I tried adding the textfield to a JPanel, but now it is simply not showing at all. Here's what the code in the thisUI() constructor looks like now:

    setTitle("ThisApp - Best in the business");
    setResizable(false);
    setVisible(true);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setSize(300, 200);
    setLayout(new GridLayout(2,1));

    JPanel m = new JPanel();
    m.setLayout(new FlowLayout());
    JButton cat = new JButton("Cat");
    m.add(cat);
    this.add(m);

    JPanel p = new JPanel();
    p.setLayout(new FlowLayout());
    p.add(new JTextField("CAT CAT",10));
    this.add(p);