Java Swing Scroll through drawing

Question

Im trying to add a JScrollpane to my JPanel. The problem is that the scrollpane doesn't recognize that my drawing is outside the frame. So how do I add the JScrollpane correctly?

Main class:

public MainFrame() extends JFrame{

public MainFrame() {

        Container container = getContentPane(); 
        container(new BorderLayout());
        container.add(new JScrollPane(new Drawing()));

        setSize(1280,720);
        setVisible(true);
    }

Drawing class:

public class Drawing() extends JPanel {

    @Override
    protected void paintComponent(Graphics g) {
       g.drawLine(10, 100, 30000, 10);
    }
}

Answer 1

There are a couple of errors in your code, let's step through each of them:

1. You're extending JFrame, and you should avoid it, see: Extends JFrame vs. creating it inside the program for more information about it. You're actually not changing its behavior so it's not needed to extend it.

2. For your JScrollPane to show the whole line, you need to change your window's size to be the same size of your line (as shown in this answer by @MadProgrammer).

3. Related to point 2, avoid the use of setSize(...) and instead override getPreferredSize(): See Should I avoid the use of set(Preferred|Maximum|Minimum)Size methods in Java Swing? for more information

4. You forgot to call super.paintComponent(...) method in your paintComponent() method.

5. Related to points 2, 3, you need to call pack() so Swing calculates the best preferred size for your component.

See this example:

import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.SwingUtilities;

public class LongDraw {
    private JFrame frame;
    private Drawing drawing;

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new LongDraw()::createAndShowGui);
    }

    private void createAndShowGui() {
        frame = new JFrame(getClass().getSimpleName());

        drawing = new Drawing();

        JScrollPane scroll = new JScrollPane(drawing);

        frame.add(scroll);

        frame.pack();
        frame.setVisible(true);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }

    class Drawing extends JPanel {
        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);

            Graphics2D g2d = (Graphics2D) g;
            g2d.drawLine(10, 100, 3000, 10);
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(3000, 500);
        }
    }
}

Which produces something similar to this: