I found this on the oracle forums.
public class TextFieldWithPrompt extends JTextField{
@Override
protected void paintComponent(java.awt.Graphics g) {
super.paintComponent(g);
if(getText().isEmpty() && ! (FocusManager.getCurrentKeyboardFocusManager().getFocusOwner() == this)){
Graphics2D g2 = (Graphics2D)g.create();
g2.setBackground(Color.gray);
g2.setFont(getFont().deriveFont(Font.ITALIC));
g2.drawString("zip", 5, 10); //figure out x, y from font's FontMetrics and size of component.
g2.dispose();
}
}
https://forums.oracle.com/forums/thread.jspa?threadID=1349874