Don't. Keep them separate, one to present the output, and one to process the data.
There are several routes you can go with though. You can take a Document type, or a StringBuilder and append to that for example, or you can simply return a string. I'd do neither and opt for using single property accessor's instead, but if I were to group them I'd return a string:
public String display()
StringBuilder sb = new StringBuilder();
sb.append(") has $");
Unfortunately I don't believe there is a really generic interface to work with when it comes to the swing JTextArea. Document I think is the lowest you can really go, and its still a javax.swing.text type. There's some patterned approaches you can follow as well, but those are more advanced and require a good understanding of OOP to implement.