Develop Reference

How do I connect the dots to get a polygon in JAVA?

I'm making an application for creating convex
polygon.
I imagined it to be so that I first set the vertices of the polygon and then create it.
I was able to make the addition of points (vertices). Now I need help connecting the dots with a line.
This is what it looks like:
It looks like this
And I would like when I click the Draw Polygon button that these points connect and it looks like a polygon, like this:
and it should look like this when I click the button
Here's the code so you can run it yourself:
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.border.EmptyBorder;
import javax.swing.border.EtchedBorder;
public class MyPaint{
public static void main(String[] args){
final PadDraw drawPad = new PadDraw();
JFrame frame = new JFrame("Draw Polygon");
frame.setVisible(true);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setBounds(100, 100, 450, 300);
Container contentPane = frame.getContentPane();
((JComponent) contentPane).setBorder(new EmptyBorder(5, 5, 5, 5));
contentPane.setLayout(new BorderLayout(0, 0));
frame.setContentPane(contentPane);
JPanel buttonPanel = new JPanel();
buttonPanel.setBorder(new EtchedBorder(EtchedBorder.LOWERED, null, null));
contentPane.add(buttonPanel, BorderLayout.SOUTH);
JButton buttonDrawPolygon = new JButton("Draw Polygon");
buttonPanel.add(buttonDrawPolygon);
JButton buttonReset = new JButton("Reset");
buttonPanel.add(buttonReset);
contentPane.add(drawPad, BorderLayout.CENTER);
}
}
class PadDraw extends JComponent{
private Image image;
private Graphics2D graphics2D;
private int currentX , currentY , oldX , oldY ;
public PadDraw(){
addMouseListener(new MouseAdapter(){
public void mousePressed(MouseEvent e){
oldX = e.getX();
oldY = e.getY();
}
});
addMouseListener(new MouseAdapter(){
public void mousePressed(MouseEvent e){
currentX = e.getX();
currentY = e.getY();
if(graphics2D != null) {
graphics2D.drawLine(oldX, oldY, currentX, currentY);
repaint();
oldX = currentX;
oldY = currentY;
}
System.out.println(oldX + " " + oldY);
}
});
}
public void paintComponent(Graphics g){
if(image == null){
image = createImage(getSize().width, getSize().height);
graphics2D = (Graphics2D)image.getGraphics();
graphics2D.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
graphics2D.setStroke(new BasicStroke(5));
}
g.drawImage(image, 0, 0, null);
}
}

Introduction
Oracle has a helpful tutorial, Creating a GUI With Swing. Skip the Learning Swing with the NetBeans IDE section.
Here's your revised GUI
Here's the GUI with four points
Here's the GUI with a polygon
I'm not showing it, but the Reset button clears the drawing area.
Explanation
When I create a Swing GUI, I use the model-view-controller pattern. This pattern allows me to separate my concerns and focus on one part of the Swing application at a time.
A Swing application model consists of one or more plain Java getter/setter classes.
A Swing view consists of a JFrame and one or more JPanels.
Swing controllers are the listeners that are attached to JButtons and drawing JPanels.
Model
For this application, I created a PolygonModel class. The class holds a boolean that tells me whether or not to draw the polygon and a java.util.List of java.awt.Point instances. The Point class holds an X and Y int value.
View
All Swing applications must start with a call to the SwingUtilities invokeLater method. This method ensures that the Swing components are created and executed on the Event Dispatch Thread.
I broke up your main method into a couple of methods. I separate the creation of the JFrame from the creation of the JPanels. This allows me to separate my concerns and focus on one part of the GUI at a time.
The JFrame methods must be called in a specific order. This is the order I use for most of my Swing applications.
I changed your PadDraw class to extend a JPanel. I moved the MouseAdapter code to its own class. Your drawing panel should draw. Period. Nothing else.
The paintComponent method always starts with a call to the super.paintComponent method. This maintains the Swing paint chain and helps to eliminate unwanted drawing artifacts.
The drawing JPanel is cleared before every repaint. Therefore, you have to completely redraw your image each time. That's why we store the List of Point instances in the model.
Controller
I created three controller classes.
The PointListener class extends MouseAdapter. Notice how simple the mousePressed method becomes with an application model.
The two JButtons each have their own ActionListener. Since they are so simple, I made each of them lambdas.
Code
Here's the complete runnable code. I made all the additional classes inner classes so I could make them public and post the code as one block.
import java.awt.BasicStroke;
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Point;
import java.awt.RenderingHints;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.util.ArrayList;
import java.util.List;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
import javax.swing.border.EtchedBorder;
public class PolygonImage implements Runnable {
public static void main(String[] args) {
SwingUtilities.invokeLater(new PolygonImage());
}
private final PolygonModel model;
private final PadDraw drawPad;
public PolygonImage() {
this.model = new PolygonModel();
this.drawPad = new PadDraw(this, model);
}
#Override
public void run() {
JFrame frame = new JFrame("Draw Polygon");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.add(drawPad, BorderLayout.CENTER);
frame.add(createButtonPanel(), BorderLayout.SOUTH);
frame.pack();
frame.setLocationByPlatform(true);
frame.setVisible(true);
}
private JPanel createButtonPanel() {
JPanel buttonPanel = new JPanel();
buttonPanel
.setBorder(new EtchedBorder(EtchedBorder.LOWERED, null, null));
JButton buttonDrawPolygon = new JButton("Draw Polygon");
buttonDrawPolygon.addActionListener(event -> {
model.setConnectPoints(true);
repaint();
});
buttonPanel.add(buttonDrawPolygon);
JButton buttonReset = new JButton("Reset");
buttonReset.addActionListener(event -> {
model.setConnectPoints(false);
model.clearList();
repaint();
});
buttonPanel.add(buttonReset);
return buttonPanel;
}
public void repaint() {
drawPad.repaint();
}
public class PadDraw extends JPanel {
private static final long serialVersionUID = 1L;
private final PolygonModel model;
public PadDraw(PolygonImage view, PolygonModel model) {
this.model = model;
this.addMouseListener(new PointListener(view, model));
this.setPreferredSize(new Dimension(450, 300));
}
#Override
public void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2d = (Graphics2D) g;
g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
RenderingHints.VALUE_ANTIALIAS_ON);
g2d.setColor(Color.black);
// Draw points
for (Point p : model.getPoints()) {
int radius = 6;
int diameter = radius + radius;
g2d.fillOval(p.x - radius, p.y - radius, diameter, diameter);
}
// Draw polygon
if (model.isConnectPoints()) {
g2d.setStroke(new BasicStroke(5));
List<Point> points = model.getPoints();
if (points.size() >= 1) {
Point old = points.get(0);
for (int index = 1; index < points.size(); index++) {
Point p = points.get(index);
g2d.drawLine(old.x, old.y, p.x, p.y);
old = p;
}
Point p = points.get(0);
g2d.drawLine(p.x, p.y, old.x, old.y);
}
}
}
}
public class PointListener extends MouseAdapter {
private final PolygonImage view;
private final PolygonModel model;
public PointListener(PolygonImage view, PolygonModel model) {
this.view = view;
this.model = model;
}
#Override
public void mousePressed(MouseEvent event) {
model.addPoint(event.getPoint());
view.repaint();
}
}
public class PolygonModel {
private boolean connectPoints;
private final List<Point> points;
public PolygonModel() {
this.points = new ArrayList<>();
this.connectPoints = false;
}
public void setConnectPoints(boolean connectPoints) {
this.connectPoints = connectPoints;
}
public boolean isConnectPoints() {
return connectPoints;
}
public void clearList() {
this.points.clear();
}
public void addPoint(Point point) {
this.points.add(point);
}
public List<Point> getPoints() {
return points;
}
}
}

How to move around an object drawn on a panel with button click

I am trying to build a bounce game in Java. My project has three classes ie the Main class which creates a new window(frame) where the game buttons and bounce objects are drawn.
The GameInterface class which represents the properties of the frame being drawn and the RightPanel class which I created so that I could override the paint(Graphics) method to draw my bounce object. So far this is what I have managed to draw with the code.
You can see that I have two JPanels, one that holds my buttons and the other one that accepts the drawing of a round ball on it ie RightPanel
I need help with the Button Event listeners to move the ball up and down and when user holds the button down, it needs to keep moving down until reaches the down order, sam for the up button.
The code am using is provided below.
GameInterface class
import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
public class GameInterface extends JFrame {
//we need this panel declaration in the class level for reference from other methods
RightPanel rightpanel;
//define the physical properties of the window
public GameInterface(){
setSize(new Dimension(600, 600));
setResizable(false);
setTitle("Bounce Game");
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setBackground(Color.black);
//define a new JSplitPane and use it to add two JPanels
JPanel leftpanel= new JPanel();
//add buttons to the left panel programatically
JButton up= new JButton("Move up");
//set the event listeners for the buttons
up.addActionListener(new AbstractAction() {
#Override
public void actionPerformed(ActionEvent e) {
//move my ball up
//clear and redraw the ball while in a new position, use a timer or
something
}
});
JButton down = new JButton("Move down");
down.addActionListener(new AbstractAction() {
#Override
public void actionPerformed(ActionEvent e) {
//move my ball down
// rightpanel.getGraphics.fillColor(Color.RED);
}
});
leftpanel.add(up);
leftpanel.add(down);
//add a new RightPanel with a drawn red object
rightpanel= new RightPanel();
JSplitPane splitpane= new JSplitPane(JSplitPane.HORIZONTAL_SPLIT,leftpanel,rightpanel);
this.add(splitpane);
setVisible(true);
}
}
RightPanel class
import javax.swing.*;
import java.awt.*;
public class RightPanel extends JPanel {
//define the position where the circle will be drawn
private int positionX=150;
private int positionY=150;
//I had an idea where we need a timer and then on delay we
//decrement positionX by 3 for move down but can't figure out how to clear RightPanel
private int radius=100;//as the shape is a circle
//override the paint method to draw the bounce ball on the second panel
#Override
public void paint(Graphics g) {
g.setColor(Color.RED);
g.fillOval(positionX,positionY,radius,radius);
}
}
Main class
public class Main
{
public static void main(String args[]){
new GameInterface();
}
}
How do I add logic to my code to make it move the circle up an down, Thank You.
I tried using a timer object to clear the panel and then redraw the ball in the new position of the ball but it draws a vertical bar, not clearing the original ball drawn.

Never call getGraphics() on a component.
Override paintComponent not paint
Call the super.paintComponent(g) in your override.
Give the RightPanel class setter methods that allow you to change the positionX and positionY locations for drawing,
In the button listener, call an appropriate setter method, and then call repaint() on on the RightPanel instance after changing the positions.
For example:
The key code below is here in the ActionListener where you update the position values and call repaint:
moveRightBtn.addActionListener(e -> {
// get and update the x position
int x = drawOval.getPositionX();
x += DELTA;
// call the setter method
drawOval.setPositionX(x);
// request that Java repaint the JPanel
drawOval.repaint();
});
and in the drawing JPanel's paintComponent method where you call the super's method and draw the oval:
#Override
protected void paintComponent(Graphics g) {
// this is needed to do house-keeping painting, to clear "dirty" pixels
super.paintComponent(g);
// this is needed to draw smooth graphics
Graphics2D g2 = (Graphics2D) g;
g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
g2.setColor(OVAL_COLOR);
g2.fillOval(positionX, positionY, RADIUS, RADIUS);
}
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import javax.swing.*;
#SuppressWarnings("serial")
public class MoveCircle extends JPanel {
private static final int DELTA = 5;
private DrawOval drawOval = new DrawOval();
public MoveCircle() {
JButton moveRightBtn = new JButton("Move Right");
moveRightBtn.addActionListener(e -> {
// get and update the x position
int x = drawOval.getPositionX();
x += DELTA;
// call the setter method
drawOval.setPositionX(x);
// request that Java repaint the JPanel
drawOval.repaint();
});
JPanel buttonPanel = new JPanel();
buttonPanel.add(moveRightBtn);
setLayout(new BorderLayout());
add(drawOval);
add(buttonPanel, BorderLayout.LINE_START);
}
public static void main(String[] args) {
SwingUtilities.invokeLater(() -> {
MoveCircle mainPanel = new MoveCircle();
JFrame frame = new JFrame("GUI");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.add(mainPanel);
frame.pack();
frame.setLocationRelativeTo(null);
frame.setVisible(true);
});
}
}
#SuppressWarnings("serial")
class DrawOval extends JPanel {
private static final int RADIUS = 100;
private static final int PANEL_WIDTH = 600;
private static final int PANEL_HEIGHT = 450;
private static final Color OVAL_COLOR = Color.RED;
private int positionX = 0;
private int positionY = 0;
public DrawOval() {
setPreferredSize(new Dimension(PANEL_WIDTH, PANEL_HEIGHT));
}
public int getPositionX() {
return positionX;
}
public void setPositionX(int positionX) {
this.positionX = positionX;
}
public int getPositionY() {
return positionY;
}
public void setPositionY(int positionY) {
this.positionY = positionY;
}
#Override
protected void paintComponent(Graphics g) {
// this is needed to do house-keeping painting, to clear "dirty" pixels
super.paintComponent(g);
// this is needed to draw smooth graphics
Graphics2D g2 = (Graphics2D) g;
g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
g2.setColor(OVAL_COLOR);
g2.fillOval(positionX, positionY, RADIUS, RADIUS);
}
}

Why JLabel isn't refreshing when setText() method is called within jpanel.paintComponent()?

After searching for more than 3 hours for similar problem and didn't find any clue i hope i will get my rescue hear.
I'm trying to write a problem that stores in queue up to 5 lines that are draw inside JPanel using paintComponent() method. Also i want to update JLabel text to count size of queue. I'm trying to update the JLabel by setText method inside Jpanel.paintComponent() method.
The problem that i have encountered and can't overcome on them are:
1. When paintComponent runs the line: label.setText("... "+ lineQueue.size()); inside DrawPanel class i get an exception means that label is equal to null. How does it possible if DrawPanel constructor initialized it?
2. In order to overcome first problem i put if (label != null) than label.setText("... " + lineQueue.size()) but label text isn't updated no matter what happen to JFrame. Can someone explain me what is the problem? It makes me crazy :(
My Code (4 files):
public class Point
{
private int x;
private int y;
public Point (int x, int y)
{
this.x = x;
this.y = y;
}
public int getX()
{
return x;
}
public int getY()
{
return y;
}
}
import java.awt.Color;
import java.awt.Graphics;
import java.util.Random;
import javax.swing.JPanel;
public class MyLine
{
private Point p1;
private Point p2;
private Color color;
public MyLine (Point point1 ,Point point2, Color color)
{
p1 = point1;
p2 = point2;
this.color = color;
}
public void drawLine (Graphics g)
{
g.setColor(color);
g.drawLine (p1.getX(),p1.getY(),p2.getX(),p2.getY());
}
}
import java.awt.Color;
import java.awt.Graphics;
import java.util.*;
import javax.swing.JPanel;
import javax.swing.JLabel;
import java.util.Queue;
public class DrawPanel extends JPanel
{
private LinkedList<MyLine> lineQueue;
private JLabel label;
public DrawPanel(JLabel label)
{
label = this.label;
lineQueue = new LinkedList <MyLine>();
}
public void paintComponent (Graphics g)
{
super.paintComponent(g);
Random rand = new Random();
Point p1 = new Point (rand.nextInt(400),rand.nextInt(400));
Point p2 = new Point (rand.nextInt(400),rand.nextInt(400));
Color color = new Color (rand.nextInt(256), rand.nextInt(256), rand.nextInt(256));
if (lineQueue.size() >= 5)
lineQueue.remove();
lineQueue.add(new MyLine (p1,p2,color));
ListIterator<MyLine> iterator = lineQueue.listIterator(0);
while (iterator.hasNext() == true)
iterator.next().drawLine(g);
if (label!=null)
{
label.setText("... "+ lineQueue.size());
}
}
}
import java.awt.Color;
import javax.swing.*;
import java.awt.BorderLayout;
public class TestDrawLine
{
public static void main(String[] args)
{
JFrame frame = new JFrame ();
frame.setLayout(new BorderLayout());
frame.setSize(400,350);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
JLabel label = new JLabel();
DrawPanel drawPanel = new DrawPanel(label);
drawPanel.setBackground(Color.BLACK);
frame.add (drawPanel,BorderLayout.CENTER);
JPanel southPanel = new JPanel();
southPanel.add (label);
frame.add (southPanel,BorderLayout.SOUTH);
frame.setVisible(true);
}
}

label = this.label;
This initializes the argument with the instance variable. You need to do the exact inverse:
this.label = label;
which initializes the instance variable with the argument.
In order to overcome first problem i put if (label != null) than label.setText("... " + lineQueue.size())
That can't possibly fix the issue. All this does is that it doesn't do anything anymore instead of causing an exception. The label is still null, and you're just not doing anything with it anymore.
That said: the paintComponent() method is not supposed to modify the state of your component (or of anything else). It's only supposed to paint the component. That method will be called many times, whenever swing decides that the component needs to be repainted.

Can not add JComponent to JPanel

Have problem with adding class extended from JComponent to MyPanel class.
After choose from JComboBox list and press Start/Restart, the right panel doesnt update. Mean after adding to the Panel MyComponent class where is all drawing stuff. If someone can take a look and tell me when i am doing the mistakes or if its the different way to do that, please help me :)!
Run app class :
import javax.swing.JFrame;
public class RunApp {
public static void main(String[] args) {
JFrame mainFrame = new MyPanel();
mainFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
mainFrame.setExtendedState(JFrame.MAXIMIZED_BOTH);
mainFrame.setVisible(true);
}
}
MyPanel class :
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Toolkit;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JPanel;
public class MyPanel extends JFrame {
private JPanel leftPanel = null;
private JPanel rightPanel = null;
private JPanel mainPanel = null;
private JButton start = null;
private JButton restart = null;
private JComboBox<String> menuBox = new JComboBox<>();
private Toolkit kit = Toolkit.getDefaultToolkit();
private Dimension screenSize = kit.getScreenSize();
private int screenHeight = screenSize.height;
private int screenWidth = screenSize.width;
public MyPanel() {
mainPanel = new JPanel();
mainPanel.setBackground(Color.orange);
mainPanel.setPreferredSize(getPreferredSize());
leftPanel = new JPanel();
leftPanel.setBackground(Color.blue);
leftPanel.setPreferredSize(new Dimension(screenWidth/6, screenHeight));
menuBox.addItem("Gaussian Wave - non Dispersive");
menuBox.addItem("Gaussian Wave - Dispersive");
start = new JButton("Start");
restart = new JButton("Restart");
leftPanel.add(menuBox);
leftPanel.add(start);
leftPanel.add(restart);
rightPanel = new JPanel();
rightPanel.setBackground(Color.red);
rightPanel.setPreferredSize(new Dimension(screenWidth -( screenWidth/5 ), screenHeight));
start.addActionListener(new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
if(menuBox.getItemAt(menuBox.getSelectedIndex()).equals("Gaussian Wave - non Dispersive")) {
rightPanel.add(new MyComponent());
rightPanel.revalidate();
rightPanel.repaint();
} else if(menuBox.getItemAt(menuBox.getSelectedIndex()).equals("Gaussian Wave - Dispersive")) {
rightPanel.add(new MyComponent());
rightPanel.revalidate();
rightPanel.repaint();
}
}
});
restart.addActionListener(new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
rightPanel.removeAll();
rightPanel.revalidate();
rightPanel.repaint();
rightPanel.setBackground(Color.RED);
}
});
mainPanel.add(leftPanel);
mainPanel.add(rightPanel);
add(mainPanel);
}
#Override
public Dimension getPreferredSize() {
return new Dimension(screenWidth, screenHeight);
}
}
Like i say the MyComponent class is not working when i add it in this way. Normally when i add this class directly like :
public MyPanel() {
add(new MyComponent);
}
Working perfect, but i want to add it after choose from the list to the divided screen. Thx !
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.geom.GeneralPath;
import java.util.function.Function;
import javax.swing.JComponent;
public class MyComponent extends JComponent {
private int wx, wy;
private double xMin, xMax, yMin, yMax, xInc, time;
private Function<Double, Double>gaussFunction;
private Function<Double, Double>dispersiveGaussFunction;
public MyComponent() {
init();
}
#Override
public void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2 = (Graphics2D) g;
g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
makeGraphics(g2);
}
private void makeGraphics(Graphics2D g2) {
GeneralPath path = new GeneralPath(GeneralPath.WIND_EVEN_ODD);
if(time < 5) {
countGaussianWave(this.time);
double x = xMin;
double y = gaussFunction.apply(x);
path.moveTo(
mapX(x), mapY(y)
);
x+=xInc;
while(x < xMax) {
y = gaussFunction.apply(x);
path.lineTo(
mapX(x), mapY(y)
);
x+=xInc;
}
g2.draw(path);
repaint();
this.time+=0.001;
if(this.time > 3.5) {
this.time = -3.5;
System.out.println(time);
}
}
}
private void init() {
wx = 700;
wy = 700;
xMin = -1;
xMax = 1;
yMin = -1;
yMax = 1;
xInc = 0.01;
setTime(time);
}
private double mapX(double x) {
double fx = wx / 2;
double sx = wx / (xMax - xMin);
return x * fx + sx;
}
private double mapY(double y) {
double fy = wy / 2;
double sy = wy / (yMax - yMin);
return -y * sy + fy;
}
public void countGaussianWave(double time) {
double lambda = 1;
this.gaussFunction = x -> {
return x = Math.exp(-(Math.pow((x-time ), 2)))
* (Math.cos((2*Math.PI*(x-time))/lambda)
+ Math.sin((2*Math.PI*(x-time))/lambda)); // need complex
};
}
// public void countDispersiveGaussianWave(double time) {
//
// double lambda = 1;
// double k = (2 * Math.PI) / lambda;
//
// this.dispersiveGaussFunction = x -> {
// return x = (1/(Math.sqrt(1 + 2 * time)) *
// Math.exp(-(Math.pow((1 / Math.pow(1 + 4 * time, 2)) * (x - k * time), 2)))
// * Math.exp(Math.pow(, b));
// );
// };
// }
public double getTime() {
return time;
}
public void setTime(double time) {
this.time = time;
}
}
app ss

Don't use "==" when comparing objects.
Instead you should be using the equals(...) method.
When you add (or remove) a component to a visible GUI the basic code is:
panel.add(...);
panel.revalidate();
panel.repaint();
Basically you need to make sure the layout manager is invoked otherwise the component has a size of (0, 0) so there is nothing to paint.
Also, you need to override the getPreferredSize() method to return the size of the component. Otherwise the size is (0, 0) so there is nothing to paint.
Edit
I'm guessing your components isn't showing because of your poor GUI design and hardcoding of values. A FlowLayout is a terrible layout to use for ALL the components added to the frame. You need to make an effort to logically organize the components on panels with appropriate layout managers. When lots of components are added the components will wrap to a second row. However, the preferred size does not automatically change so you may not see all the components. So
rightPanel.setPreferredSize(new Dimension(...) );
Don't hardcode a preferred size. Each component is responsible for determining its own size which is why you override the getPreferredSize() method. Then the layout manages can do their job properly. So get rid of all the setPreferrededSize() statements.
The default layout for a Jframe is a BorderLayout. I would stick with that layout. There is no need for your mainPanel as the content pane for the frame is already a JPanel.
So I would rename your "leftPanel" and maybe call it "topPanel". Then you can just add the fixed components to that panel and add that panel to the frame:
JPanel topPanel = new JPanel();
topPanel.add( comboBox );
topPanel.add( startButton );
topPanel.add( restartButton );
frame.add(topPanel, BorderLayout.PAGE_START);
So these components will appear at the top of the frame.
Now there is also no need for the "rightPanel". Instead you can just add your component directly to the frame.
frame.add( new MyComponent(), BorderLayout.CENTER);
frame.revalidate();
frame.repaint();
This component will now appear in the center of the frame and take up all the extra space available in the frame.

To start, you can simplify the start buttons ActionListener logic like so
if(combobox.getSelectedItem().equals("Text")
{
doSomething();
}
Also, after adding or removing new components, you need to call Revalidate & Repaint
If you're trying to paint in one of the components, make sure to #Override it and call it's super method super.paintComponent(g);
You also have 2 action listeners assigned to the same button. I suggest removing one of those
Along with #Camickr's answer of Overriding the getPreferredSize() method, you should change the restart button's ActionListener to this
rightPanel.removeAll();
rightPanel.revalidate();
rightPanel.repaint();
rightPanel.setBackground(Color.RED);
You can Override the getPreferredSize(); method like so:
#Override
public Dimension getPreferredSize()
{
int width = Toolkit.getDefaultToolkit().getScreenSize().width / 2;
int height = Toolkit.getDefaultToolkit().getScreenSize().height;
return new Dimension(width, height);
}

Problems in mixing two JPanels in 1 JFrame

I was mixing two JPanels in one frame and it gives me this output!
Here's my code in which I add the two JPanels:
import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import javax.swing.*;
import java.util.*;
public class Board extends JFrame{
private int width=500;
private int height=450;
Obstacles asd= new Obstacles();
Human human;
private Dimension mindim= new Dimension(width+10,height+10);
Board(){
human = new Human();
this.setTitle("Athwart");
//setLayout(null);
human.add(asd); //!!!we see here, I add asd (which is inherited from a JPanel)
// to another existing JPanel
this.setMinimumSize(mindim); //
this.getContentPane().setLayout(new BorderLayout());
this.getContentPane().add(human);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); //
this.setLocationRelativeTo(null); //
this.setResizable(true); //
pack(); //
setVisible(true);
human.requestFocus(); //
}
}
This is how my Obstacles class looks like.
import javax.swing.*;
import java.awt.*;
public class Obstacles extends JPanel {
private int width=500;
private int height=450;
private Dimension mindim= new Dimension(width+10,height+10);
Obstacles()
{
this.setBackground(Color.white);
// this.addKeyListener(this);
// this.setFocusable(true);
// this.setRequestFocusEnabled(true);
setSize(mindim);
this.setVisible(true);
}
public void paintComponent(Graphics g)
{
super.paintComponent(g); //
g.fillRect(0, 0, 60, 30);
g.setColor(Color.black);
g.draw3DRect(0, 0, 60, 30, true);
g.setColor(Color.black);
}
}
So as you can see the height of the component is 30 and the width is 60 but the Image above shows not even a half of it!
Is there anything I can do to make that two JPanels mixed together? By the way,
I tried using BoxLayout earlier but it didn't worked. Is there something wrong or it's just my IDE is not working properly? Cheers and thanks for an awesome reply. I'm just a starting gui programmer and I really don't know how to handle things. And yeah, if you would ask the complete code, I'll edit this if it would matter. :)

Finally, you put in a requirement:
I was just trying to put an image of a rectangle inside a jframe together with another Jpanel of an image of a circle. to see if how i would be that far in mixing two Jpanels together without overlapping.
Yes, this can be done, say by using a JLayeredPane, you could layer one JPanel over another, but you need to make sure that the upper JPanel is not opaque (setOpaque(false)).
But having said that, I still stand by my comment, that you look to be going about this wrong. You should not create a JPanel for drawing one thing and try to combine multiple JPanels because this can lead to an unholy mess. Instead you should consider creatomg one drawing JPanel, and give it logical objects, say non-GUI Obstacle objects, put them in a collection such as an ArrayList, and then in the drawing JPanel, iterate through all the Obstacles in the drawing JPanel's paintComponent method, drawing each Obstacle as it directs.
Edit
For example:
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Point;
import java.awt.RenderingHints;
import java.util.ArrayList;
import java.util.List;
import javax.swing.*;
public class ObstacleDrawer extends JPanel {
private static final int PREF_W = 800;
private static final int PREF_H = PREF_W;
private List<Obstacle> obstacleList = new ArrayList<>();
public ObstacleDrawer() {
}
public void addObstacle(Obstacle obstacle) {
obstacleList.add(obstacle);
}
#Override
public Dimension getPreferredSize() {
if (isPreferredSizeSet()) {
return super.getPreferredSize();
}
return new Dimension(PREF_W, PREF_H);
}
#Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2 = (Graphics2D) g;
// smooth out the drawing
g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
RenderingHints.VALUE_ANTIALIAS_ON);
// iterate through the obstacle list, drawing each obstacle
for (Obstacle obstacle : obstacleList) {
obstacle.draw(g2);
}
}
private static void createAndShowGui() {
ObstacleDrawer mainPanel = new ObstacleDrawer();
mainPanel.addObstacle(new CircleObstacle(new Point(200, 200), 100, Color.red));
mainPanel.addObstacle(new CircleObstacle(new Point(400, 300), 150, Color.blue));
JFrame frame = new JFrame("ObstacleDrawer");
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(new Runnable() {
public void run() {
createAndShowGui();
}
});
}
}
interface Obstacle {
public Point getCenter();
public void setCenter(Point center);
public int getWidth();
public void setWidth(int width);
public Color getColor();
public void setColor(Color color);
public void draw(Graphics2D g2);
}
class CircleObstacle implements Obstacle {
private Point center;
private int width;
private Color color;
public CircleObstacle(Point center, int width, Color color) {
this.center = center;
this.width = width;
this.color = color;
}
#Override
public Point getCenter() {
return center;
}
#Override
public void setCenter(Point center) {
this.center = center;
}
#Override
public int getWidth() {
return width;
}
#Override
public void setWidth(int width) {
this.width = width;
}
#Override
public Color getColor() {
return color;
}
#Override
public void setColor(Color color) {
this.color = color;
}
#Override
public void draw(Graphics2D g2) {
Color oldColor = g2.getColor();
g2.setColor(color);
int x = center.x - width / 2;
int y = center.y - width / 2;
int height = width;
g2.fillOval(x, y, width, height);
g2.setColor(oldColor);
}
}

When you want to mix JPanels, the best way to do it is to nest them inside another JPanel. For nesting, the best layouts (in my experience) are BoxLayout and GridLayout. The example below tries to replicate the drawing in your JFrame.
JPanel outsidePanel = new JPanel();
Obstacle obstacle1 = new Obstacle();
Human human1 = new Human();
outsidePanel.setLayout(new BoxLayout(BoxLayout.Y_AXIS));
outsidePanel.add(human1);
outsidePanel.add(obstacle1);
Furthermore, I recommend you to actually use BorderLayout in your JFrame, and then add this Panel to the CENTER location of the BorderLayout. Following your example:
this.setLayout(new BorderLayout());
this.add(outsidePanel(), BorderLayout.CENTER);
Usually this will give you the best results. More about nesting panels in the BoxLayout documentation (http://docs.oracle.com/javase/7/docs/api/javax/swing/BoxLayout.html). Hope this helps.