Double e float -> casas após a ,

import java.awt.Component;
import java.awt.Container;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import java.text.NumberFormat;
import java.text.ParseException;
import java.util.Locale;

import javax.swing.ButtonGroup;
import javax.swing.JButton;
import javax.swing.JCheckBox;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;

public class NumberFormatTest {
  public static void main(String[] args) {
    JFrame frame = new NumberFormatFrame();
    frame.show();
  }
}

class NumberFormatFrame extends JFrame {
  public NumberFormatFrame() {
    setSize(400, 200);
    setTitle("NumberFormatTest");

    addWindowListener(new WindowAdapter() {
      public void windowClosing(WindowEvent e) {
        System.exit(0);
      }
    });

    getContentPane().setLayout(new GridBagLayout());

    ActionListener listener = new ActionListener() {
      public void actionPerformed(ActionEvent event) {
        updateDisplay();
      }
    };

    JPanel p = new JPanel();
    addCheckBox(p, numberCheckBox, cbGroup, listener, true);
    addCheckBox(p, currencyCheckBox, cbGroup, listener, false);
    addCheckBox(p, percentCheckBox, cbGroup, listener, false);

    GridBagConstraints gbc = new GridBagConstraints();
    gbc.fill = GridBagConstraints.NONE;
    gbc.anchor = GridBagConstraints.EAST;
    add(new JLabel("Locale"), gbc, 0, 0, 1, 1);
    add(p, gbc, 1, 1, 1, 1);
    add(parseButton, gbc, 0, 2, 1, 1);
    gbc.anchor = GridBagConstraints.WEST;
    add(localeCombo, gbc, 1, 0, 1, 1);
    gbc.fill = GridBagConstraints.HORIZONTAL;
    add(numberText, gbc, 1, 2, 1, 1);

    locales = NumberFormat.getAvailableLocales();
    for (int i = 0; i < locales.length; i++)
      localeCombo.addItem(locales[i].getDisplayName());
    localeCombo.setSelectedItem(Locale.getDefault().getDisplayName());
    currentNumber = 123456.78;
    updateDisplay();

    localeCombo.addActionListener(listener);

    parseButton.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent event) {
        String s = numberText.getText();
        try {
          Number n = currentNumberFormat.parse(s);
          if (n != null) {
            currentNumber = n.doubleValue();
            updateDisplay();
          } else {
            numberText.setText("Parse error: " + s);
          }
        } catch (ParseException e) {
          numberText.setText("Parse error: " + s);
        }
      }
    });
  }

  public void add(Component c, GridBagConstraints gbc, int x, int y, int w,
      int h) {
    gbc.gridx = x;
    gbc.gridy = y;
    gbc.gridwidth = w;
    gbc.gridheight = h;
    getContentPane().add(c, gbc);
  }

  public void addCheckBox(Container p, JCheckBox checkBox, ButtonGroup g,
      ActionListener listener, boolean v) {
    checkBox.setSelected(v);
    checkBox.addActionListener(listener);
    g.add(checkBox);
    p.add(checkBox);
  }

  public void updateDisplay() {
    Locale currentLocale = locales[localeCombo.getSelectedIndex()];
    currentNumberFormat = null;
    if (numberCheckBox.isSelected())
      currentNumberFormat = NumberFormat.getNumberInstance(currentLocale);
    else if (currencyCheckBox.isSelected())
      currentNumberFormat = NumberFormat
          .getCurrencyInstance(currentLocale);
    else if (percentCheckBox.isSelected())
      currentNumberFormat = NumberFormat
          .getPercentInstance(currentLocale);
    String n = currentNumberFormat.format(currentNumber);
    numberText.setText(n);
  }

  private Locale[] locales;

  private double currentNumber;  private JComboBox localeCombo = new JComboBox();

  private JButton parseButton = new JButton("Parse");

  private JTextField numberText = new JTextField(30);

  private JCheckBox numberCheckBox = new JCheckBox("Number");

  private JCheckBox currencyCheckBox = new JCheckBox("Currency");

  private JCheckBox percentCheckBox = new JCheckBox("Percent");

  private ButtonGroup cbGroup = new ButtonGroup();

  private NumberFormat currentNumberFormat;
}

DecimalFormat novoFormato = new DecimalFormat( "0.##");    

double num=novoFormato.format(2.456789);