We Keep Coding

Java how to draw and fill a Polygon which has holes

I am currently trying to draw and fill a Polygon which has a hole in it in Java. Normally this would not be a big problem, since I would draw the exterior ring and then draw the interior ring with the color of the background.
But the problem is, that the polygon is displayed above a image which should be "seen" through the hole.
I am writing the code in Java and am using JTS Topology Suite for my geometry data.
This is my current code, which just paints the border and fills the polygon with a color.
private void drawPolygon(com.vividsolutions.jts.geom.Polygon gpoly, Color color, Graphics2D g2d){
java.awt.Polygon poly = (java.awt.Polygon)gpoly;
for(Coordinate co : gpoly.getExteriorRing().getCoordinates() {
poly.addPoint(co.x, co.y);
}
g2d.setColor(col);
g2d.fill(poly);
g2d.setColor(Color.BLACK);
g2d.draw(poly);
}
Sadly java.awt.Polygon does not support Polygons with holes.

Use the Polygon as the basis for an Area (e.g. called polygonShape).
Create an Ellipse2D for the 'hole', then establish an Area for it (ellipseShape).
Use Area.subtract(Area) something like:
Area polygonWithHole = polygonShape.subtract(ellipseShape);

There are some ways to draw shapes or areas that are more complex than a simple polygon (another answer already mentioned Area).
Besides those, you could try to tessellate your final polygon. There are lots of algorithms to do this. For more complex shapes, the algorithms get a bit more complex as well. Basically, you're dividing your final shape into little polygons (usually triangles, but it also can be something else) and then draw those polygons.
You can take a look at your possibilities by searching for "Tessellation Algorithm", there are also some already implemented libraries for Java.

You can use java.awt.geom.Path2D to render a "compound shape" with a hole in it:
If you have java.awt.Shape objects defining the outside & inside edges of the shape, use append(shape, false) to add each shape.
If you have a set of path points for the outside edge and a set of path points for the inside edge, use lineTo() to add the first set of points - creating a closed loop by either ending with the same point you started with, or calling closePath() to automatically close the loop. Then use moveTo() to create a break before adding the inner set of points via more lineTo() calls.
In either case, you must create the path passing Path.WIND_NON_ZERO to the constructor - otherwise the hole won't be left unfilled.
See How to create shape with a hole? for a longer code example.

You could fill the polygon first, and then draw the holes on top, giving the illusion that it filled everything but the holes.

LibGDX collision bounding Rectangle

I'm creating some rectangles surrounding my sprites for my player and bullet class to detect collisions with the overlaps method in Intersector class of LibGDX.
I have a question:
When I instantiate Player and Bullet, I create a bounding box around the sprite using sprite.getBoundingRectangle() which returns a Rectangle object. I update the movement of these somewhere else in the main class.
When I update the movement of the bullet/player sprite, do I also need to update the position of the bounding box surrounding the bullet/player sprite? Or since the bounding Rectangle surrounds the sprite, will the box automatically move with the sprite?
Thanks

As per getBoundingRectangle javadoc:
Returns the bounding axis aligned Rectangle that bounds this sprite. The rectangles x and y coordinates describe its bottom left corner. If you change the position or size of the sprite, you have to fetch the triangle again for it to be recomputed.
Indeed, if you open Sprite source code, you'll see that the bounding rectangle is updated only when getBoundRectangle is called.

Setting the pivot point of a JPanel to its center

I am building a Java application that is going to feature two circles of random sizes that need to be clicked by the user. The time between the click on the first and the second circle is going to be measured. Unfortunately, since I am new to Java so things have been slow for me. Currently I have my application draw circles and measure time between clicks using System.nanoTime() but now I am running into a problem.
Because the circles need to be a fixed distance away from eachother I want to use the center of the circles as the origin points. So basically I want to be able to provide coordinates for the circle so that the center of the circle should be at those coordinates. The distance between the circles then describes the distance between the centers. My circle currently is embedded into a JPanel but if I set the JPanel's position it moves the top left to that position.
Of course I have done some searching read that I may need to play around with either AffineTransform or Graphics2D.translate() which I have tried in paintComponent() but this got a bit confusing so then I tried to override setlocation and subtract the radius from the position. It sort of works but it is not the most clean solution. Can aonyone give me some pointers on how to do this?
Thanks in advance.

If I understand the problem statement, all such pairs of circles will lie on opposite sides of a circle centered in the enclosing panel, as shown here. Simply choose a random 0 ≤ θ < π and find its opposite at π - θ. Note how the example's rendering scales as the panel is resized.
As an aside, the example uses setPreferredSize() to establish the dimensions of the drawing panel, but you may want to override getPreferredSize() instead.
Addendum: The example uses fillOval() to render the circles, but you can use draw() with any desired Shape; the latter provides several contains() methods suitable for hit testing, as mentioned here.

You have the coordinates for the two center for the circle (x1, y1) and (x2, y2).
The size of the radius is random.
Once you have the radius of the two, r1 and r2, simply position them at (x1-r1, y1-r1) and (x2-r2, y2-r2).
You can use java.awt.Point to represent the center, and use
center.translate(-radius, -radius)
and use the new translated value as position for the drawing.
Maybe you think it is not a clean solution, but why not? Everything in Java is painted by giving the top left corner for the position, so is the use of the center that is not clean :).
To calculate the left top position by doing -radius is clean :)

Mouse pointer detection over a Path2D

I have constructed a Path2D that represents an unclosed shape consisting of straight lines:
I want to be able to detect when the mouse is clicked and the mouse pointer is near to (within a few pixels of) the path. Using the contains method does not work because the algorithm treats the unclosed shape as implicitly closed (i.e. by drawing a straight line between the start and end points).
Does anyone know of another mechanism for achieving this?

Create a BasicStroke (the width controls your pixel-distance-tolerance)
Don't draw with it, only use its createStrokedShape method to create a second shape from your shape. This second shape describes the outline of the shape that would be filled if you would draw your first shape with the BasicStroke.
Use the contains method of this second shape
From Stroke.createStrokedShape API documentation:
Returns an outline Shape which encloses the area that should be
painted when the Shape is stroked according to the rules defined by
the object implementing the Stroke interface.

Graphics2D - Rotating Shapes on a Graphics2D object

I have a Graphics2D object which I use to draw on my Canvas. I draw multiple shapes on the Canvas and want to transform only one (or part) of them.
I'll try to keep this simple:
void render(Graphics2D g) {
... // Draw shape 1
... // Draw shape 2
... // Draw shape 3
}
How would I go about rotating shape 2 while leaving shape 1 and 3 intact? By "rotate" I mean rotating around its center point, which we can define as x and y for example.
I've been looking for a way to do this for a while now, but couldn't find anything that works the way I want it to.
Is there any simple way to do this?

Rather than rotating the shape around it's centre point, rotate and then translate the canvas. To rotate around the centre of the shape at (x, y), first translate the canvas by (-x, -y) and then rotate the canvas -d degrees and draw the shape as normal at (0,0).
When you're done, rotate back then translate back (note that with these geometric transformations the order is important, translating and then rotating will give you a completely different outcome).
This means that you can still draw an object at any rotation without having to recalculate the coordinates yourself.

AffineTransform afx = new AffineTransform();
afx.rotate(angleRad, s.getCenter().x, s.getCenter().y);
//afx.rotate(angleRad);
java.awt.Shape ss = afx.createTransformedShape(s.getPrimativeShape());
return ss;
s is my wrapper class for java.awt.Shape and does some stuff with it.... But what you want is in line 2.
afx.rotate(Angle,xAnchorPoint,yAnchorPoint);
afx.rotate rotates the object about the point (xAnchorPoint;yAnchorPoint).
Hope this is what you wanted

In order to rotate a shape, use one of the Graphics2D.rotate methods.
Cache the transform used for both shape 1 and shape 3. Before drawing shape 3, ensure that you reset the transform to the cached one, since using rotate for shape 2 will alter the current transform coordinates.
Steps:
Cache current transform
Draw shape 1
Rotate
Draw shape 2
Set transform to cached one
Draw shape 3