How to associate Document and JTextPane ?

Question

Hello,
I'm a little lost and confused about JTextPane.
I thought that to associate a document and a JTextPane, the method JTextPane.setDocument(doc) had to be used.
But by looking at the sample : http://bioportal.weizmann.ac.il/course/prog2/tutorial/uiswing/components/example-1dot4/TextSamplerDemo.java, I haven't seen any such instruction. The code is the following :
        JTextPane textPane = new JTextPane();
        StyledDocument doc = textPane.getStyledDocument();
        addStylesToDocument(doc);
        try {
            for (int i=0; i < initString.length; i++) {
                doc.insertString(doc.getLength(), initString,
doc.getStyle(initStyles[i]));
} catch (BadLocationException ble) {
System.err.println("Couldn't insert initial text into text pane.");
I understand that the document "doc" is associated with  "textPane" , but I do not understand, how Java knows that in this JTextPane, the document to be displayed must be  "doc" ?  However it works (miracle ???).   
I'm wondering whether it is  not because the Document Listener retrieves the JTextPane information from the Document, and then triggers the  JtextPane update from the document.  But I'm not sure.
Thanks to gurus to let me understand.   
Gege                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   

Answer 1

Thanks a lot Camickr for your answer. But does this mean that when I type :
StyleDdocument doc = textPane.getStyledDocumen();
I do not really create a document, but I only get the reference of the empty document ? If it is right, then I understand why the following code does not work.
Among my various tries, I sightly modified the TextSamplerDemo sample to add a second JTextPane (that I did not add to the JPanel).
JTextPane textPane = new JTextPane();
JTextPane textPane2 = new JTextPane();
StyledDocument doc = textPane.getStyledDocument();
doc = textPane2.getStyledDocument();
addStylesToDocument(doc);In such a case, textPane becomes empty, but when I add te instruction : textPane.setDocument(doc); it starts working again.
Thanks again for your help, I hope you have understood my English (which is not my natural language).
gege