Java Swing JFrame Layout

The default layout on a JFrame is a BorderLayout. Calling the add method on a Container with such a layout is equivalent to a call add(..., BorderLayout.CENTER). Each of the locations of the BorderLayout can contain only one element. Hence making two calls

mainframe.add(jb);
mainframe.add(link);

results in a CENTER containing the last component you added. If you want to avoid this you can either add it to different locations, or use another layout manager (for example a FlowLayout) by calling JFrame#setLayout

Add your components to a JPanel and then add that panel to the ContentPane of JFrame.

JFrame window = new JFrame();
JPanel mainframe = new JPanel();

window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
window.setBounds(0,0,200,200);

JButton jb = new JButton();
jb.setText("Leech");

mainframe.add(jb);

JTextField link = new JTextField(50);
mainframe.add(link);

window.getContentPane().add(mainframe);
window.pack();
window.setVisible(true);

Instead of adding directly Components to the JFrame, use a JPanel as container with the desired LayoutManager.

Here you can find several tutorials on layout managers.

Basically in Swing the LayoutManager is responsible for laying out the children Components (establishing their position and their size), so every container component you use inside your app, should be configured with the appropiate LayoutManager.