…

String[] petStrings = { "Bird", "Cat", "Dog", "Rabbit", "Pig" };

//Create the combo box, select item at index 4.

//Indices start at 0, so 4 specifies the pig.

JComboBox petList = new JComboBox(petStrings);

petList.setSelectedIndex(4);

petList.addActionListener(this);

Tham khảo thêm tại https://docs.oracle.com/javase/tutorial/uiswing/examples/components/index.html#ComboBoxDemo