How to add objects to an ArrayList? - oop

Question

I am very new on programming, and I am trying to build a little game so I can teach myself OOP, however I have an problem that I cant solve it at all.
I have a class ComponentsPanel from which I am calling another (JavaFish) as an ArrayList. The addFish() method adds to the ArrayList a JavaFish. When I call addFish() method from the constructor works perfectly fine.
The problem is, when I call it from the start(), from another class and package, it dose not add any element to the ArrayList.
Main class
package framework;
public class Core {
JFrame window;
ComponentsPanel panel;
int width = 600;
int height = 400;
public void start() {
window = new JFrame();
window.setTitle("Java Game");
window.setPreferredSize(new Dimension(width, height));
window.setMinimumSize(new Dimension(width, height));
window.setMaximumSize(new Dimension(width, height));
window.setResizable(false);
panel = new ComponentsPanel();
window.add(panel, BorderLayout.CENTER);
window.setVisible(true);
window.pack();
}
public void stop() {
window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}
}
ComponentsPanel
package framework;
public class ComponentsPanel extends JPanel {
ArrayList<JavaFish> javaFishes = new ArrayList<JavaFish>();
public ComponentsPanel() {
javaFishes = new ArrayList<JavaFish>();
//addFish(new JavaFish(100, 100, 25, 25));
}
public void addFish(JavaFish javaFish) {
javaFishes.add(javaFish);
repaint();
}
public void start() {
addFish(new JavaFish(100, 100, 25, 25));
}
#Override
public void paint(Graphics g) {
for (JavaFish jFish : javaFishes) {
jFish.draw(g);
}
}
}
Fish class
package framework;
public class JavaFish {
BufferedImage img;
private int x_pos;
private int y_pos;
private int sizeX;
private int sizeY;
public JavaFish(int x_pos, int y_pos, int sizeX, int sizeY) {
this.x_pos = x_pos;
this.y_pos = y_pos;
this.sizeX = sizeX;
this.sizeY = sizeY;
}
public void draw(Graphics g) {
BufferedImage JavaFish = LoadImage("img/JavaFish.png");
Graphics2D g2d = (Graphics2D) g;
g2d.drawImage(JavaFish, x_pos, y_pos, sizeX, sizeY, null);
}
BufferedImage LoadImage(String FileName) {
img = null;
try {
img = ImageIO.read(new File (FileName));
} catch (IOException e) {
e.printStackTrace();
}
return img;
}
}
Simulation class
package user;
import framework.ComponentsPanel;
import framework.Core;
public class Simulation {
Core c;
ComponentsPanel panel;
boolean endSim = false;
public Simulation() {
c = new Core();
panel = new ComponentsPanel();
}
private void populate() {
panel.start()
}
private void updateWorld() {
while (!endSim) {
c.start();
break;
}
c.stop();
}
public static void main(String[] args) {
Simulation simulation = new Simulation();
simulation.populate();
simulation.updateWorld();
}
}

Answer 1

start() method present in ComponentsPanel is not a lifecycle method. So unless you invoke start() method manually it won't get invoked. You can add a print statement for tracking the calls.

Answer 2

#Cristea Thank you for the updated code.
This will work if you call the addFish() method on the ComponentsPanel instance created in Core class's start() method as below.
public void start() {
window = new JFrame();
window.setTitle("Java Game");
window.setPreferredSize(new Dimension(width, height));
window.setMinimumSize(new Dimension(width, height));
window.setMaximumSize(new Dimension(width, height));
window.setResizable(false);
panel = new ComponentsPanel(); // This panel is passed to the JFrame
panel.addFish(new JavaFish(100, 100, 25, 25)); // so do addFish() here.
window.add(panel, BorderLayout.CENTER);
window.setVisible(true);
window.pack();
}
The problem with your code was, you are creating two instances of ComponentsPanel class. On the first instance(let's call it panel-1 ) you created, you are calling addFish() method, through the method below. This start() method is called on panel-1.
// panel you used to call this start() method was not used in JFrame
public void start() {
addFish(new JavaFish(100, 100, 25, 25));
}
Then you are creating 2nd instance of ComponentsPanel class(let's call it panel-2) from the Core class's start() method and panel-2 does not do addFish(). But this instance (panel-2)is what is added to the JFrame, which will be used for the purpose of paint(). Since panel-2 does not do addFish(), paint() does not find any JavaFishes.
Remember, instance variable javaFishes is specific to each instance of ComponentsPanel. javaFishes added by panel-1 would not be available for panel-2.