There are many ways to do this, here's one:

/**
 * Find first occurence of file of certain type within specified folder
 * @return first file found or throws NoSuchElementException
 */
File findFirstOfType(File folder, String ext) throws NoSuchElementException {
return Arrays.asList(rootOutputDirectory.listFiles())
	.stream()
	.filter(f -> f.getName().endsWith("." + ext))
	.findFirst().get();
}
 
// Usage example
findFirstOfType(rootOutputDirectory, "zip");

I did ask chatbot https://chat.openai.com/ whether this was correct JAVA code.
This was the answer:

Yes, the code looks correct. It takes in a File object representing a folder and a string representing a file extension, and returns the first file in the folder that has the specified extension. If no such file is found, it will throw a NoSuchElementException.

Here's a breakdown of what the code does:

It converts the list of files in the folder to a stream using Arrays.asList(folder.listFiles()).stream().
It filters the stream to only include files that have the specified extension using .filter(f -> f.getName().endsWith("." + ext)).
It returns the first element in the stream using findFirst().get().
If the stream is empty (i.e., there are no files in the folder with the specified extension), findFirst().get() will throw a NoSuchElementException.