Styjun

I have a gameloop with a triangle that moves around the screen randomly. I want to add a button that when clicked will add another randomly moving triangle. I'm using event listeners and no matter where I place it I'm still getting an exception

Here's my code

import javax.swing.*;
import drawing.Canvas;
import tools.Utils;
import triangle.DynamicTriangle;

import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.ArrayList;

public class TriangleProgram 
 private JFrame frame; 
 private Canvas canvas; 
 private JPanel lowerPanel; 
 private JButton addTriangleButton; 
 private ArrayList<DynamicTriangle> triangles;

 public void gameLoop() 
 int deltaTime = 20;

 triangles.add(new RandomTriangleA(canvas, 150, 150));

 for (DynamicTriangle dynamicTriangle : triangles) 
 dynamicTriangle.drawTriangle();
 
 while (true)
 
 for (DynamicTriangle dynamicTriangle : triangles) 
 dynamicTriangle.undrawTriangle();
 

 for (DynamicTriangle dynamicTriangle : triangles) 
 dynamicTriangle.update(100);
 dynamicTriangle.wrapPosition();
 

 for (DynamicTriangle dynamicTriangle : triangles) 
 dynamicTriangle.drawTriangle();
 

 Utils.pause(deltaTime);
 canvas.repaint();
 

 

 public TriangleProgram() 
 frame = new JFrame();
 frame.setTitle("Canvas");
 frame.setSize(800, 600);
 frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
 frame.setVisible(true);

 canvas = new Canvas();
 frame.add(canvas, BorderLayout.CENTER);

 lowerPanel = new JPanel();
 lowerPanel.setLayout(new FlowLayout());
 frame.add(lowerPanel, BorderLayout.SOUTH);

 triangles = new ArrayList<DynamicTriangle>();

 addTriangleButton = new JButton("Add Triangle"); 
 lowerPanel.add(addTriangleButton);
 frame.revalidate();

 addTriangleButton.addActionListener(new ButtonListener());

 gameLoop();

 

 public static void main(String[] args) 
 System.out.println("Running TriangleProgram...");
 new TriangleProgram();
 

 class ButtonListener implements ActionListener 

 @Override
 public void actionPerformed(ActionEvent ae) 
 System.out.println(triangles.size());
 triangles.add(new RandomTriangleA(canvas, 150, 150));
 

 

I expect every time I press the button that a new triangle is added to the canvas, but instead I get the stack trace:

Running TriangleProgram...
1
Exception in thread "main" java.lang.IndexOutOfBoundsException: Index -1 out-of-bounds for length 0
 at java.base/jdk.internal.util.Preconditions.outOfBounds(Preconditions.java:64)
 at java.base/jdk.internal.util.Preconditions.outOfBoundsCheckIndex(Preconditions.java:70)
 at java.base/jdk.internal.util.Preconditions.checkIndex(Preconditions.java:248)
 at java.base/java.util.Objects.checkIndex(Objects.java:372)
 at java.base/java.util.ArrayList.remove(ArrayList.java:517)
 at java.base/java.util.Collections$SynchronizedList.remove(Collections.java:2424)
 at drawing.Canvas.removeMostRecentLine(Canvas.java:162)
 at triangle.Triangle.undrawTriangle(Triangle.java:131)
 at TriangleProgram.gameLoop(TriangleProgram.java:29)
 at TriangleProgram.<init>(TriangleProgram.java:69)
 at TriangleProgram.main(TriangleProgram.java:75)

 triangle.add(new Triangle(canvas, 150, 150));

Well here you never even declare triangle so not sure how this code even runs to give you an exception. I'm assuming you meant triangles
Also you can probably(?) condense your loops.

Edit: Also when you declare the list triangles, why is it a list of DynamicTriangle but when you initialize and add to list it's Triangle.

Not sure if DynamicTriangle is subclass of Triangle or just typo but if that's the case it shouldn't compile.

If it's a typo then make your ArrayList<DynamicTriangle> triangles use Triangle instead.

If DyanmicTriangle is a subclass of Triangle then you want to declare and initialize triangles as ArrayList<Triangle>() and when you add you do .add(new DynamicTriangle(...