//定义下拉框组件
JComboBox<String> cmb = new JComboBox<String>();//添加内容
cmb.addItem("北京");
cmb.addItem("上海");
cmb.addItem("广州");
cmb.addItem("深圳");//获取内容
String city = cmb.getSelectedItem().toString();
System.out.println(city);

结果：
点击按钮，触发事件