So up until this point I've just used points.add(new Point(x,y))
to add points to my ArrayList
. However, I found out that I need the first point to be in index=1 in order to make it possible to multiply the number for each step. So I tried setting the counter
from 0
to 1
, and as expected I knew I would get a error because of the range, but I've tried changing up the condition within the while-loop, but nothing seems to work.
Here's my code:
ArrayList<Point> points = new ArrayList<>();
int counter = 1;
int nPoints = 12;
while (counter <= nPoints) {
x = (int) (centerX + r * Math.cos(start));
y = (int) (centerY + r * Math.sin(start));
points.set(counter, (new Point(x, y)));
//points.add(new Point(x,y));
counter++;
}
This is the error that I am getting:
Exception in thread "main" java.lang.IndexOutOfBoundsException: Index: 1, Size: 0
at java.util.ArrayList.rangeCheck(Unknown Source)
at java.util.ArrayList.set(Unknown Source)
I've deleted much of the code that isn't relevant for this problem in order to make it easier to read.
Edit:
public void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2d = (Graphics2D) g;
a = getWidth() /2;
b = getHeight() /2;
g2d.setColor(Color.RED);
//draw cirle
g2d.drawOval(a-r, b-r, 2*r, 2*r);
//draw lines
for (int i= 1; i < points.size();i ++) {
g2d.drawLine(points.get(i).x, points.get(i).y, points.get(i+1).x, points.get(i+1).y);
}
g2d.dispose();
}
I've created a circle in which I'm trying to draw lines between points along the circumference. Here I want the 1st point to connect to 2, 2 to 4, 3 to 6, 4 to 8 and so on... So the pattern here is is that it is multiplied by 2 for each time. So my initial thought was that I could use the i
inside the for
-loop to multiply by 2 each time. But since I have my first point in i=0 inside the ArrayList, I am having trouble.
Your trying to set an index
that doesn't exist.
The size of your ArrayList
is 0 but you are trying to access index 1
which doesn't exist.
You can use ArrayList.add()
to add another element to your ArrayList
. If you use set()
the index
must actually exist in the ArrayList
.
To insert an element at index 0
use points.add(null);
before loop.
So the final code looks like:
ArrayList<Point> points = new ArrayList<>();
int counter = 1;
int nPoints = 12;
points.add(null); // <= this line was added
while (counter <= nPoints) {
x = (int) (centerX + r * Math.cos(start));
y = (int) (centerY + r * Math.sin(start));
//points.set(counter, (new Point(x, y))); // <= it's not correct use
points.add(new Point(x,y));
counter++;
}
Use add
instead of set
. You can not set (replace) an item that does not exist. From the docs on set
:
Replaces the element at the specified position in this list with the specified element.
You can use index + 1
for your calulations, but list indices should start at 0.
Edit:
//draw lines
for (int i= 0; i < points.size() - 1;i ++){
g2d.drawLine(points.get(i).x,
points.get(i).y, points.get(i+1).x,
points.get(i+1).y);
}
ArrayList
s in Java do not extend to the desired length if you try to set a value outside of it's range. You are trying to set item at index 1, when the array has 0 length. Try this:
ArrayList<Point> points = new ArrayList<>();
points.add(null);
for (int counter = 0; counter < nPoints; counter++) {
x = (int) (centerX + r * Math.cos(start));
y = (int) (centerY + r * Math.sin(start));
points.add(new Point(x,y));
}
points.set(0, points.get(1));
EDIT: you are generating the same Point
nPoints times. Do something with the angle, for example:
x = (int) (centerX + r * Math.cos(start + counter * 0.01));
y = (int) (centerY + r * Math.sin(start + counter * 0.01));
EDIT: replacing the 0th element with the 1st
EDIT: now that you provided the paint code, I see that your algorithm does not necessarily use 1-based indices. By the way I highly doubt that you got a null point out of the list. I suspect that even your list is not saved, and got null pointer exception on the list, not on the point. Anyway, try this:
ArrayList<Point> points = new ArrayList<>();
for (int counter = 0; counter < nPoints; counter++) {
x = (int) (centerX + r * Math.cos(start));
y = (int) (centerY + r * Math.sin(start));
points.add(new Point(x,y));
}
// ...
public void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2d = (Graphics2D) g;
a = getWidth() /2;
b = getHeight() /2;
g2d.setColor(Color.RED);
//draw cirle
g2d.drawOval(a-r, b-r, 2*r, 2*r);
//draw lines
for (int i= 1; i < points.size();i ++) {
g2d.drawLine(points.get(i - 1).x, points.get(i - 1).y, points.get(i).x, points.get(i).y);
}
g2d.dispose();
}
In my case that helped:
if (!listName.isEmpty()) {
return listName.get(0);
}
Hope it works by you ☺️
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