1. Introduction

Java 8 introduced the java.util.Optional class to represent a value that may or may not be present to avoid NullPointerException. Converting an Optional to a java.util.ArrayList can be useful in scenarios where we want to handle optional values as lists. For example, finding the sum from a list or transforming the data. In this example, I will demonstrate the Optional ArrayList conversion with the following examples.

• Convert Optional to ArrayList via isPresent.
• Convert Optional to ArrayList via orElse and orElseGet.
• Convert Optional to ArrayList via Stream.
• Convert Optional to ArrayList via Stream and find the sum of the items.
• Convert Optional to ArrayList via Stream and loop the items.

2. Setup

In this step, I will create a maven project with the Junit library.

pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
	<modelVersion>4.0.0</modelVersion>
	<groupId>org.zheng.demo</groupId>
	<artifactId>convert_optional_to_arraylist</artifactId>
	<version>0.0.1-SNAPSHOT</version>
	<build>
		<plugins>
			<plugin>
				<artifactId>maven-compiler-plugin</artifactId>
				<version>3.8.1</version>
				<configuration>
					<release>17</release>
				</configuration>
			</plugin>
		</plugins>
	</build>

	<dependencies>
		<!-- junit 5 -->
		<dependency>
			<groupId>org.junit.jupiter</groupId>
			<artifactId>junit-jupiter-engine</artifactId>
			<version>5.5.2</version>
			<scope>test</scope>
		</dependency>
	</dependencies>
</project>

3. Junit Test Class For Optional ArrayList Conversion

In this step, I will create a TestConvertOptionalToArrayList.java test class to show a few ways to convert an Optional to an ArrayList.

• test_ifPresent – when the Optional has value, then add it to ArrayList with the optional.ifPresent method.
• test_orElse – when the Optional has no value, then add the default value to ArrayList via optional.orElse.
• test_orElseGet – when the Optional has no value, then add the default value to ArrayList via optional.orElseGet.
• test_stream_getSum – convert the Optional to ArrayList via the Stream function and get the sum of the list items
• test_stream_loopItems -convert the Optional to ArrayList via the Stream function and loop the list items.
• test_stream_withData – convert a valued Optional to ArrayList .
• test_stream_withNoData – convert an empty Optional to ArrayList .

TestConvertOptionalToArrayList.java

package org.zheng.demo;

import static org.junit.jupiter.api.Assertions.assertTrue;

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

import org.junit.jupiter.api.Test;

public class TestConvertOptionalToArrayList {

	private static final String A_STRING = "Hello, Mary!";
	private static final String Z_STRING = "Default";
	private Optional<List<Integer>> emptyIntegers = Optional.empty();
	private Optional<String> emptyOptional = Optional.empty();

	private Optional<List<Integer>> optionalIntegers = Optional.of(List.of(30, 60));
	private Optional<String> optionalValue = Optional.of(A_STRING);

	private Integer getSum(final List<Integer> values) {
		return values.stream().mapToInt(Integer::intValue).sum();
	}

	private void loopItems(final List<Integer> values) {
		values.stream().forEach((value) -> System.out.println("Value: " + value));
	}

	@Test
	void test_ifPresent() {
		List<String> arrayList = new ArrayList<>();
		optionalValue.ifPresent(arrayList::add);

		assertTrue(arrayList.contains(A_STRING));
	}

	@Test
	void test_orElse() {
		List<String> arrayList = new ArrayList<>();
		arrayList.add(emptyOptional.orElse(Z_STRING));

		assertTrue(arrayList.contains(Z_STRING));
	}

	@Test
	void test_orElseGet() {
		List<String> arrayList = new ArrayList<>();
		arrayList.add(emptyOptional.orElseGet(() -> Z_STRING));

		assertTrue(arrayList.contains(Z_STRING));
	}

	@Test
	void test_stream_getSum() {
		optionalIntegers.ifPresentOrElse(values -> loopItems(values), () -> System.out.println("No values present"));

		System.out.println("Loop Item via List");
		loopItems(optionalIntegers.stream().flatMap(List::stream).collect(Collectors.toList()));

		System.out.println("Sum Items via List: "
				+ getSum(optionalIntegers.stream().flatMap(List::stream).collect(Collectors.toList())));
	}

	@Test
	void test_stream_loopItems() {
		emptyIntegers.ifPresentOrElse(values -> loopItems(values), () -> System.out.println("No values present"));

		loopItems(emptyIntegers.stream().flatMap(List::stream).collect(Collectors.toList()));
	}

	@Test
	void test_stream_withData() {
		List<String> arrayList = optionalValue.stream().collect(Collectors.toList());

		assertTrue(arrayList.contains(A_STRING));
	}

	@Test
	void test_stream_withNoData() {
		List<String> arrayList = emptyOptional.stream().collect(Collectors.toList());
		
		assertTrue(arrayList.isEmpty());
	}
}

• Line 23, 27: Stream function to get sum and loop items.
• Line 33, 41, 49: Add the Optional value to ArrayList via ifPresent, orElse, orElseGet methods.
• Line 56, 67: The Optional value is List itself, use it as ArrayList .
• Line 59, 69: Convert the ArrayList of Optional to Streamand flat it to ArrayList.
• Line 74, 81: Convert Optional to ArrayList via Stream.

4. Demo Optional ArrayList Conversion

In this step, I will run the Junit tests and capture the output here.

TestConvertOptionalToArrayList Output

Value: 30
Value: 60
Loop Item via List
Value: 30
Value: 60
Sum Items via List: 90
No values present

Also capture the screenshot of the Junit test result.

5. Conclusion

In this example, I demonstrated the Optional ArrayList conversion with the examples

• Convert Optional to ArrayList via isPresent.
• Convert Optional to ArrayList via orElse and orElseGet.
• Convert Optional to ArrayList via Stream.
• Convert Optional to ArrayList via Stream and find the sum of the items.
• Convert Optional to ArrayList via Stream and loop the items.

6. Download

This was an example of a maven project which converting an Optional to an ArrayList.