Preferred:

String x = JComboBox.getSelectedItem().toString();

or

String x = String.valueOf(JComboBox.getSelectedItem());

The second method protects against null values as well.

Avoid using casting:

String x = (String)JComboBox.getSelectedItem();

This would work fine if the item is indeed a string, but will fail if it can (also) be any other data type. To be safe, use either of the first two methods.