That code needs to be in a listener that is attached to the JRadioButton such as an ActionListener or ItemListener. And you don't even need the if blocks since all you'd need is one line of code inside of the listener:
radioStock.addItemListener(new ItemListener() {
@Override
public void itemStateChanged(ItemEvent itemEvent) {
stockField.setEnabled(itemEvent.getStateChange() == ItemEvent.SELECTED);
}
});
For more on use of JRadioButtons, please check out the tutorial: button tutorial.
Edit my SSCCE
import java.awt.event.*;
import javax.swing.*;
public class ItemListenereg {
private static void createAndShowGui() {
final JRadioButton radioStock = new JRadioButton("Stock", true);
final JTextField stockField = new JTextField(10);
JPanel panel = new JPanel();
panel.add(radioStock);
panel.add(stockField);
radioStock.addItemListener(new ItemListener() {
@Override
public void itemStateChanged(ItemEvent itemEvent) {
stockField.setEnabled(itemEvent.getStateChange() == ItemEvent.SELECTED);
}
});
JOptionPane.showMessageDialog(null, panel);
}
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable() {
public void run() {
createAndShowGui();
}
});
}
}