Why do my components disappear when added to a panel?

Question

So I have one jFrame object ,which in turn has 3 JPanels attached to it. These JPanels are placed on NORTH,CENTER,SOUTH. I added some buttons and labels to CENTER and SOUTH.

I added in some fieldsets on NORTH and CENTER to give it a nicer look. Now when I try to run the program ,it doesn't show anything. My buttons and labels disappear and my fieldsets are nowhere to be found. What am I doing wrong?

Thank you for you time.

public General() {
    super("OmniTool");
    initComponents();
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setLocation(300,100);
    setVisible(true);
    setResizable(true);
    setSize(900, 700);

    //radiobuttons ivm dependencies
    ButtonGroup dependenciesInfo = new ButtonGroup();
    dependenciesInfo.add(rdbYesDependencies);
    dependenciesInfo.add(rdbNoDependencies);
    //boolean om te controleren of alle textareas al ingevuld zijn 
    generalInfoComplete = false;

    //creating 3 panels and add them on the same frame + giving them their location 
    JPanel oudeMod = new JPanel();
    JPanel nieuweMod = new JPanel();
    JPanel generate = new JPanel();
    this.add(oudeMod,BorderLayout.NORTH);
    this.add(nieuweMod,BorderLayout.CENTER);
    this.add(generate,BorderLayout.SOUTH);
    oudeMod.setBorder(BorderFactory.createTitledBorder("Use exisiting modfolder"));
    nieuweMod.setBorder(BorderFactory.createTitledBorder("Create new modfolder"));

    //adding buttons and textfields to proper panels

    nieuweMod.add(jLabel2);
    nieuweMod.add(jLabel3);
    nieuweMod.add(jLabel4);
    nieuweMod.add(jLabel5);
    nieuweMod.add(jLabel7);
    nieuweMod.add(jLabel8);
    nieuweMod.add(jLabel9);
    nieuweMod.add(jLabel10);
    nieuweMod.add(btnBrowseMain);
    nieuweMod.add(btnBrowseMods);
    nieuweMod.add(btnDependencies);
    nieuweMod.add(rdbNoDependencies);
    nieuweMod.add(rdbYesDependencies);
    nieuweMod.add(txtDependencies);
    nieuweMod.add(txtMainDir);
    nieuweMod.add(txtModName);
    nieuweMod.add(txtModsDir);
    generate.add(btnGenerate);
}

EDIT1: These jFrames are on 1 jtabbedPane.

Answer 1

if "this" is the JFrame, this.add(...) won't work. You should add your components to the contentPane of the JFrame.

Answer 2

"So I have one jFrame object ,which in turn has 3 JPanels attached to it. These JPanels are placed on NORTH,CENTER,SOUTH. I added some buttons and labels to CENTER and SOUTH."

Each BorderLayout position can only hold one component. If you add a JPanel to the CENTER then add the JButton to the CENTER only the last component added (the JButton) will appear.

Add whatever components you need to a max 5 JPanels and position those JPanel at the different BorderLayout positions. Or you can use a different layout. Or you can use a main JPanel container and just add the JPanel to the frame. You have many options.

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Also do all your JFrame housekeeping after you add all your components. Put them at the end of your constructor.

setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setLocation(300,100);
setVisible(true);
setResizable(true);
setSize(900, 700);

Don't set size either. Just pack(). And setVisible() should be the last thing you do.

setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
pack();
setLocationRelativeTo(null);
setResizable(true);
setVisible(true);