Develop Reference

flickering my jframe even i used doubleBuffer

when I call a repaint method its flickering, I was searching on the internet about doubleBuffered but its stil flickering all of objects on the same time,
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.io.*;
import java.util.ArrayList;
import java.util.Random;
public class GuiGame extends JFrame implements ActionListener {
private final Flower flower;
private final ArrayList<MyObjects> objekty;
private Image dbImage;
private Graphics dbGraphics;
public GuiGame() {
this.setFocusable(true);
Timer timer = new Timer(40, this);
timer.start();
this.setVisible(true);
/*
public void paint(Graphics g) {
this.dbImage = this.createImage(this.getWidth(), this.getHeight());
this.dbGraphics = this.dbImage.getGraphics();
this.paintComponents(g);
g.drawImage(this.dbImage, 0 ,0, this);
}
*/
public void paint(Graphics g) {
//when i use doubleBuffering this method was called paintComponents
g.drawImage(background.getImage(), 0, 0, this);
g.drawImage(flower.getImage(), flower.getPozX(), flower.getPozY(), this);
String skore = "Score: " + player.getScore() ;
g.drawString(skore, 20, 50);
this.paintObjects(g);
}
public void paintObjects(Graphics g) {
if (this.objekty != null) {
for (objekty o : this.objekty) {
o.move();
g.drawImage(o.getImage(), o.getPozX(), o.getPozY(), this);
}
}
}
when I used doubleBuffering I tried to slow down a timer e.g. to 1000, it was blank page for most of the time and my objects was there only for moment.
when I do not used it, only last objects which i draw were flickering.
how can I avoid?

I would recommend you start by taking a look at Painting in AWT and Swing and Performing Custom Painting.
Swing components, when used correctly, are double buffered by default.
Painting is very complex, there's a lot of work that goes into a paint pass. When you override a "paint" method, you either need to honour the existing workflow (ie, call the super.paintXxx method) or be prepared to take over the full responsibility of the method. Your code has circumvented whole sections of the paint workflow, this is never a good idea.
It's not generally recommend that you extend from top level containers like JFrame, you're not adding any new functionality and they are generally complex components to start with. JFrame for example, is actually a composite component, that is, it has a number of child components added to it that form it's core functionality
By overriding paint (of JFrame), you start competing with these components, and because a child component can be painted without the parent been involved, this can lead to any number of issues.
Instead, start with a JPanel and override it's paintComponent method, this guarantees that you will be operating in a double buffered workflow.
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.imageio.ImageIO;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.Timer;
public class Main {
public static void main(String[] args) {
new Main();
}
public Main() {
EventQueue.invokeLater(new Runnable() {
#Override
public void run() {
try {
JFrame frame = new JFrame();
frame.add(new MainPane());
frame.pack();
frame.setLocationRelativeTo(null);
frame.setVisible(true);
} catch (IOException ex) {
Logger.getLogger(Main.class.getName()).log(Level.SEVERE, null, ex);
}
}
});
}
public class MainPane extends JPanel {
private int xDelta = 1;
private int xPos = 0;
private BufferedImage ufo;
public MainPane() throws IOException {
ufo = ImageIO.read(getClass().getResource("/images/ufo.png"));
Timer timer = new Timer(5, new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
xPos += xDelta;
if (xPos + ufo.getWidth() > getWidth()) {
xPos = getWidth() - ufo.getWidth();
xDelta *= -1;
} else if (xPos < 0) {
xPos = 0;
xDelta *= -1;
}
repaint();
}
});
timer.start();
}
#Override
public Dimension getPreferredSize() {
return new Dimension(400, 400);
}
#Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2d = (Graphics2D) g.create();
paintUFO(g2d);
g2d.dispose();
}
protected void paintUFO(Graphics2D g2d) {
int y = (getHeight() - ufo.getHeight()) / 2;
g2d.drawImage(ufo, xPos, y, this);
}
}
}

Java Swing: Making a growing circle by mouse click on JPanel

I'm a beginner in Java and this time I'm trying to learn more by finding code examples and editing them, for example from this website. I have a JFrame and each time it (or more precise the JPanel in it) is clicked on, a circle is drawn which will grow/expand outwards like a water ripple. Each circle starts with a certain radius and will be removed or redrawn when reaching a bigger radius (I chose radius "r" from 10 to 200). I have two programs for this which almost work but are missing something. If I'm correct, I might also know what their problems are but I can't figure out how to solve them:
One generates growing circles but all of them have the same size, no matter when they're generated, since I can't find out how to assign the radius to a single circle. The code is adapted from here:
import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Point;
import java.awt.RenderingHints;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.util.ArrayList;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.Timer;
public class Ripples extends JPanel implements ActionListener{
public int r = 10;
private ArrayList<Point> p;
public Ripples() {
p = new ArrayList<>();
addMouseListener(new MouseAdapter() {
#Override
public void mousePressed(MouseEvent e) {
p.add(new Point(e.getX(), e.getY()));
}
});
}
#Override
public void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2 = (Graphics2D) g;
g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
g2.setColor(Color.CYAN);
for (Point pt : p) {
g2.drawOval(pt.x-r, pt.y-r, 2*r, 2*r);
}
}
#Override
public void actionPerformed(ActionEvent evt) {
if(r<200){
r++;
} else {
r = 10;
}
repaint();
}
public static void Gui() {
JFrame f = new JFrame();
Ripples p = new Ripples();
p.setBackground(Color.WHITE);
f.setContentPane(p);
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
f.setSize(500,300);
f.setVisible(true);
Timer t = new Timer(20,p);
t.start();
}
public static void main(String[] args) {
Gui();
}
}
The other program(I've forgotten where I got it from) has circles with different radii depending on when they were generated, however the circles "flicker", because -as far as I understand- they are all processed at the same time, because the code doesn't include an Array to store and update each circle individually like the one above does:
import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.util.ArrayList;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.Timer;
public class Ripples extends JPanel {
int x,y;
int r = 10;
public Ripples(int x, int y) {
this.x = x;
this.y = y;
Timer t = new Timer(20, new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
if (r<200) {
r++;
} else {
r=10;
}
revalidate();
repaint();
}
});
t.start();
}
#Override
public void paintComponent(Graphics g) {
super.paintComponent(g);
g.setColor(Color.CYAN);
Graphics2D g2 = (Graphics2D) g;
g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
g2.drawOval(x-r,y-r,2*r,2*r);
}
public static void Gui() {
JFrame f = new JFrame("Water Ripples");
JPanel p0 = new JPanel();
p0.setBackground(Color.WHITE);
f.add(p0);
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
f.setBounds(100,100,600,500);
f.setVisible(true);
f.addMouseListener(new MouseAdapter() {
#Override
public void mousePressed(MouseEvent e) {
Ripples rG = new Ripples(e.getX(), e.getY());
rG.setBackground(Color.WHITE);
f.add(rG);
}
});
}
public static void main(String[] args) {
Gui();
}
}
So how can I solve this so that I get the circles growing independent from each other? I'd prefer a solution/improvement/hint for the upper code because I think its structured better than the second one. Also, I apologize for not splitting the code into more classes and for possibly not sticking to naming conventions. I appreciate your help, thank you very much!

I added a Circle class to your Ripples code. This allows the ActionListener to treat each circle independently.
I started the GUI with a call to the SwingUtilities invokeLater method. This method ensures that the Swing components are created and executed on the Event Dispatch Thread.
Here's the code.
import java.awt.BasicStroke;
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.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.util.ArrayList;
import java.util.List;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
import javax.swing.Timer;
public class Ripples extends JPanel implements ActionListener {
private List<Circle> circles;
public Ripples() {
circles = new ArrayList<>();
addMouseListener(new MouseAdapter() {
#Override
public void mousePressed(MouseEvent event) {
circles.add(new Circle(event.getPoint()));
}
});
}
#Override
public void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2 = (Graphics2D) g;
g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
RenderingHints.VALUE_ANTIALIAS_ON);
g2.setColor(Color.CYAN);
g2.setStroke(new BasicStroke(3f));
for (Circle circle : circles) {
Point p = circle.getCenter();
int radius = circle.getRadius();
g2.drawOval(p.x - radius, p.y - radius,
2 * radius, 2 * radius);
}
}
#Override
public void actionPerformed(ActionEvent evt) {
for (Circle circle : circles) {
circle.incrementRadius();
}
repaint();
}
public static void createGUI() {
JFrame f = new JFrame("Ripples");
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
Ripples p = new Ripples();
p.setBackground(Color.WHITE);
p.setPreferredSize(new Dimension(500, 500));
f.setContentPane(p);
f.pack();
f.setLocationByPlatform(true);
f.setVisible(true);
Timer t = new Timer(20, p);
t.start();
}
public static void main(String[] args) {
SwingUtilities.invokeLater(new Runnable() {
#Override
public void run() {
createGUI();
}
});
}
public class Circle {
private int radius;
private final Point center;
public Circle(Point center) {
this.center = center;
this.radius = 10;
}
public void incrementRadius() {
radius += 1;
radius = (radius > 200) ? 10 : radius;
}
public int getRadius() {
return radius;
}
public Point getCenter() {
return center;
}
}
}
Edited to add:
I reworked the Ripples class code to separate the concerns. I created a DrawingPanel class to hold the drawing panel, a RipplesListener class to hold the MouseAdapter code, an Animation class to hold the Runnable that runs the animation of the circles, a RipplesModel class to hold the List of Circle instances, and finally, the Circle class.
I could have used a Swing Timer for the animation, but I'm more familiar with creating and running my own animation thread.
Yes, this code is more complicated than the original example. The coding style used here can be carried into larger, more complex Swing GUI development.
Here's the revised code. I hope it's a better example.
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.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import java.util.ArrayList;
import java.util.List;
import java.util.Random;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
public class Ripples implements Runnable {
public static void main(String[] args) {
SwingUtilities.invokeLater(new Ripples());
}
private Animation animation;
private DrawingPanel drawingPanel;
private RipplesModel model;
public Ripples() {
model = new RipplesModel();
}
#Override
public void run() {
JFrame frame = new JFrame("Ripples");
frame.setDefaultCloseOperation(JFrame.DO_NOTHING_ON_CLOSE);
frame.addWindowListener(new WindowAdapter() {
#Override
public void windowClosing(WindowEvent event) {
stopAnimation();
frame.dispose();
System.exit(0);
}
});
drawingPanel = new DrawingPanel(model);
frame.add(drawingPanel, BorderLayout.CENTER);
frame.pack();
frame.setLocationByPlatform(true);
frame.setVisible(true);
animation = new Animation(this, model);
new Thread(animation).start();
}
public void repaint() {
drawingPanel.repaint();
}
private void stopAnimation() {
if (animation != null) {
animation.setRunning(false);
}
}
public class DrawingPanel extends JPanel {
private static final long serialVersionUID = 1L;
private RipplesModel model;
public DrawingPanel(RipplesModel model) {
this.model = model;
setBackground(Color.WHITE);
setPreferredSize(new Dimension(500, 500));
addMouseListener(new RipplesListener(model));
}
#Override
public void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2 = (Graphics2D) g;
g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
RenderingHints.VALUE_ANTIALIAS_ON);
g2.setStroke(new BasicStroke(3f));
List<Circle> circles = model.getCircles();
for (Circle circle : circles) {
Point p = circle.getCenter();
int radius = circle.getRadius();
g2.setColor(circle.getColor());
g2.drawOval(p.x - radius, p.y - radius,
2 * radius, 2 * radius);
}
}
}
public class RipplesListener extends MouseAdapter {
private RipplesModel model;
public RipplesListener(RipplesModel model) {
this.model = model;
}
#Override
public void mousePressed(MouseEvent event) {
model.addCircle(new Circle(event.getPoint(),
createColor()));
}
private Color createColor() {
Random random = new Random();
int r = random.nextInt(255);
int g = random.nextInt(255);
int b = random.nextInt(255);
return new Color(r, g, b);
}
}
public class Animation implements Runnable {
private volatile boolean running;
private Ripples frame;
private RipplesModel model;
public Animation(Ripples frame, RipplesModel model) {
this.frame = frame;
this.model = model;
this.running = true;
}
#Override
public void run() {
while (running) {
sleep(20L);
incrementRadius();
}
}
private void incrementRadius() {
List<Circle> circles = model.getCircles();
for (Circle circle : circles) {
circle.incrementRadius();
}
repaint();
}
private void sleep(long delay) {
try {
Thread.sleep(delay);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
private void repaint() {
SwingUtilities.invokeLater(new Runnable() {
#Override
public void run() {
frame.repaint();
}
});
}
public synchronized void setRunning(boolean running) {
this.running = running;
}
}
public class RipplesModel {
private List<Circle> circles;
public RipplesModel() {
this.circles = new ArrayList<>();
}
public void addCircle(Circle circle) {
this.circles.add(circle);
}
public List<Circle> getCircles() {
return circles;
}
}
public class Circle {
private int radius;
private final Color color;
private final Point center;
public Circle(Point center, Color color) {
this.center = center;
this.color = color;
this.radius = 10;
}
public void incrementRadius() {
radius = (++radius > 200) ? 10 : radius;
}
public Color getColor() {
return color;
}
public int getRadius() {
return radius;
}
public Point getCenter() {
return center;
}
}
}

I'd prefer a solution/improvement/hint for the upper code
The second code is better because it uses:
a custom class to contain information about the object to be painted
an ArrayList to contain the objects to be painted
a Timer for the animation.
because I think its structured better than the second one.
Not a good reason. Use the code that provides the functionality that you require.
Restructure the code yourself. That is part of the learning experience.
Issues with the second code:
It doesn't compile. Why post code that doesn't compile? This implies you haven't even tested it.
the initial radius is assigned when the Position object is created.
When the Timer fires you need to iterate through the ArrayList to update the radius of each Position object.
The radius of the Position object is used in the painting code.
As an added change, maybe call the Position class Ripple. Then you can add another custom property for the Color of the ripple. Then when you add the Ripple to the ArrayList you randomly generate a Color. Then in the painting method you use the Color property of the Ripple class. This is how you make objects and painting more flexible and dynamic.

Moving an image to the position of the cursor where is clicked on a JFrame

i need some help here with an exercise of my Java class. What i'm trying to do is that when i click on any part of the JFrame, an image moves closer to the pointer. Right now i have done the part that once i click on the JFrame the image is in the same position as the pointer but it does like it's "teleporting" and i'm trying to make it more like a constant movement to the pointer position.
So far this is my code i have atm:
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class ControlaRaton extends MouseAdapter {
JLabel label;
public ControlaRaton(JLabel label){
this.label = label;
}
public void mouseClicked(MouseEvent evt){
Point pos = evt.getPoint();
System.out.println(pos.x+" "+pos.y);
//System.out.println("Boton: "+evt.getButton());
label.setLocation(pos.x-20,pos.y-50);
}
}
Any ideas on how to do it that way? I was thinking maybe using a Thread but i don't know exactly how to implement it here :s

This is pretty simple approach, basically all this does is uses a Swing Timer to move an entity towards the last known click point
Have a look at
Concurrency in Swing
How to use Swing Timers
How to Write a Mouse Listener
for more details
Swing is NOT thread safe, when performing these types of operations, it's important to take this into consideration. The Swing API provides several ways to work threads, in this case, I've used a simple Timer as it generates updates at a regular interval and triggers updates from within the EDT, making it safe to update the UI from within
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Point;
import java.awt.RenderingHints;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.geom.AffineTransform;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;
import javax.imageio.ImageIO;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.Timer;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;
public class MoveTowards {
public static void main(String[] args) {
new MoveTowards();
}
public MoveTowards() {
EventQueue.invokeLater(new Runnable() {
#Override
public void run() {
try {
UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
} catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
ex.printStackTrace();
}
JFrame frame = new JFrame("Testing");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.add(new MoveTowardsPane());
frame.pack();
frame.setLocationRelativeTo(null);
frame.setVisible(true);
}
});
}
class MoveTowardsPane extends JPanel {
private final BufferedImage image;
private Point imagePosition = new Point(150, 150);
private Point mousePoint;
private double imageAngleRad = 0;
public MoveTowardsPane() {
BufferedImage i = null;
try {
i = ImageIO.read(getClass().getResource("/sprite.png"));
} catch (IOException e) {
e.printStackTrace();
}
image = i;
addMouseListener(new MouseAdapter() {
#Override
public void mouseClicked(MouseEvent e) {
mousePoint = e.getPoint();
double dx = e.getX() - imagePosition.getX();
double dy = e.getY() - imagePosition.getY();
imageAngleRad = Math.atan2(dy, dx);
repaint();
}
});
Timer timer = new Timer(40, new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
if (mousePoint != null) {
int centerX = imagePosition.x + (image.getWidth() / 2);
int centerY = imagePosition.y + (image.getHeight() / 2);
if (mousePoint.x != centerX) {
imagePosition.x += mousePoint.x < centerX ? -1 : 1;
}
if (mousePoint.y != centerY) {
imagePosition.y += mousePoint.y < centerY ? -1 : 1;
}
repaint();
}
}
});
timer.start();
}
#Override
public Dimension getPreferredSize() {
return new Dimension(400, 400);
}
#Override
protected void paintComponent(Graphics gr) {
super.paintComponent(gr);
Graphics2D g = (Graphics2D) gr.create();
g.setRenderingHint(
RenderingHints.KEY_RENDERING,
RenderingHints.VALUE_RENDER_QUALITY);
int cx = image.getWidth() / 2;
int cy = image.getHeight() / 2;
AffineTransform oldAT = g.getTransform();
g.translate(cx + imagePosition.x, cy + imagePosition.y);
g.rotate(imageAngleRad);
g.translate(-cx, -cy);
g.drawImage(image, 0, 0, null);
g.setTransform(oldAT);
g.dispose();
}
}
}
Why doesn't this use a JLabel? A lot of reasons, JLabel isn't really well fitted for the task, as it needs to take into account a lot of other information. This example also "turns" the sprite towards the click point, something which isn't easily achieved with a JLabel.
In principle, the theory is still the same for moving a component.
See
Painting in AWT and Swing
Performing Custom Painting
for more details about how this approach works

Redraw your circle on mouse move
void mouseMoved(MouseEvent evt){
Point pos = evt.getPoint();
System.out.println(pos.x+" "+pos.y);
//System.out.println("Boton: "+evt.getButton());
label.setLocation(pos.x-20,pos.y-50);
}
});

If you want to move the label just a bit with each click you could do the following:
public void mouseClicked(MouseEvent evt){
Point clickedPos = evt.getPoint();
Point newPosForLabel = calculateNewPos(clickedPos, labelPos);
label.setLocation(newPosForLabel);
}
private Point calculateNewPos(Point clickedPos, Point labelPos) {
// calculate newPos based on labelPos and clickedPos
return newPos;
}
else use the timer from Hannes or from the link given by MadProgrammer:

How to move a rectangle using keys in java

I have been trying to find out how to move a rectangle with the arrow keys but there seems to be a problem.
I am using a KeyListener to detect all of the key inputs.
I do not know how to use a KeyBinding therefore I do not wish for the solution to have it.
I am planning to learn it right after mastering KeyListener. Please give me suggestions on how to fix it.
package expo;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Rectangle;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.swing.JFrame;
import javax.swing.JPanel;
public class Expo extends JPanel implements KeyListener{
int x = 0;
int y = 0;
#Override
public void keyTyped(KeyEvent e) {
//System.out.println("Key Typed");
}
#Override
public void keyPressed(KeyEvent e) {
if(e.getKeyCode() == KeyEvent.VK_DOWN){
System.out.println("Key Pressed" + e.getKeyCode());
y = y + 2;
}
}
#Override
public void keyReleased(KeyEvent e) {
//System.out.println("Key Released");
}
public void paint(Graphics g){
g.setColor(Color.BLUE);
g.drawRect(x ,y,100,100);
repaint();
}
public static void main(String[] args) throws InterruptedException {
Expo expo = new Expo();
JFrame f = new JFrame();
f.setVisible(true);
f.setSize(500,500);
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
f.addKeyListener(expo);
f.add(expo);
f.repaint();
}
}

Use the key bindings API over KeyListener, it solves the focus related issues that KeyListener suffers from. How to Use Key Bindings
Don't break the paint chain. If you override one of the paint methods, you must call it's super implementation.
Avoid overriding paint, it's normally to high in the paint process and because of the way painting works, doesn't always get called when a child component is painted, which can cause some interesting issues. Convention recommends using paintComponent instead. See Painting in AWT and Swing and Performing Custom Painting for more details
Try calling JFrame#setVisible last, after you have established the UI, you will find it causes less issues
As an example...
import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.KeyEvent;
import javax.swing.AbstractAction;
import javax.swing.Action;
import javax.swing.ActionMap;
import javax.swing.InputMap;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.KeyStroke;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;
public class Expo extends JPanel {
int x = 0;
int y = 0;
public Expo() {
bindKeyStrokeTo(WHEN_IN_FOCUSED_WINDOW, "down", KeyStroke.getKeyStroke(KeyEvent.VK_DOWN, 0), new AbstractAction() {
#Override
public void actionPerformed(ActionEvent e) {
y += 2;
if (y + 100 > getHeight()) {
y = getHeight() - 100;
}
repaint();
}
});
bindKeyStrokeTo(WHEN_IN_FOCUSED_WINDOW, "up", KeyStroke.getKeyStroke(KeyEvent.VK_UP, 0), new AbstractAction() {
#Override
public void actionPerformed(ActionEvent e) {
y -= 2;
if (y < 0) {
y = 0;
}
repaint();
}
});
}
public void bindKeyStrokeTo(int condition, String name, KeyStroke keyStroke, Action action) {
InputMap im = getInputMap(condition);
ActionMap am = getActionMap();
im.put(keyStroke, name);
am.put(name, action);
}
#Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
g.setColor(Color.BLUE);
g.drawRect(x, y, 100, 100);
}
#Override
public Dimension getPreferredSize() {
return new Dimension(500, 500);
}
public static void main(String[] args) throws InterruptedException {
EventQueue.invokeLater(new Runnable() {
#Override
public void run() {
try {
UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
} catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
ex.printStackTrace();
}
JFrame frame = new JFrame("Testing");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.add(new Expo());
frame.pack();
frame.setLocationRelativeTo(null);
frame.setVisible(true);
}
});
}
}
That's all cool and everything, but now, when you hold down the key, the rectangle moves, pauses and then begins to move steadily!?
This is actually quite normal. Instead, what you can do, is establish an "update loop" which constantly monitors the state of a set of flags, makes decisions about what to do when those flags are set and updates the UI.
So, what this does, is sets up a Swing Timer which ticks every 40 milliseconds, checks the state of the current "vertical key state", updates the y position accordingly and schedules a repaint, this allows for a much more smoother movement as we're not reliant on the repeating key stroke.
This also demonstrates the power of the key bindings API, as it establishes a single Action to handle both the up and down movements and the associated key releases...neat
import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.KeyEvent;
import javax.swing.AbstractAction;
import javax.swing.Action;
import javax.swing.ActionMap;
import javax.swing.InputMap;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.KeyStroke;
import javax.swing.Timer;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;
public class Expo extends JPanel {
int x = 0;
int y = 0;
public enum VerticalKey {
UP, DOWN, NONE;
}
public enum HorizontalKey {
LEFT, RIGHT, NONE;
}
private VerticalKey verticalKeyState = VerticalKey.NONE;
private HorizontalKey horizontalKeyState = HorizontalKey.NONE;
public Expo() {
bindKeyStrokeTo(WHEN_IN_FOCUSED_WINDOW, "pressed.down", KeyStroke.getKeyStroke(KeyEvent.VK_DOWN, 0), new VerticalAction(VerticalKey.DOWN));
bindKeyStrokeTo(WHEN_IN_FOCUSED_WINDOW, "released.down", KeyStroke.getKeyStroke(KeyEvent.VK_DOWN, 0, true), new VerticalAction(VerticalKey.NONE));
bindKeyStrokeTo(WHEN_IN_FOCUSED_WINDOW, "pressed.up", KeyStroke.getKeyStroke(KeyEvent.VK_UP, 0), new VerticalAction(VerticalKey.UP));
bindKeyStrokeTo(WHEN_IN_FOCUSED_WINDOW, "released.up", KeyStroke.getKeyStroke(KeyEvent.VK_UP, 0, true), new VerticalAction(VerticalKey.NONE));
Timer timer = new Timer(40, new ActionListener() {
#Override
public void actionPerformed(ActionEvent e) {
switch (verticalKeyState) {
case UP:
y -= 2;
break;
case DOWN:
y += 2;
break;
}
if (y + 100 > getHeight()) {
y = getHeight() - 100;
} else if (y < 0) {
y = 0;
}
repaint();
}
});
timer.start();
}
public void bindKeyStrokeTo(int condition, String name, KeyStroke keyStroke, Action action) {
InputMap im = getInputMap(condition);
ActionMap am = getActionMap();
im.put(keyStroke, name);
am.put(name, action);
}
#Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
g.setColor(Color.BLUE);
g.drawRect(x, y, 100, 100);
}
public void setVerticalKeyState(VerticalKey verticalKeyState) {
this.verticalKeyState = verticalKeyState;
System.out.println(verticalKeyState);
}
public void setHorizontalKeyState(HorizontalKey horizontalKeyState) {
this.horizontalKeyState = horizontalKeyState;
}
public class VerticalAction extends AbstractAction {
private VerticalKey verticalKey;
public VerticalAction(VerticalKey verticalKeys) {
this.verticalKey = verticalKeys;
}
#Override
public void actionPerformed(ActionEvent e) {
setVerticalKeyState(verticalKey);
}
}
#Override
public Dimension getPreferredSize() {
return new Dimension(500, 500);
}
public static void main(String[] args) throws InterruptedException {
EventQueue.invokeLater(new Runnable() {
#Override
public void run() {
try {
UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
} catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
ex.printStackTrace();
}
JFrame frame = new JFrame("Testing");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.add(new Expo());
frame.pack();
frame.setLocationRelativeTo(null);
frame.setVisible(true);
}
});
}
}

Just change these two pieces of code:
#Override
public void keyPressed(KeyEvent e) {
if(e.getKeyCode() == KeyEvent.VK_DOWN){
System.out.println("Key Pressed" + e.getKeyCode());
y = y + 2;
repaint();//add this line to update the UI
}
}
and
public void paint(Graphics g){
super.paint(g);//you should always call the super-method first
g.setColor(Color.BLUE);
g.drawRect(x ,y,100,100);
}
This would fix the issue. Though i'd recommend to override paintComponent instead of paint (read the section about "The Paint Methods" in this article: Painting in AWT and Swing). Basically the changes are i made change the following:
In keyPressed this line: repaint(); will update the UI after the square moved. Actually the square was moved before aswell, but the changes won't show until the UI is updated. In paint this line: super.paint(g); makes the panel do it's default painting first, which includes clearing the complete panel. And i've removed the repaint(); call, since it's completely useless.
NOTE:
if you use paintComponent instead of paint, you'll have to change the first call from super.paint(g) to super.paintComponent(g) to avoid a stackoverflow.

Drawing a polygon based on clicks

I am trying to write a program that allows the user to draw a polygon by drawing a line every time they click within the frame. On the first click, a small square should be drawn. Every click following, a line should be drawn from where the endpoint of the last line is to where the user has clicked. Once the user clicks within the square that was made originally, the polygon will be completed and the square will disappear. My code is as follows; it runs but it does not operate correctly.
import java.awt.Rectangle;
import java.awt.event.ActionListener;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.awt.geom.Point2D;
import java.awt.geom.Point2D.Double;
import java.util.ArrayList;
import javax.swing.JFrame;
public class DrawPolygonComponent {
public static void main(String[] args) {
final JFrame frame = new JFrame();
frame.setSize(300, 400);
frame.setTitle("Draw a Polygon");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
final ArrayList<Point2D.Double> path = new ArrayList();
//addRectangle(frame, 10, 10);
//
class ClickListener implements MouseListener {
#Override
public void mouseClicked(MouseEvent me) {
if (path.isEmpty()) {
System.out.println("First");
addRectangle(frame, me.getX(),me.getY());
path.add(new Point2D.Double(me.getX(),me.getY()));
} else {
System.out.println("Second");
Point2D.Double prev = path.get(path.size()-1);
addLine(frame, (int) prev.x, (int) prev.y,me.getX(),me.getY());
path.add(new Point2D.Double(me.getX(),me.getY()));
frame.repaint();
}
}
public void mousePressed(MouseEvent me) {}
public void mouseReleased(MouseEvent me) {}
public void mouseEntered(MouseEvent me) {}
public void mouseExited(MouseEvent me) {}
}
MouseListener listener = new ClickListener();
frame.addMouseListener (listener);
frame.setVisible (true);
}
public static void addRectangle(JFrame frame, int x , int y) {
RectangleComponent r = new RectangleComponent(x, y, 10, 10);
frame.add(r);
}
public static void addLine(JFrame frame, int x1, int y1, int x2, int y2) {
LineComponent line = new LineComponent(x1, y1, x2, y2);
frame.add(line);
}
}
/////////other
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Rectangle;
import javax.swing.JComponent;
public class LineComponent extends JComponent {
private int px;
private int py;
private int x;
private int y;
public LineComponent(int px, int py, int x, int y){
this.px=px;
this.py=py;
this.x=x;
this.y=y;
}
#Override
public void paintComponent(Graphics g){
Graphics2D g2 = (Graphics2D)g;
g2.drawLine(px,py,x,y);
}
}
////////other
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Rectangle;
import javax.swing.JComponent;
public class RectangleComponent extends JComponent {
private Rectangle box;
public RectangleComponent(int x,int y, int l, int w){
box = new Rectangle(x,y,l,w);
}
public void paintComponent(Graphics g){
Graphics2D g2 = (Graphics2D)g;
g2.fill(box);
g2.draw(box);
}
}

You've attached your mouse listener to the frame, but not provided any means for the frame to paint your paths...
The component you have setup to "apparently" paint the polygon has not been added to the frame.
Instead.
Create a custom component, using something like JPanel. Attach the mouse listener to this component. Override it's paintComponent method. When a mouse event occurs (that should generate a new line), call repaint to request an update to the component.
Within the paintComponent method, re-draw all the lines.
Take a look at Java Applet Polygon array and How can I draw a polygon using path2d and see if a point is within it's area? and drawPolygon keeps drawing lines from starting (mousePressed) location to current (mouseDragged) location for some conceptional ideas
Ps You may also like to check out Performing Custom Painting