Develop Reference

Animating a shape in a JPanel

first questing for me here. Been searching forever but cant seem to find the answer.
Im working on a school assignment. Got given an ui and are supposed to make the different panels in it do different things in separate threads. Anyway, Im trying to make a triangle rotate inside one of the JPanels. I have managed to draw it and somewhat rotate it, but when I try to make a loop to update it it just blinks and then disappears again. Heres the code Ive written.
StartAssignment, starts the application
public class StartAssignment1 {
public static void main(String[] args) {
Controller controller = new Controller();
}
Controller, recieves calls from the ui and executes various functions in other classes
public class Controller {
private GUIAssignment1 gui = new GUIAssignment1(this);
private RotateShape rotateShape;
private Thread t1;
public Controller() {
SwingUtilities.invokeLater(new Runnable() {
public void run() {
gui.Start();
}
});
}
public void startT1(JPanel panel) {
rotateShape = new RotateShape(panel);
t1 = new Thread(rotateShape);
t1.start();
}
public void t1Shutdown() {
rotateShape.shutdown();
}
RotateShape, where Im trying to rotate the damned thing
public class RotateShape implements Runnable {
JPanel panel;
private volatile boolean t1Running = true;
public RotateShape(JPanel panel) {
this.panel = panel;
}
private void draw() {
Graphics2D g2 = (Graphics2D) panel.getGraphics();
g2.rotate(Math.toRadians(10));
g2.drawPolygon(new int[] {50, 100, 150}, new int[] {150, 50, 150}, 3);
}
public void shutdown() {
t1Running = false;
System.out.println("Shutdown");
}
#Override
public void run() {
while (t1Running) {
try {
draw();
Thread.sleep(500);
System.out.println("loop working");
panel.repaint();
panel.revalidate();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}

I'm not certain because I don't have access to your GUI code and therefore cannot test it myself, but here's an idea:
It looks like you're rotating your Graphics2D object by a fixed amount on every re-render (i.e. every invocation of draw()). It's possible that the internal JPanel code initiates the Graphics2D at a default rotation before every re-render, which may be why your rotation only causes the shape to "somewhat rotate."
Maybe try storing a variable for the radian offset (e.g. double offset;) as a member field of the RotateShapeclass, incrementing it on every re-render, then calling g2.rotate(Math.toRadians(offset));?
Addendum 1:
The reason that it may draw on top of the previous render is because the Graphics2D object is not clearing the canvas between re-renders. One way to fix this is to "clear" the canvas at the beginning of the draw() method by filling a rectangle that covers the whole canvas.
Addendum 2:
When you call panel.repaint(), it triggers the paintComponent() method of the JPanel object. So, by calling repaint() and your own draw() method, you are doing two separate renders, which may cause some graphical errors. If you were able to extends the JPanel class and use that, you should define an override to replace draw():
#Override
public void paintComponent(Graphics g) {
super.paintComponent(g);
// TODO: add your rendering operations here
}
This way, calls to repaint() should trigger this method.

I can't get this shape to move using Keylistener in java

I'm a beginner at java and I'm trying to make a square that moves in response to arrow keys. I've tried a lot of things but it just won't work. There are probably obvious errors in my code/ in my understanding of the code so if anyone could point them out that would be really helpful.
Main class:
import java.awt.;
import javax.swing.;
public class mainClass2 {
public static void main(String[] args) {
JFrame window = new JFrame();
window.setSize(600,400);
window.setVisible(true);
window.getContentPane().setBackground(Color.WHITE);
window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
boolean constant = true;
graphics2 DC = new graphics2();
window.add(DC);}
}
Graphics class:
import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
public class graphics2 extends JComponent implements KeyListener{
public static int x1;
public static int y1;
public static int xvelocity = 0;
public static int yvelocity = 0;
public void paintComponent(Graphics g){
graphics2 thing = new graphics2();
this.addKeyListener(thing);
System.out.println(x1);
System.out.println(y1);
Graphics2D g2 =(Graphics2D)g;
Rectangle rect = new Rectangle(x1,y1,200,200);
g2.setColor(Color.red);
rect.setLocation(x1,y1);
g2.fill(rect);
repaint();
}
#Override
public void keyPressed(KeyEvent e) {
if(e.getKeyCode() == KeyEvent.VK_RIGHT) {
x1+=10;
// TODO Auto-generated method stub
if(e.getKeyCode()== KeyEvent.VK_LEFT) {
x1-=10;
}
}
}
#Override
public void keyReleased(KeyEvent e) {
// TODO Auto-generated method stub
}
#Override
public void keyTyped(KeyEvent e) {
// TODO Auto-generated method stub
}

Let's start with...
public void paintComponent(Graphics g){
graphics2 thing = new graphics2();
this.addKeyListener(thing);
System.out.println(x1);
System.out.println(y1);
Graphics2D g2 =(Graphics2D)g;
Rectangle rect = new Rectangle(x1,y1,200,200);
g2.setColor(Color.red);
rect.setLocation(x1,y1);
g2.fill(rect);
repaint();
}
You're creating a new instance of graphics2 and adding a KeyListener to that instance, but since that instance is never added to anything which could possibly display it, it will never be eligible for receiving key events.
Painting should paint the current state and work as quickly as possible. Creating short lived objects in paintComponent (including the creation of your rect) is generally a bad idea, as it could put unnecessary strain on your system and degrade performance.
Also, calling repaint(); inside paintComponent is a very, very bad idea. paintComponent called be called for any number of reasons, doing this is going to setup a cycle of updates which will consume the CPU and bring your system to its knees (yes, I've done it before).
To schedule regular updates, a better choice would be to use a Swing Timer, see How to use Swing Timers
Another issue is, you component isn't focusable, so it will never be able to receive keyboard focus and won't receive key events.
KeyListener is a poor choice, which has well documented limitations (just search "KeyListener doesn't work"). Instead, you should be using the Key Bindings API which will solve the limitations of the KeyListener
You may also want to have a look at How to use Key Bindings instead of Key Listeners and Key bindings vs. key listeners in Java

Add your graphics2 as a key listener and a component to your JFrame (Which I assume you are using) instead:
graphics2 graphics = new graphics2();
JFrame frame = new JFrame("Your Frame");
frame.add(graphics);
frame.addKeyListener(graphics);
Remove this line of code from graphics2 above:
this.addKeyListener(this);

Repaint function messes up the whole frame

I have a problem when trying to draw some elements using paint method in Swing.
As title says, my whole frame collapses and does some weird repeating.
I made a separate JPanel so I can manipulate drawn shapes:
public class PanelPovrsina extends JPanel{
private ArrayList<Oblik> listaOblika;
public PanelPovrsina() {
// svi oblici
this.listaOblika = new ArrayList<Oblik>();
this.listaOblika.add(new Kvadrat(new Tacka(50, 50), 50, "zuta", "crvena"));
this.setBackground(Color.WHITE);
this.setVisible(true);
}
public void paint(Graphics g) {
if(this.listaOblika.isEmpty()) return;
Iterator<Oblik> it = this.listaOblika.iterator();
while(it.hasNext()) {
it.next().crtajUBoji(g);
}
repaint(); // this causes problems!
}
public ArrayList<Oblik> getListaOblika() {
return this.listaOblika;
}
}
Here is the frame with this code:
And here it is without repaint method:
No, I know repaint method is essential in order to dynamically add shapes and actually draw, but I can't make this work correctly.
Also, as you can see from the code above, background of panel is set to white, but my frame would'n render it.
Hope there is enough information to solve my problem, if not, I will add code of my JFrame!
Thank you!

You should never override the paint method, as it handles a number of other things behind the scenes. You should override paintComponent instead.

As #Joe C answered, I should have been using paintComponent method, not paint! Working code:
public class PanelPovrsina extends JPanel{
private ArrayList<Oblik> listaOblika;
public PanelPovrsina() {
// svi oblici
this.listaOblika = new ArrayList<Oblik>();
this.listaOblika.add(new Kvadrat(new Tacka(50, 50), 50, "zuta", "crvena"));
this.setBackground(Color.PINK);
this.setVisible(true);
}
#Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
for (Oblik obl : this.listaOblika) {
obl.crtajUBoji(g);
}
repaint();
}
public ArrayList<Oblik> getListaOblika() {
return this.listaOblika;
}
}

Does "Thread.sleep()" do anything else than pause the program for a certain amount of time?

I have created a program that draws a thick line.
import javax.swing.*;
import java.awt.*;
public class Movement {
int xGrid = 50;
public static void main(String[] args) {
Movement m = new Movement();
m.animate();
}
public void animate() {
JFrame frame = new JFrame("Movement");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
ScreenDisplay display = new ScreenDisplay();
frame.getContentPane().add(display);
frame.setSize(400, 400);
frame.setVisible(true);
for (int aL = 0; aL < 200; aL++) {
xGrid++;
display.repaint();
try {
Thread.sleep(50);
} catch (Exception ex) { }
}
}
class ScreenDisplay extends JPanel {
public void paintComponent(Graphics g) {
g.setColor(Color.RED);
g.fillOval(xGrid, 175, 50, 50);
}
}
}
Because of the method "Thread.sleep(50)", the speed of the program slows down a little.
So I got a little curious and removed the "sleep()" method.
What I expected to output was the exact same output, just extremely fast.
However, it just prints out one circle in the frame.
I don't really know why it outputs just one circle, none of the researches I've done back up the answer.
Can anyone please explain why?

From Component.repaint documentation:
Repaints this component.
If this component is a lightweight component, this method
causes a call to this component's paint
method as soon as possible. Otherwise, this method causes
a call to this component's update method as soon
as possible.
By the looks of it, your for loop finishes so quickly that by the time the component calls the repaint method, it has already finished and therefore only paints the final circle stored in the buffer.

My JComponent will only repaint() once I manually resize the window, how do I get it to work as it should?

I have tried my best to read around on the topic but I cannot find/understand how to apply the answers to my piece code (and those I have applied don't work)
I have used an example from "Ivor Horton's Beginning Java 2 JDK book" and it is the first time I'm using repaint() but it doesn't seem to work unless I resize the window. It tries to repaint but leaves the screen blank.
Here's the code please let me know if there is something I'm doing wrong.
public class CurveDrawings extends JFrame{
public CurveDrawings (String title){
setTitle(title);
setDefaultCloseOperation(EXIT_ON_CLOSE);
pane = new CurvePane(); //Create pane containing curves
Container content = getContentPane(); //Get the content pane
//Add the pane containing the curves to the content pane for the applet
content.add(pane);
MouseHandler handler = new MouseHandler();
pane.addMouseListener(handler);
pane.addMouseMotionListener(handler);
}
//Class defining pane on which to draw
class CurvePane extends JComponent{
public CurvePane(){
quadCurve = new QuadCurve2D.Double(
startQ.x, startQ.y, control.x, control.y, endQ.x, endQ.y
);
cubicCurve = new CubicCurve2D.Double(
startC.x, startC.y, controlStart.x, controlStart.y,
controlEnd.x, controlEnd.y, endC.x, endC.y
);
}
}
class Marker{//not needed for my problem}
class MouseHandler extends MouseInputAdapter{
public void mousePressed(MouseEvent e){
if(ctrlQuad.contains(e.getX(),e.getY())){
selected = ctrlQuad;}
else if(ctrlCubic1.contains(e.getX(),e.getY())){
selected = ctrlCubic1;}
else if(ctrlCubic2.contains(e.getX(),e.getY())){
selected = ctrlCubic2;}
}
public void mouseReleased (MouseEvent e){
selected = null;
}
public void mouseDragged (MouseEvent e){
System.out.println("DRAGGED");
if(selected != null){
//Set the marker to current cursor position
selected.setLocation(e.getX(),e.getY());
pane.validate();
pane.repaint();
}
}
Marker selected = null;
}
public void paint (Graphics g){
Graphics2D g2D = (Graphics2D)g; //Get a 2D device context
//Code to draw each component
}
//Points for quadratic curve
//Points for cubic curve
//More unnecessary code}
Incase it is any help here's the 'Launcher' class for the application
Thanks in advance.

You will need to call
super.paint(g);
in your paint method in your CurveDrawings class to avail of the painting functionality already provided in the JFrame super class.
Note, the standard way of using custom painting in Swing is to use custom components that are based on javax.swing.JComponent that use and override paintComponent. This approach would leverage Swing's optimized painting model.
See: Performing Custom Painting