Provide a class for authoring a simple letter.

Question

Provide a class for authoring a simple letter. In the constructor, supply the names of the sender and the recipient: public Letter(String from, String to) Supply a method public void addLine(String line) to add a line of text to the body of the letter. Supply a method public String getText() that returns the entire text of the letter. The text has the form:
Dear recipient name :
blank line
first line of the body
second line of the body
last line of the body
blank line
Sincerely,
blank line
sender name
Also supply a program LetterPrinter that prints this letter:
Dear John:
I am sorry we must part.
I wish you all the best.
Sincerely,
Mary
Construct an object of the Letter class and call addLine twice.
Hints: (1) Use the concat method to form a longer string from two shorter strings. (2) The special string "\n" represents a new line. For example, the statement
body = body.concat("Sincerely,").concat("\n");
adds a line containing the string "Sincerely," to the body.
Complete the following class in your solution:
This class models a simple letter.
public class Letter
Constructs a letter with a given sender and recipient.
@param from the sender
@param to the recipient
public Letter(String from, String to)
Adds a line to the body of this letter.
public void addLine(String line)
Gets the text of this letter.
public String getText()
private String sender;
private String recipient;
private String body;
}

Answer 1

I've answered the question and I've gotten the right answer. However, when I uploaded in our online homewok (which calls WileyPlus, "Launch LabRat") it gives me and error because it's a machine not a human to correct my answer. So, I thought people here could help me to come up with some new codes that would actually work in the "Launch LabRat".
Here's how I answered the problem (it works perfectly in the BlueJ, but "LabRat" fails the solution):
This class models a simple letter.
public class Letter
Constructs a letter with a given sender and recipient.
@param from the sender
@param to the recipient
public Letter(String from, String to)
sender = from;
recipient = to;
text = text.concat("Dear ").concat( recipient).concat(":").concat("\n").concat("\n");
Adds a line to the body of this letter.
public void addLine(String line)
text = text.concat(line).concat("\n");
Gets the text of this letter.
public String getText()
text = text.concat("\n").concat("Sincerely, ").concat("\n").concat("\n").concat(sender);
return text;
private String sender;
private String text = "";
private String recipient;
This is the Tester
public class LetterTester
public static void main(String[] args)
Letter departure = new Letter("Mary", "John");
departure.addLine("I am sorry we must part.");
departure.addLine("I wish you all the best.");
String departureLetter = departure.getText();
System.out.println(departureLetter);
}