
Create a Customer class:
class Customer{
private String firstname;
private String lastname;
public String getFirstname() {
return firstname;
}
public String getLastname() {
return lastname;
}
Customer(String firstname, String lastname){
this.firstname = firstname;
this.lastname = lastname;
}
}– Example: How to get a string list of fullname of Customer List?
List custList = Arrays.asList(
new Customer("Jack", "Smith"),
new Customer("Joe", "Johnson"),
new Customer("Richard", "Brown"),
new Customer("Thomas", "Wilson")
);
List fullnameList = new ArrayList();
for(Customer c: custList) {
String fullname = c.getFirstname() + " " + c.getLastname();
fullnameList.add(fullname);
}
System.out.println(fullnameList);
// [Jack Smith, Joe Johnson, Richard Brown, Thomas Wilson]
Java 8 Stream Map Examples to Convert or Transform a List
With Java 8, We have a powerful Stream API map() method that help us transform or convert each element of stream with more readable code:
Stream map(Function mapper);
-> It returns a stream consisting of the results of applying the given function to the elements of this stream.
Now we can convert or transform a List with following way:
List -> Stream -> map() -> Stream -> List
Stream Map Example with Integer List
– Example: Transform a Java number list to a double value number list by using stream map.
Integer List -> Stream -> map() -> Stream -> Another Integer List
– Code:
package com.ozenero.stream;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
public class StreamMapExamples {
public static void main(String[] args) {
List intLst = Arrays.asList(1, 2, 3, 4);
// How to double value for each element in a List
// n -> n*2
// we use map function of stream:
// List -> Stream -> map() -> Stream -> List
List newLst = intLst.stream()
.map(n->n*2)
.collect(Collectors.toList());
System.out.println(newLst);
// [2, 4, 6, 8]
}
}
In above code, we use collect() method to transform stream elements into another container such as a List.
Stream Map Example with String List
– Example: Convert a Java string list to an uppercase string list by Stream Map.
String List -> Stream -> map() -> Stream -> Uppercase String List
– Code:
List strLst = Arrays.asList("one", "two", "three", "four");
// How to upper-case for each element in a List
// We use map() method of Stream
// List -> Stream -> map() -> Stream -> List
List newLst = strLst.stream()
.map(str->str.toUpperCase())
.collect(Collectors.toList());
System.out.println(newLst);
// [ONE, TWO, THREE, FOUR]
Stream Map Example with Custom Object List
– Example: Transform an Java object list to another string list.
Customer List -> Stream -> map() -> Stream -> String List
– Code:
List custList = Arrays.asList(
new Customer("Jack", "Smith"),
new Customer("Joe", "Johnson"),
new Customer("Richard", "Brown"),
new Customer("Thomas", "Wilson")
);
List fullnameList = custList.stream()
.map(c -> c.getFirstname() + " " + c.getLastname())
.collect(Collectors.toList());
System.out.println(fullnameList);
// [Jack Smith, Joe Johnson, Richard Brown, Thomas Wilson]
Parallel & Sequential Stream Map Examples
Stream provides a parallel API processing parallel(), so We can combine it with map() method to leverage the multiple cores on your computer for performance processing data.
List custList = Arrays.asList(
new Customer("Jack", "Smith"),
new Customer("Joe", "Johnson"),
new Customer("Richard", "Brown"),
new Customer("Thomas", "Wilson")
);
custList.stream()
.parallel()
.map(c -> c.getFirstname() + " " + c.getLastname()) //concurrently mapping
.forEach(System.out::println); //concurrently System.out:println
/*
Richard Brown
Thomas Wilson
Joe Johnson
Jack Smith
*/
– In above code, we use forEach() method to perform an action System.out::println for each stream element.
To switch from concurrently to sequential processing, We can use the sequential() API of Stream:
List custList = Arrays.asList(
new Customer("Jack", "Smith"),
new Customer("Joe", "Johnson"),
new Customer("Richard", "Brown"),
new Customer("Thomas", "Wilson")
);
custList.stream()
.parallel()
.map(c -> c.getFirstname() + " " + c.getLastname())
.sequential()
.forEach(System.out::println);
/*
Jack Smith
Joe Johnson
Richard Brown
Thomas Wilson
*/
There are a difference order of items in the output between concurrently processing and sequential processing with forEach() method: result of the sequential processing remains the same order of original list while the parallel processing does NOT.
Java 8 Stream Map combines with Filter and Reduce
Java Stream Map with Filter
We can combines Stream Map with Filter mechanics.
Example:
Integer List -> Stream -> Filter odd number -> Map: double value of filtered (odd) number -> Stream -> Integer List
– Code:
List intLst = Arrays.asList(1, 2, 3, 4, 5, 6, 7);
//
// n -> n*2
List newLst = intLst.stream()
.parallel()
.filter(i -> i%2 == 1)
.map(i->{
System.out.println(i);
return i*2;
})
.collect(Collectors.toList());
System.out.println(newLst);
/*
5
3
7
1
[2, 6, 10, 14]
*/
See more about Stream Filter at post: link
Java Stream Map with Reduce
Question: How to sum all number of the final stream after mapping?
We can do the calculation by using reduce() function of Java Stream:
Stream.reduce(Integer identity, BinaryOperator accumulator)
– Code:
List intLst = Arrays.asList(1, 2, 3, 4, 5, 6, 7);
// How to double value for each element in a List
// n -> n*2
Integer total = intLst.stream()
.parallel()
.filter(i -> i%2 == 1)
.map(i->{
System.out.println(i);
return i*2;
})
.reduce(0, (i1, i2) -> i1 + i2);
System.out.println("Total: " + total);
/*
5
1
3
7
Total: 32
*/
32 is a result of reduce funtion by sum all all numbers in final stream after mapping: (5*2 + 1*2 + 3*2 + 7*2).
Java 8 Stream Map Examples with Array
To apply Stream Map in Array with Java 8, We do 2 steps:
- Create Stream from Array Objects.
- Apply Stream Mapping for Array Objects as the same way we had done with above List Objects.
Example Code:
Customer[] customers = new Customer[] {
new Customer("Jack", "Smith"),
new Customer("Joe", "Johnson"),
new Customer("Richard", "Brown"),
new Customer("Thomas", "Wilson")
};
Arrays.stream(customers)
.parallel()
.map(c -> c.getFirstname() + " " + c.getLastname())
.sequential()
.forEach(System.out::println);
/*
Jack Smith
Joe Johnson
Richard Brown
Thomas Wilson
*/
Conclusion
We had done how to use Java 8 Stream Map with Examples:
- How to transform Java Array or List with basic looping
- Apply Java 8 Stream Map to Integer, String, Custom Object List
- Combine
map()method withfilter()andreduce()functions of Java 8 Stream<> - Apply Stream Map to Java Array
- Explore how to use parallel & sequential streams with
map()method
Thanks for reading! See you later!
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