Develop Reference

Why is paint not being called?

I've created a class with three methods. I originally started with slightly different coding, and have changed it after looking at several videos and questions until I ended up with this. For some reason, the square I am trying to make simply will not appear on my screen. Please make any suggestions you have. I started relatively recently, which I'm sure is obvious. Please excuse any conventional errors I have made. As a side note, I figured out that the problem is that the method "paint" isn't being called. As I'm new I have no idea how to call it after trying a few different ways. If you have any clarifying questions about my code please ask. Thanks!
public class TheGame extends JPanel {
public void screen() {
JFrame f = new JFrame();
f.setTitle("Grid Game");
f.setResizable(false);
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
f.setBounds(250, 250, 500, 500);
f.add(this);
f.setVisible(true);
}
public void paint(Graphics2D g) {
g.setColor(Color.BLACK);
g.fillRect(50, 50, 200, 200);
}
public static void main(String[] args) {
TheGame game = new TheGame();
game.screen();
}

You need to override the paintComponent() method:
#Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
g.setColor(Color.BLACK);
g.fillRect(50, 50, 200, 200);
}

What did super.paintComponent(g) do?

So in the past few days I've tried to implement an easier version of a graph plotter.
One big problem I was confronted with was a bug that occured on repainting.
Basically I've designed my program in one class which is responsible for drawing the whole coordinate system and the given function after clicking a JButton in an other class. The other class contains the JButton which is pressed. After pressing the JButton it calls a function in the coordinate system class which repaints the picture. Both of those classes are extending JPanel.
The bug was that when I was doing the repainting on pressing the button, the button was drawn on the coordinate System and not in its original place, so in other words on the other JPanel even though I didn't change a thing about placements and stuff. Both classes were added to a JFrame which use a GridLayout.
Can anyone tell me why super.paintComponent(g); solved that bug?
Edit: Added Code
Window class
public class main {
public static void main(String[] args) throws SemanticFailureException {
int x = 800;
int y = 600;
JFrame frame = new JFrame();
frame.setLayout(new GridLayout());
frame.setTitle("Function plotter");
frame.setSize(2*x, 2*y);
Surface test = new Surface(x, y, 10);
CommandDraw test1 = new CommandDraw(x/2,y,test);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setLocationRelativeTo(null);
frame.add(test);
frame.add(test1);
frame.setVisible(true);
}
}
Coordinate System class: (changed drawing the coordinate system to a rectangle for simplicity, the bug still occures with only drawing a rectangle)
public class Surface extends JPanel {
/**
*
*/
private static final long serialVersionUID = 1L;
boolean drawFunct;
public Surface(int x1, int y1, int coordLength) {
setSize(x1,y1);
drawFunct = false;
}
public void paintComponent(Graphics g) {
super.paintComponent(g); // without this the jbutton occures on the left
// create Graphics object to get more functions
Graphics2D g2 = (Graphics2D) g;
// draw Plotter
drawFunction(g2);
if (drawFunct)
g2.drawLine(0, 0, 80, 80);
}
public void drawFunction(Graphics2D g) {
g.drawRect(40, 40, 30, 30);
}
public void redraw() {
drawFunct = true;
repaint();
}
}
The Class with the JButton:
public class CommandDraw extends JPanel {
/**
*
*/
private static final long serialVersionUID = 1L;
JButton makeDraw;
JTextField inputPoly;
Surface surf;
public CommandDraw(int x, int y, Surface surf) {
this.surf = surf;
setSize(x,y);
setLayout(new FlowLayout());
makeDraw = new JButton("draw Function");
makeDraw.setBackground(Color.LIGHT_GRAY);
makeDraw.setFocusable(false);
makeDraw.addActionListener( new ActionListener() {
#Override
public void actionPerformed(ActionEvent arg0) {
surf.redraw();
}
});
add(makeDraw);
inputPoly = new JTextField("Input polynomial");
inputPoly.setHorizontalAlignment(JTextField.CENTER);
add(inputPoly);
}
}

Can anyone tell me why super.paintComponent(g); solved that bug?
Because the call to paintComponent(g) of the superclass (JPanel) will guarantee that panel will be rendered as expected before you do your paint operations. You need to make sure that your JPanel will behave, in the Graphics perspective, like any other JPanel.
A template for a customized JPanel to draw should be something like:
import java.awt.Graphics;
import java.awt.Graphics2D;
import javax.swing.JPanel;
public class MyDrawPanel extends JPanel {
#Override
protected void paintComponent( Graphics g ) {
super.paintComponent( g );
// create a new graphics context based on the original one
Graphics2D g2d = (Graphics2D) g.create();
// draw whatever you want...
g2d.dispose();
}
}
EDIT
You need to call super.paintComponent(g) to tell that the JPanel paintComponent(Graphics) version should be execute before you start doind your customized things. It's something like to ask the JPanel to prepare its paint capabilities to be used. A good starting point to these kind of doubts is the documentation: https://docs.oracle.com/javase/10/docs/api/javax/swing/JComponent.html#paintComponent(java.awt.Graphics)
The dispose() method is being called in the copy of the original Graphics. If you dispose the Graphics that is passed to the method, you may have some issues too. You could use the original Graphics to perform you painting operations, but you shouldn't, so a better practice is to make a copy of the original Graphics and dispose it after using.
Take a look here too: How does paintComponent work?

Display Two Classes Java

so I am new to designing graphics in Java I was wondering if anyone could help me here. I have two classes and I want to display both of them at the same time in a JFrame. But only one or the other get displayed.
public class Tutorial extends JPanel implements ActionListener {
Background bc = new Background();
Timer tm = new Timer(5,this);
int x =0, velX = 2;
public void paintComponent(Graphics g){
super.paintComponent(g);
g.setColor(Color.RED);
g.fillRect(x, 30, 50, 30);
tm.start();
}
#Override
public void actionPerformed(ActionEvent e) {
if(x<0 || x>550){
velX = -velX;
}
x = x+ velX;
repaint();
}
public static void main(String [] args){
Background bc = new Background();
Tutorial t = new Tutorial();
JFrame jf = new JFrame();
jf.setTitle("Tutorial");
jf.setSize(600,400);
jf.setVisible(true);
jf.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
jf.add(t);
jf.add(bc);
}
My second class
public class Background extends Canvas {
public void paint(Graphics g){
g.setColor(Color.GREEN);
g.fillRect(0,0,600,125);
g.fillRect(0,250,600,125);
g.setColor(Color.black);
g.fillRect(0,125,600,125);
}
For some reason I can only get either Background to be displayed or Tutorial?
Can anyone point me in the right direction or tell me where I am going wrong. I want to be able to display multiple things like these classes in the one window

The default layout manager of a JFrame is a BorderLayout.
By using the single-argument JFrame.add() function, you're adding both of the components to the BorderLayout.CENTER portion of your JFrame. This means that you'll only see one of the components.
The solution is to either use a different layout manager, or to add the components to different sections of your BorderLayout.
More info here: http://docs.oracle.com/javase/tutorial/uiswing/layout/visual.html

Java not drawing desired graphics

This is a pretty simple program that just draws a white rectangle in a pop-up window. The program compiles and runs no problem, and the windoe pops up, but there is nothing in it, it's just grey. Why isn't anything drawing?
import java.awt.*;
import javax.swing.*;
public class DrawPanel extends JPanel {
public void paintcompnent(Graphics g) {
int width = getWidth();
int height = getHeight();
super.paintComponent(g);
g.setColor(Color.WHITE);
g.fillRect(10, 10, 200, 200);
}
public static void main(String[] args) {
DrawPanel panel = new DrawPanel();
JFrame window = new JFrame();
window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
window.add(panel);
window.setSize(550,550);
window.setVisible(true);
}
}

public void paintcompnent(Graphics g)
Should be
public void paintComponent(Graphics g)

You have not correctly overridden the method. It should be:
#Override
protected void paintComponent(Graphics g) {}
Note the use of the #Override annotation, which will prevent you from making this mistake in the future.

After fixing the overridden method name, it still won't paint anything because you are filling the rectangle in white and then painting white lines on top. You need to use setColor() to set something other than white to see the lines.

PaintComponent not working for drawing shapes

I was studying graphics and tried to use PaintComponent to draw some shapes, following is the code. I am trying for an hour but still its not working really can't get reason. What is the resolution to this simple problem?
public class MyPainting extends JPanel
{
public void PaintComponent (Graphics g)
{
super.paintComponent(g);
g.setColor(Color.RED);
g.drawRect(100, 100, 10, 20);
}
public static void main (String [] args)
{
MyPainting p = new MyPainting();
JFrame f= new JFrame();
f.setSize(300,300);
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
f.add(p);
f.setVisible(true);
}
}
When I run program there is empty JFrame, I did try it g.drawString, ImageIcon but every time nothing is visible.

The method PaintComponent is not defined in any of the super classes of JPanel. You want paintComponent
#Override
public void paintComponent (Graphics g)
and add the #Override annotation to allow compiler check for the correct method.