In this tutorial, I will show you ways to convert Flux into List/Map example that uses `collectList()`, `collectSortedList()`, `collectMap()`, `collectMultimap()` function…

## Ways to convert Flux into Collection

We will use `Flux` methods such as:

• `collectList()`: accumulate sequence into a `Mono<List>`.
• `collectSortedList()`: accumulate sequence and sort into a `Mono<List>`.
• `collectMap()`: convert sequence into a `Mono<Map>`.
• `collectMultimap()`: convert sequence into a `Mono<Map>` that each Map’s key can be paired with multi-value (in a `Collection`).

Then the `Mono` result above will be converted into a real List/Map using `block()` method.

## Declare & Initialize Flux

There are many ways to initialize a Flux, in this tutorial, we’re gonna use a simple way with `Flux.just()` function.

``````Flux<String> flux = Flux.just(
"Site_0:bezkoder.com",
"Description_0:Java Technology",
"Description_1:Project Reactor");
``````

## Convert Flux into List

### Flux collectList()

`collectList()` will accumulates sequence into a `Mono<List>`, then we use `block()` method to subscribe to the Mono and block indefinitely until a next signal is received.

``````List<String> list1 = flux.collectList().block();
list1.forEach(System.out::println);
``````

Result:

``````Site_0:bezkoder.com
Description_0:Java Technology
Description_1:Project Reactor
``````

### Flux collectSortedList()

`collectSortedList()` accumulates sequence and sort into a `Mono<List>`, then we use `block()` method to subscribe to the `Mono` and block it.

``````List<String> list2 = flux.collectSortedList().block();
list2.forEach(System.out::println);
``````

Result:

``````Description_0:Java Technology
Description_1:Project Reactor
Site_0:bezkoder.com
``````

## Convert Flux into Map

### Flux collectMap()

Function prototype:

``````Mono<Map> collectMap(keyExtractor, valueExtractor)
``````

– First, the function converts sequence into a `Mono<Map>`.
– Finally, the `Mono` becomes real List/Map by `block()` method.

``````Map<String, String> map1 = flux
.collectMap(
item -> item.split(":")[0],
item -> item.split(":")[1])
.block();
map1.forEach((key, value) -> System.out.println(key + " -> " + value));
``````

Result:

``````Site_0 -> bezkoder.com
Description_1 -> Project Reactor
Description_0 -> Java Technology
``````

### Flux collectMultimap()

Function prototype:

``````Mono<Map<Object, Collection>> collectMultimap(keyExtractor, valueExtractor)
``````

`collectMultimap()`: convert sequence into a `Mono<Map>` that each Map’s key can be paired with multi-value (in a `Collection`).

For example, we’re gonna get a Map with `Site` and `Description` as keys:

``````Map<String, Collection<String>> map2 = flux
.collectMultimap(
item -> item.split("_[0-9]+:")[0],
item -> item.split(":")[1])
.block();
map2.forEach((key, value) -> System.out.println(key + " -> " + value));
``````

Check the result:

``````Site -> [bezkoder.com]
Description -> [Java Technology, Project Reactor]
``````

## Implementation

### Technology

– Java 8
– Maven 3.6.1
– Reactor Core 3.4.0 with the 2020.0.1 release train.

### Source Code

``````package com.bezkoder.reactor;

import java.util.Collection;
import java.util.List;
import java.util.Map;

import reactor.core.publisher.Flux;

public class FluxCollection {
public static void main(String[] args) {
Flux<String> flux = Flux.just("Site_0:bezkoder.com", "Description_0:Java Technology",
"Description_1:Project Reactor");

System.out.println("=== flux.collectList() ===");
List<String> list1 = flux.collectList().block();
list1.forEach(System.out::println);

System.out.println("\n=== flux.collectSortedList() ===");
List<String> list2 = flux.collectSortedList().block();
list2.forEach(System.out::println);

System.out.println("\n=== flux.collectMap() ===");
Map<String, String> map1 = flux
.collectMap(
item -> item.split(":")[0],
item -> item.split(":")[1])
.block();
map1.forEach((key, value) -> System.out.println(key + " -> " + value));

System.out.println("\n=== flux.collectMultimap() ===");
Map<String, Collection<String>> map2 = flux
.collectMultimap(
item -> item.split("_[0-9]+:")[0],
item -> item.split(":")[1])
.block();
map2.forEach((key, value) -> System.out.println(key + " -> " + value));
}
}
``````

The Result

``````=== flux.collectList() ===
Site_0:bezkoder.com
Description_0:Java Technology
Description_1:Project Reactor

=== flux.collectSortedList() ===
Description_0:Java Technology
Description_1:Project Reactor
Site_0:bezkoder.com

=== flux.collectMap() ===
Site_0 -> bezkoder.com
Description_1 -> Project Reactor
Description_0 -> Java Technology

=== flux.collectMultimap() ===
Site -> [bezkoder.com]
Description -> [Java Technology, Project Reactor]
``````

## Appendix: Getting Reactor

### Reactor installation in Maven

– First, import the BOM by adding the following to `pom.xml`:

``````<dependencyManagement>
<dependencies>
<dependency>
<groupId>io.projectreactor</groupId>
<artifactId>reactor-bom</artifactId>
<version>2020.0.1</version>
<type>pom</type>
<scope>import</scope>
</dependency>
</dependencies>
</dependencyManagement>
``````

``````<dependencies>
<dependency>
<groupId>io.projectreactor</groupId>
<artifactId>reactor-core</artifactId>
</dependency>
</dependencies>
``````

– First, apply the plugin from the Gradle Plugin Portal:

``````plugins {
id "io.spring.dependency-management" version "1.0.7.RELEASE"
}
``````

– Next use dependency-management to import the BOM:

``````dependencyManagement {
imports {
mavenBom "io.projectreactor:reactor-bom:2020.0.1"
}
}
``````

``````dependencies {