Why oval can't be drawn?

Question

Im trying to understand a Paint Graphics, but my oval can't be drawn. Can someone tell me what am I doing wrong and oval is not drawing? Where did I make a mistake?

Main class:

import java.awt.EventQueue;
public class Main {
public static void main(String[] args) {
    EventQueue.invokeLater(new Runnable() {
        @Override
        public void run() {
            Frame frame = new Frame();

        }
    });
}

Frame class:

public class Frame extends JFrame {
private static final long serialVersionUID = 1L;

public static Grafika grafika;

public Frame() {
    JFrame frame = new JFrame("Title");
    grafika = new Grafika();

    frame.setSize(500, 500);
    frame.setLocationRelativeTo(null);
    frame.setDefaultCloseOperation(EXIT_ON_CLOSE);
    frame.setVisible(true);
    frame.setLayout(null);
    // frame.addKeyListener(this);
    frame.add(grafika);
}
}

And last Grafic class:

public class Grafika extends JComponent {

int x = 200;
int y = 200;

public void paint(Graphics g) {

    Graphics2D oval = (Graphics2D) g;
    oval.setColor(Color.BLACK);
    oval.fillOval(x, y, 100, 100);
    oval.setRenderingHint(RenderingHints.KEY_ANTIALIASING, 
    RenderingHints.VALUE_ANTIALIAS_ON);

}
}

Answer 1

Several issues, but the biggest is that you're using a null layout on your JFrame, and then adding a JComponent whose preferred size and size are both 0,0. So while you're adding your Grafika to the JFrame, it doesn't have a chance of being displayed.

Suggestions:

• Never use null layout, except in very specific exceptional circumstances.
• Give your Grafika component a preferred size, best by overriding getPreferredSize(), but at this stage, I think that it would be OK to call setPreferredSize(...) on it.
• Add it to the JFrame, pack() the JFrame and then lastly, only after all components have been added to the JFrame, make it visible.

Also

• You should be overriding paintComponent not paint
• You should call the super painting method within your override.
• Always use the @Override annotation when you think that you're overriding a parent method. You could be wrong, and you want the compiler to tell you.
• Set the RenderingHints before drawing. Else the hints will have no effect on the drawing.
• Avoid giving your classes names that clash with the names of core Java classes, such as Frame. This will potentially confuse others and your future self.

e.g.,

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

import javax.swing.*;

public class MyGrafika extends JComponent {
    private static final int PREF_W = 500;
    private static final int PREF_H = PREF_W;
    private static final Color OVAL_COLOR = Color.RED;
    private int ovalX = 200;
    private int ovalY = 200;
    private int ovalWidth = 100;
    private int ovalHeight = 100;

    public MyGrafika() {
        setPreferredSize(new Dimension(PREF_W, PREF_H));
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D) g;
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
        g2.setColor(OVAL_COLOR);
        g2.fillOval(ovalX, ovalY, ovalWidth, ovalHeight);
    }

    private static void createAndShowGui() {
        MyGrafika mainPanel = new MyGrafika();

        JFrame frame = new JFrame("MyGrafika");
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        frame.getContentPane().add(mainPanel);
        frame.pack();
        frame.setLocationByPlatform(true);
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> createAndShowGui());
    }
}