I need to write java code that can monitor and detect network disconnection without pressing any button in apache netbeans

Question

Presently I have followed some of the codes that had been solved here and I was able to detect network when I press my button to check. It works fine. When I disconnect from network, I click the button to check again. It works fine. However, what I want to achieve now is that once I load the application, without pressing any button, java should detect network if available and if at any time I disconnect the network, the application should detect and send message.When the network is connected again, the application should automatically detect and send message. Please how can I write this code? Thank you