Dragging and dropping shapes in Java

Here's my basic drag and drop implementation in a little over 100 lines of code. What you'll need (to do):

• extend MouseAdapter and override mousePressed, mouseReleased and mouseDragged
• a JPanel with the above listener added as mouse listener and mouse motion listener
• a list of shapes to paint and drag around

First, the list of shapes. Here's a very simple class that acts as a square. All it has are its x and y coordinates, getters & setters for them and then a method that draws the shape.

Square

Now let's think about how the drag and drop works. What do you actually do? Whell, you press your mouse button to select your shape. Then you drag the shape where you want it to be. Then you release the mouse button when you are done dragging to deselect the shape. That's exactly the role our above overriden methods will have: select - move - deselect.

We'll keep a field, selected (of type Square) that knows which shape is selected - if any. mouseReleased will always set it to null. This is the easiest method.

mousePressed isn't much harder though - here we want to know which shape was selected. To do that, we save the coordinates where the button was pressed and check whether any of the shapes in our list resides at those coordinates. If yes, assign our selected field the value of the respective shape. If none of the shapes is in that area, the selected will stay null.

Finally, mouseDragged. If no shape was selected, then it's easy: just return. If we did select a shape, then we need to update its coordinates. As we drag the mouse on the screen, the shape beneath it needs to, well, stay beneath it. For this we are going to take our pointer's coordinates and assign them to our shape. In Java, squares (rectangles) are drawn from the top left corner, so I adjusted my new coordinates for the shape so that its center will always be where the mouse is pointing. Then, of course, we repaint.

Here's the full code:

MyPanel

Finally, put the panel in an applet or frame and there you go.

Unboxing and autoboxing in Java

I think this is one of the small Java features that don't get that much attention. But it's still nice nonetheless.

Autoboxing means converting from a primitive type to the corresponding wrapper class. Like below (note how much better the code looks when you're adding a new element to an ArrayList):

Autoboxing

Unboxing is the opposite: converting from a wrapper class to a primitive type. Here are a few examples (this one was a bit more interesting :D ):

Unboxing

Here's the article from the Java tutorials.

RSA in/and Python

I'm writing a very basic application that uses the principles behind RSA encryption to send data. Code is here on Bitbucket. Obviously released under my favourite license, WTFPL.

I need to generalize it so that it can be used on different machines, randomly generate the public key instead of using hard coded numbers, accept the message as command line argument and so on. But I probably won't bother much with this, since my main goal was to experiment with the brilliant mathematics behind RSA encryption. Here's the Wikipedia section.

It took me quite some time to accept that RSA relies on some of the most basic number theoretic properties. This is its brilliancy. Here is an example. What you might also find brilliant is the way you can break this brilliant algorithm using a quantum computer. I may write some time about Shor's Algorithm on my Quantum Blog.

Oh, and the other cool thing about this application is Python. First, it's not Java. I had enough of it. Then, well it seems very friendly to mathematical code. And to pseudocode too. In fact, whenever I have to write pseudocode, I use Python syntax - or something very close to it.