Java: Using Threads For File Searching

I am trying to use threads in order to search an already defined text file. What this program should do is take in command line arguments from the user and depending on what they specify, search a file either using a single thread or multithreads in order to find specified strings.

So an example of a command line argument would be:

java threads m dog cat

What this would do is assign 1 thread to dog and 1 thread to cat and simultaneously search the file for both strings.

However, my issue is that I'm not entirely to familiar with threads. If my user wants to use a single-thread, how do I use that to search a file? What I want to happen is if my user wants to use single-threading, I would want a single thread to be assigned to the word. So this means I'd want to open the file, search, find the Strings, and then close it. How would I go about this? I know I cant use the same thread twice, so I'm a bit confused as how I would accomplish this. The only code I have so far is:

    public static void main(String[] args) {
    String arguments[] = new String[args.length];
    
    for(int i = 0; i < args.length; i++){
        arguments[i] = args[i];
    }
    
    if(args[0].equalsIgnoreCase("m")){
        
    } else if (args[0].equalsIgnoreCase("s")){
        
    }
    
}

Let's start by creating the search function itself. I assume that by "searching a file" you mean finding all lines that contains some word.

I will use Java 8 Nio API, but you are free to use whatever you want to read files

public static void searchFile(File file, String query) throws IOException {
    Files.newBufferedReader(file.toPath())
        .lines().filter(line -> line.contains(query))
        .forEach(line -> System.out.println("Found " + query + ": " + line));
}

Now we have a function that searches a file, let's use it:

static File file = new File("yourfile.txt");

public static void main(String[] args) throws IOException {
    boolean async = args[0].equals("m") // If args[0] is "m", then use multithreading, else use single thread
    // Start iterating from args[1], not args[0]
    for (int i = 1; i < args.length; i++) {
        // If in async mode, start a new thread for each searchFile(...) call
        if (async) new Thread(() -> searchFile(file, args[i])).start();
        // Else just call searchFile(...) from current thread
        else searchFile(file, args[i]);
    }
}

So you have this cool lambda-syntax for creating a thread and executing some code in it, method() becomes new Thread(() -> method()).start() and that's pretty much it for simple multithreading.