Trying to build a Ping functionality, that wil return a dialog to the user displaying the host is up or not when a button it's clicked

Question

Below is the code I'm working on. The code will execute a Swing-based GUI, with a simple JFrame that holds a button which when pressed will run the Ping utility in the background. If the host is either reachable or not, a dialog will be displayed with a successful/unsuccess message.

import javax.swing.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.IOException;
import java.net.InetAddress;

public class App {

private JTextArea clickThisButtonTextArea;
public JButton button1;
private JPanel panelMain;

public App() {
    button1.addActionListener(new ActionListener() {
        @Override
        public void actionPerformed(ActionEvent actionEvent) {
            try {
                sendPing("8.8.8.8");
                //JOptionPane.showMessageDialog(null, sendPing("192.168.0.1"));
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    });
}

public String sendPing(String ipAddr) throws IOException {
    InetAddress ip = InetAddress.getByName(ipAddr);
    boolean ipReach = ip.isReachable(5000);
    System.out.println("Sending Ping Request to " + ipAddr);
    if (ip.isReachable(5000)) {
        JOptionPane.showMessageDialog(null, "Host is reachable!");
        System.out.println("Host is reachable");
    } else {
        JOptionPane.showMessageDialog(null, "Sorry, no host!");
        System.out.println("Sorry ! We can't reach to this host");
    }
    return null;
}

public static void main(String[] args) {
    JFrame frame = new JFrame("App");
    frame.setContentPane(new App().panelMain);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.pack();
    frame.setVisible(true);
    frame.setLocationRelativeTo(null);
    frame.setSize(250,250);


}

}

The image below shows a tcpdump of traffic flowing out after the button is pressed vs. a ping ran from the terminal.

Any insights will be greatly appreciated.

Answer 1

So, running your code generates

Exception in thread "main" java.lang.NullPointerException
    at sotest.App.<init>(App.java:25)
    at sotest.App.main(App.java:54)

This is because button1 is never initialised. I can fix it by updating the constructor like so:

public App() {

    button1 = new JButton("Ping");        
    button1.addActionListener(new ActionListener() {
        @Override
        public void actionPerformed(ActionEvent actionEvent) {
            try {
                sendPing("8.8.8.8");
                //JOptionPane.showMessageDialog(null, sendPing("192.168.0.1"));
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    });
}

But now when I run it, I get...

Exception in thread "main" java.awt.IllegalComponentStateException: contentPane cannot be set to null.
    at java.desktop/javax.swing.JRootPane.setContentPane(JRootPane.java:598)
    at java.desktop/javax.swing.JFrame.setContentPane(JFrame.java:679)
    at sotest.App.main(App.java:56)

This is because panelMain has never been initialised. I can fix it by updating the constructor like so:

public App() {

    panelMain = new JPanel();
    
    button1 = new JButton("Ping");
    button1.addActionListener(new ActionListener() {
        @Override
        public void actionPerformed(ActionEvent actionEvent) {
            try {
                sendPing("8.8.8.8");
                //JOptionPane.showMessageDialog(null, sendPing("192.168.0.1"));
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    });
}

Ah, but now when I run it, it's not displaying anything. This is because I've not added the button to the panel, once again, we update the constructor like so:

public App() {

    panelMain = new JPanel();
    
    button1 = new JButton("Ping");
    button1.addActionListener(new ActionListener() {
        @Override
        public void actionPerformed(ActionEvent actionEvent) {
            try {
                sendPing("8.8.8.8");
                //JOptionPane.showMessageDialog(null, sendPing("192.168.0.1"));
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    });
    
    panelMain.add(button1);
}

All of this is pretty basic Java and would suggest you might need to spend some more time reading through Creating a GUI With JFC/Swing.

Your next problem is going to more tricky to solve, but you can get a start with Concurrency in Swing