java.lang.NullPointerException while drawing in java.awt - graphics

Question

I want to draw some balls and interact with them, but when I run my program I see my balls staying still and i get java.lang.NullPointerException error. Can u tell me what I'm doing wrong?
Main class:
public class Main extends JPanel {
// creating big ball
Big big = new Big(this);
// creating small balls
Small small_list = new Small(40);
// method for moving big ball
private void moveBig() throws InterruptedException{
big.move();
}
// same but small balls
private void moveSmall() throws InterruptedException{
// ERROR
small_list.move();
}
// paiting
#Override
public void paint(Graphics g){
super.paint(g);
Graphics2D g2d = (Graphics2D) g;
g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
big.paint(g2d);
big.paintLine(g2d);
small_list.paint(g2d);
}
public static void main(String[] args) throws InterruptedException {
// TODO code application logic here
JFrame window = new JFrame("test");
Main main = new Main();
window.add(main);
window.setSize(500,500);
window.setVisible(true);
window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
while(true){
main.moveBig();
// ERROR
main.moveSmall();
main.repaint();
Thread.sleep(15);
}
}
}
Big ball class:
public class Big {
// coordinates of first apperence
int x = 200;
int y = 200;
int upAnddown = 1;
int leftAndright = 1;
private static final int sizeOfbig = 30;
// list to draw a line after ball
private List<Point> listofpoints = new ArrayList<>();
private Main main;
public Big(Main main){
this.main=main;
}
void move() throws InterruptedException{
if (x+upAnddown < 5) {
upAnddown=1;
}
if (x+upAnddown > main.getWidth()-14) {
upAnddown=-1;
}
if (y+leftAndright < 5) {
leftAndright=1;
}
if (y+leftAndright > main.getHeight()-14) {
leftAndright=-1;
}
(...)
Small ball class:
public class Small {
int x = 20;
int y=20;
private static final int sizeOfSmall = 10;
int upAnddown = 1;
int leftAndright = 1;
private Main main;
public Small(Main main){
this.main=main;
}
public Small(int j){
ArrayList<Small> my_array = new ArrayList<Small>(j);
for (int i = 0; i < my_array.size(); i++) {
i=j;
my_array.add(new Small(i));
}
}
void move() throws InterruptedException{
//////////////////////////////////////
if (x+upAnddown < 0) {
upAnddown=1;
}
// ERROR
if (x+upAnddown > main.getWidth()-30) {
upAnddown=-1;
}
if (y+leftAndright < 0) {
leftAndright=1;
}
if (y+leftAndright > main.getHeight()-30) {
leftAndright=-1;
}
(...)
ERROR:
Exception in thread "main" java.lang.NullPointerException
at test2.Small.move(Small.java:56)
at test2.Main.moveSmall(Main.java:34)
at test2.Main.main(Main.java:67)

Answer 1

The while(true) will take all UI thread time. You need to put it in a thread to avoid that.
That's why balls stay still.
new Thread(new Runnable(){
public void run(){
while(true) {
main.moveBig();
main.moveSmall();
main.repaint();
Thread.sleep(15);
}
}
}).Start();
The NullPointerException may be related to this issue, I'm not sure. I don't see anything else that could raise it.