How do I get which JRadioButton is selected from a ButtonGroup

I would just loop through your JRadioButtons and call isSelected(). If you really want to go from the ButtonGroup you can only get to the models. You could match the models to the buttons, but then if you have access to the buttons, why not use them directly?

I suggest going straight for the model approach in Swing. After you've put the component in the panel and layout manager, don't even bother keeping a specific reference to it.

If you really want the widget, then you can test each with isSelected, or maintain a Map<ButtonModel,JRadioButton>.

Typically, some object associated with the selected radio button is required. It is not necessarily a String representing the button's label. It could be an Integer containing the button's index or an object of more complicated type T. You could fill and use a Map<ButtonModel, T> as Tom Hawtin suggested, but I propose to extend the model and place the objects there. Here's an improved ButtonGroup that uses this approach.

import javax.swing.*;

@SuppressWarnings("serial")
public class SmartButtonGroup<T> extends ButtonGroup {
    @Override
    public void add(AbstractButton b) {
        throw new UnsupportedOperationException("No object supplied");
    }

    public void add(JRadioButton button, T attachedObject) {
        ExtendedModel<T> model = new ExtendedModel<>(attachedObject);
        model.setSelected(button.isSelected());
        button.setModel(model);
        super.add(button);
    }

    @SuppressWarnings("unchecked")
    public T getSelectedObject() {
        ButtonModel selModel = getSelection();
        return selModel != null ? ((ExtendedModel<T>)selModel).obj : null;
    }

    public static class ExtendedModel<T> extends javax.swing.JToggleButton.ToggleButtonModel {
        public T obj;

        private ExtendedModel(T object) {
            obj = object;
        }
    }
}

You can use this utility class instead of ButtonGroup. Create an object of this class and add buttons along with associated objects to it. For example,

SmartButtonGroup<Integer> group = new SmartButtonGroup<>();
JPanel panel = new JPanel();

for (int i = 1; i <= 5; i++) {
    JRadioButton button = new JRadioButton("Button #" + i, i == 3); // select the 3rd button
    group.add(button, i);
    panel.add(button);
}

After this, you can get the object associated with the currently selected button anytime you need by simply calling getSelectedObject(), like this:

int selectedButtonIndex = group.getSelectedObject();

In case you need just the buttons themselves, you can use the next non-generic class instead.

import javax.swing.JRadioButton;

@SuppressWarnings("serial")
public class RadioButtonGroup extends SmartButtonGroup<JRadioButton> {
    public void add(JRadioButton button) {
        super.add(button, button);
    }

    @Override
    public void add(JRadioButton button, JRadioButton attachedObject) {
        throw new UnsupportedOperationException("Use the short form of addition instead");
    }

    public JRadioButton getSelectedButton() {
        return getSelectedObject();
    }
}

You could use getSelectedObjects() of ItemSelectable (superinterface of ButtonModel) which returns the list of selected items. In case of a radio button group it can only be one or none at all.

Add the radiobuttons to a button group then:

buttonGroup.getSelection().getActionCommand

Ale Rojas's answer works fine:

As alternative you can also use the My_JRadiobutton11.addActionListener(this); on your JButton and then make your actions in the actionPerformed function like this (It just uses an extra variable which you have to instantiate (e.g Private String selection;) but it's not a big deal):

public void actionPerformed(ActionEvent arg0) {
      if(arg0.getSource() == My_JRadiobutton11){
          //my selection
          selection = "Become a dolphin";
      }else if(arg0.getSource() == My_JRadiobutton12){
          //my selection
          selection = "Become a Unicorn";
      } ..etc 
 }

jRadioOne = new javax.swing.JRadioButton();
jRadioTwo = new javax.swing.JRadioButton();
jRadioThree = new javax.swing.JRadioButton();

... then for every button:

buttonGroup1.add(jRadioOne);
jRadioOne.setText("One");
jRadioOne.setActionCommand(ONE);
jRadioOne.addActionListener(radioButtonActionListener);

...listener

ActionListener radioButtonActionListener = new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                radioButtonActionPerformed(evt);
            }
        };

...do whatever you need as response to event

protected void radioButtonActionPerformed(ActionEvent evt) {            
       System.out.println(evt.getActionCommand());
    }