Question

I need your suggestions and guidence on following task.

I have a frame which has two JComboBoxes supposed they are named combo1 and combo2, a JTable and other components.

At initial stage when frame is visible with above component. The combo1 combobox is filled with some values but no value is selected at initial stage, the combo2 combobox is disabled and the table is empty.

I have added an actionListener on combo1 as well as combo2. There are two types of values in combo1 suppose those values are type1 and type2.

Condition 1:
When we selects value type1 from Combo1 the actionListener method is called of combo1 which invokes a method which combo2 remains disabled and adds some rows to table related to selected value type1 from combo1.

Condition 2:
when we selects value type2 from combo1 the actionListener method is called of combo1 which invokes a method who makes combo2 filled with some values related to type2 and gets enabled but no value is selected from combo2 and table also should remain empty until we selects any value from combo2.

table at every addition of value to combo2 the action listener method of combo2 is gets fired. In actionListener method of combo2 which gets combo2 selected value but here there is no selected value of combo2 which leads to a NullPointerException.

So what should I do that the action listner method of combo2 will not be get executed after addition of an values to combo2.

Answer 1

You could remove the action listener before you add the new elements, and add it back once you’re done . Swing is single threaded so there is no need to worry about other threads needing to fire the listener.

Your listener could probably also check if something is selected and take appropriate action if not. Better than getting a NPE.