if statement causing jar to not run

Question

I am trying to make a program that copies a file and then pastes it to another location, with the use of a few Apache IO methods. It works perfectly in Eclipse, but when I export it to a JAR, the JAR doesn't run. The "Past Filepaths.txt" file is in my project folder. When I look in the Task Manager, I can see it start up for a few seconds, before it disappears. I've narrowed it down to a single if statement:

if (filePaths.length == 2){
        source.setText(filePaths[0]);
        dest.setText(filePaths[1]);
}

If I comment this out, the JAR runs. If I don't, it doesn't.

This is some very rough code I created in ~30 mins as a kind of script, to help me move some files around, so if it looks a little rough, I apologize.

My full code:

public class Main {
    private JFrame jf = new JFrame();
    private JTextField source, dest;
    private String sourcePath, destPath;

public Main() {
    String[] filePaths = null;
    try {
        filePaths = FileUtils.readFileToString(new File("Past Filepaths.txt"), "ASCII").split("~");
    } catch (IOException e1) {
        e1.printStackTrace();
    }

    JPanel panel = new JPanel();
    JButton jButton = new JButton("Update Files");

    source = new JTextField("", 40);
    dest = new JTextField("", 40);

    if (filePaths.length == 2){
        source.setText(filePaths[0]);
        dest.setText(filePaths[1]);
    }

    jButton.addActionListener( (e) -> {
        updateVars();
        updateFiles();
    });
    jf.setSize(500, 200);
    jf.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    jf.setLocationRelativeTo(null);
    jf.setResizable(false);
    jf.add(panel);
    panel.add(new JLabel("Source"));
    panel.add(source);
    panel.add(new JLabel("Destination"));
    panel.add(dest);
    panel.add(jButton);
    jf.setVisible(true);
}

private void updateVars(){
    sourcePath = source.getText();
    destPath = dest.getText();
}

private void updateFiles(){
    if(new File(sourcePath).exists() == false){
        JOptionPane.showMessageDialog(jf, sourcePath + " is not a valid file path!");
        return;
    }

    if(new File(destPath).exists() == false){
        JOptionPane.showMessageDialog(jf, destPath + " is not a valid file path!");
        return;
    }

    try {
        FileUtils.copyDirectory(new File(sourcePath), new File(destPath));
    } catch (IOException e) {
        e.printStackTrace();
    }

    File pastFiles = new File("Past Filepaths.txt");
    try{
        FileUtils.write(pastFiles, sourcePath + "~", "ASCII");
        FileUtils.write(pastFiles, destPath, "ASCII", true);
    }catch(Exception e){
        e.printStackTrace();
    }
}

Answer 1

This is a debugging problem, not a code problem per se. So, debug it.

Start the JVM in debug mode:

java -Xdebug -agentlib:jdwp=transport=dt_socket,address=9999,server=y,suspend=y <rest of your startup command>

Note that suspend=y will halt execution until you connect your IDE to it.

Open your IDE and set a break point where you want execution to stop, connect your IDE to debug port 9999 (as per address=9999), then step through the code to see what's happening.