I'm using string builders to append text to my JTextPane, I've set content type as pane.setContentType("text/html"); but the no text actually appears on my JTextPane.
This is an example of my append:
buildSomething.append("<b style=\"color:pink\">"+Birthday+"</span>");
Is there something I'm doing severely wrong? And how do I go about fixit it?
Every time JTextPane.setText(...) is called a new content type is determined. Start the text with "<html>" and you've got HTML.
A new document is created, in your case HTMLDocument.
@mKorbel: the following creates every time HTML for the JTextPane.
    buildSomething.append("<html>");
    buildSomething1.append("<html>");
    for (int i = 0; i < 10; i++) {
        buildSomething.append("<span style=\"color:red\">" + myBirthday + "</span>");
        buildSomething1.append("<b style=\"color:blue\">" + myBirthday + "</b>");
    }
                        @Joop Eggen
1st. loop generate
buildSomething.append("<span style=\"color:pink\">" + myBirthday + "</span>");

2nd.  loop generate same output, I think that doesn't matter if is wrapped inside <html> ..<html> or not because there is pane.setContentType("text/html");
and (not correct code that I posted here <html> ..</html>)
buildSomething1.append("<html><span style=\"color:pink\">" 
    + myBirthday + "</span></html>");

import java.awt.*;
import java.io.IOException;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.swing.*;
import javax.swing.text.BadLocationException;
import javax.swing.text.html.HTMLDocument;
public class MyTextPane implements Runnable {
    private JFrame frm;
    private JScrollPane jsp;
    private JTextPane jta;
    private StringBuilder buildSomething = new StringBuilder();
    private StringBuilder buildSomething1 = new StringBuilder();
    final String myBirthday = "Birthday";
    public MyTextPane() {
        for (int i = 0; i < 10; i++) {
            buildSomething.append("<span style=\"color:red\">" + myBirthday + "</span>");
            buildSomething1.append("<span style=\"color:blue\">" + myBirthday + "</span>");
        }
        jta = new JTextPane();
        jta.setContentType("text/html");
        jta.setCursor(Cursor.getPredefinedCursor(Cursor.WAIT_CURSOR));
        jta.setText(myBirthday);
        jsp = new JScrollPane(jta);
        jsp.setPreferredSize(new Dimension(250, 450));
        frm = new JFrame("awesome");
        frm.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frm.setLayout(new BorderLayout());
        frm.add(jsp, BorderLayout.CENTER);
        frm.setLocation(100, 100);
        frm.pack();
        frm.setVisible(true);
        new Thread(this).start();
    }
    @Override
    public void run() {
        try {
            Thread.sleep(1500);
        } catch (Exception e) {
            e.printStackTrace();
        }
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                jta.setText(null);
                HTMLDocument doc = (HTMLDocument) jta.getStyledDocument();
                try {
                    doc.insertAfterEnd(doc.getCharacterElement(doc.getLength()), buildSomething.toString());
                } catch (BadLocationException ex) {
                    Logger.getLogger(MyTextPane.class.getName()).log(Level.SEVERE, null, ex);
                } catch (IOException ex) {
                    Logger.getLogger(MyTextPane.class.getName()).log(Level.SEVERE, null, ex);
                }
            }
        });
        try {
            Thread.sleep(1500);
        } catch (Exception e) {
            e.printStackTrace();
        }
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                HTMLDocument doc = (HTMLDocument) jta.getStyledDocument();
                try {
                    doc.insertAfterEnd(doc.getCharacterElement(doc.getLength()), buildSomething1.toString());
                } catch (BadLocationException ex) {
                    Logger.getLogger(MyTextPane.class.getName()).log(Level.SEVERE, null, ex);
                } catch (IOException ex) {
                    Logger.getLogger(MyTextPane.class.getName()).log(Level.SEVERE, null, ex);
                }
            }
        });
    }
    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                MyTextPane fs = new MyTextPane();
            }
        });
    }
}
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