private YourClass that = this;
 
cboOrigin.addItemListener(new ItemListener(){
	@Override
	public void itemStateChanged(ItemEvent evt) {
		if(evt.getStateChanged() == ItemEvent.SELECTED) {
			String selected = String.valueOf(evt.getSelectedItem());
			if(selected.equals("Centímetros a Metros")) {
				// that = this. Será el padre de éste dialogo.
				// Se bloqueará el frame hasta que se cierre el dialog
				new CentimetersToMetersDialog(that);
			}
		}
	}
});
 
public class CentimetersToMetersDialog extends JDialog {
	/* ... */
}

private String origin;
private String target;
 
cboOrigin.addItemListener(new ItemListener(){
	@Override
	public void itemStateChanged(ItemEvent evt) {
		if(evt.getStateChanged() == ItemEvent.SELECTED) {
			origin = String.valueOf(evt.getSelectedItem());
			cboTarget.setDisabled(false);
		} else {
			cboTarget.setDisabled(true);
		}
	}
});
 
cboTarget.addItemListener(new ItemListener(){
	@Override
	public void itemStateChanged(ItemEvent evt) {
		if(evt.getStateChanged() == ItemEvent.SELECTED) {
			target = String.valueOf(evt.getSelectedItem());
			btnConvert.setDisabled(false);
		} else {
			btnConvert.setDisabled(true);
		}
	}
});
 
btnConvert.addActionListener(new ActionListener() {
	@Override
	public void actionPerformed(ActionEvent evt) {
		String fully = (origin + " " + target).toLowerCase();
		Double toConvert = Double.parseDouble(txtNumber.getText());
                Double converted = 0.0d;
		Converter converter = new Converter();
 
		if(fully.contains("centimetros") && fully.contains("metros")) {
			converted = converter.centimetersToMeters(toConvert);
		}
		if(fully.contains("centimetros") && fully.contains("pulgadas")) {
			converted = converter.centimetersToInch(toConvert);
		}
		/* otras conversiones */
                txtResult.setText(converted);
	}
});