Convert Optional to ArrayList

Mary ZhengJuly 15th, 2024Last Updated: July 15th, 2024

0 418 3 minutes read

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.

Download
You can download the full source code of this example here: Convert Optional to ArrayList

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Convert Optional to ArrayList

1. Introduction

2. Setup

3. Junit Test Class For Optional ArrayList Conversion

4. Demo Optional ArrayList Conversion

5. Conclusion

6. Download

Thank you!

Mary Zheng

Thank you!

1. Introduction

2. Setup

3. Junit Test Class For Optional ArrayList Conversion

4. Demo Optional ArrayList Conversion

5. Conclusion

6. Download

Thank you!

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