StringJoiner cannot be resolved to a type

It seems that you are facing an issue with the "StringJoiner" class. This class is available in the `java.util` package and was introduced in Java 8. To resolve this issue, you need to make sure that you have imported the `java.util.StringJoiner` class in your code.

Here's an example of how to import and use the `StringJoiner` class:

import java.util.StringJoiner;

public class Example {
    public static void main(String[] args) {
        StringJoiner joiner = new StringJoiner(", ");
        joiner.add("Hello");
        joiner.add("World");
        String result = joiner.toString();
        System.out.println(result);
    }
}

Make sure to check if you are using a version of Java that supports the `StringJoiner` class and that you have imported it correctly.