How can I determine how many lines are in a TextArea or JTextArea?

If you have a JTextArea with a PlainDocument (every child element represents a single ine) then you could do this:

Document doc = mytextarea.getDocument();
Element root = doc.getDefaultRootElement();
int lines = root.getElementCount();

Sandip Chitale adds

In both cases you could create a java.io.StringReader out of the getText() returned text. Wrap it in a java.io.BufferedReader and then read lines using the readLine() API.

You could walk through the String object returned by getText() and look for ' ' char to count the lines.

For JTextArea there is a direct API -

public int getLineCount();

String text = area.getText();
StringReader reader = new StringReader(text);
LineNumberReader lnr = new LineNumberReader(reader);
while (lnr.readLine() != null);
System.out.println(reader.getLineNumber());