I dont know if anybody has ever seen this before, but i need help on my computer science homework:
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Calendar;
import java.util.Date;
import java.util.List;
import java.util.regex.Pattern;
import static java.util.Objects.isNull;

public class Main {

public static void main(String[] args) {


class MSG{
private String recipient;
private String sender;
private String subject;
private String MSGText;
private Date date;

public MSG(String recipient, String sender, String subject)
{
this.recipient = recipient;
this.sender = sender;
this.subject = subject;
MSGText = "";
this.date = Calendar.getInstance().getTime();
}


public void append(String line)
{

}


public String toString()
{

return "";
}

public String getMSGHeader() {

return "";
}


public void print()
{
System.out.print(this.toString());
}

public void send(BOX sender, BOX recipient, MSG MSG) {

}

}
class BOX{
private String user;
private ArrayList<MSG> MSGs;
private ArrayList<MSG> sentMSGs;

public String getUser() {
return user;
}

public BOX(String user)
{
this.user = user;
MSGs = new ArrayList<MSG>();
sentMSGs = new ArrayList<MSG>();
}

public ArrayList<MSG> getMSGs() {
return MSGs;
}