How can I set a customized currency symbol to be applied to a number?

The biggest misconception here is that DecimalFormat will automatically format to currency values. This is not the case; the basic DecimalFormat will assume that it is strictly formating just a number. You need to use NumberFormat.getCurrencyInstance(), which returns a NumberFormat. In general, the NumberFormat will also be a DecimalFormat, but it knows to format a value as currency. The other minor error is that setMonetaryDecimalSeparator( char sep ) needs to be used instead of setDecimalSeparator(char decimalSeparator).

Of course, a large amount of work has been done with these classes, including locales, to avoid customization, but the following code shows how I would do it when necessary:

//  JCusCF.java - Custom CurrencyFormat

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.text.*;
import java.util.*;

public class JCusCF extends    JFrame
                    implements ActionListener,
                               WindowListener
{
  boolean    bPre = true;
  int        ndx = 0;

  JButton    jb = new JButton( "Go" );
  JLabel     jlI = new JLabel("Input Currency Value:"),
             jlD = new JLabel("Display:"),
             jlP = new JLabel("Parsed:");

  JPanel     jp = new JPanel();

  JTextField jtI = new JTextField( 10 ),
             jtD = new JTextField( 10 ),
             jtP = new JTextField( 10 );

  NumberFormat cfLocal = NumberFormat.
                          getCurrencyInstance();
  NumberFormat cf2 = NumberFormat.
                       getCurrencyInstance();

  String sCurSymbol = "";


  public JCusCF()
  {
    super( "JCusCF" );

    DecimalFormatSymbols dfs = null;

    addWindowListener( this );
    jb.addActionListener( this );

    jp.add(jlI);
    jp.add(jtI);
    jp.add(jb);
    jp.add(jlD);
    jp.add(jtD);
    jp.add(jlP);
    jp.add(jtP);

    getContentPane().add( jp, BorderLayout.CENTER );
    pack();
    show();

    if( cfLocal instanceof DecimalFormat )
    { // determine if symbol is prefix or suffix
      dfs =
          ((DecimalFormat)cfLocal).
             getDecimalFormatSymbols();
      sCurSymbol = dfs.getCurrencySymbol();
      String sLP = ((DecimalFormat)cfLocal).
                      toLocalizedPattern();
      ndx = sLP.indexOf( 'u00A4' );  // currency sign
      if( ndx > 0 ) { bPre = false; }
      else { bPre = true; }
    }

// code to modify getDecimalFormatSymbols
    if( cf2 instanceof DecimalFormat )
    { // determine if symbol is prefix or suffix
      dfs =
       ((DecimalFormat)cf2).getDecimalFormatSymbols();
      dfs.setCurrencySymbol("Rs");
      dfs.setGroupingSeparator('&');
      dfs.setMonetaryDecimalSeparator('$');
    ((DecimalFormat)cf2).setDecimalFormatSymbols(dfs);
      System.out.println(cf2.format(3333.454)); 
    }

  }  // end constructor


  // ActionListener Implementation
  public void actionPerformed(ActionEvent e)
  {
    Number n = null;
    String sText = jtI.getText();

    jtD.setText( "" );
    jtP.setText( "" );

    ndx = sText.indexOf( sCurSymbol );
    if( ndx == -1 )
    { 
      if( bPre ) { sText = sCurSymbol + sText; }
      else       { sText = sText + " " + sCurSymbol; }
    }
    try
    {
      n = cfLocal.parse( sText );
      jtI.setText( cfLocal.format( n ) );
      jtD.setText( cf2.format( n ) );
      n = cf2.parse( jtD.getText() );
      jtP.setText( n.toString() );
    }
    catch( ParseException pe ) { jtI.setText( "" ); }

  }  // End actionPerformed


// Window Listener Implementation
  public void windowOpened(WindowEvent e) {}
  public void windowClosing(WindowEvent e)
  {
    dispose();
    System.exit(0);
  }
  public void windowClosed(WindowEvent e) {}
  public void windowIconified(WindowEvent e) {}
  public void windowDeiconified(WindowEvent e) {}
  public void windowActivated(WindowEvent e) {}
  public void windowDeactivated(WindowEvent e) {}
// End Window Listener Implementation


  public static void main(String[] args)
  {
    new JCusCF();
  }

}  // end class JCusCF