How to Extend an ImmutableList with Additional Elements in Java?

How can I extend an ImmutableList with another List in Java?

import com.google.common.collect.ImmutableList;
import java.util.List;

public class Example {
    public static void main(String[] args) {
        ImmutableList<String> immutableList = ImmutableList.of("one", "two", "three");
        List<String> additionalList = List.of("four", "five");

        ImmutableList<String> extendedList = ImmutableList.<String>builder()
            .addAll(immutableList)
            .addAll(additionalList)
            .build();

        System.out.println(extendedList);
    }
}

Extending an ImmutableList with elements from another List in Java is a common requirement when using the Guava library. ImmutableList instances are fixed-size and cannot be changed after creation, but you can create a new ImmutableList that includes elements from an existing one along with additional elements.

ImmutableList<String> extendedList = ImmutableList.<String>builder()
    .addAll(immutableList)
    .addAll(additionalList)
    .build();

Solutions

• Use the ImmutableList.builder() method to create a new ImmutableList.
• Call addAll() for both the existing ImmutableList and the additional List elements.
• Call build() to finalize the new ImmutableList.

Mistake: Not using ImmutableList.builder() to create a new instance.

Solution: Always utilize the builder pattern to add elements to a new ImmutableList.

Mistake: Attempting to modify the original ImmutableList.

Solution: Remember that ImmutableLists are immutable; create a new list instead.