Java impassable region

Question

I have a mouse event handler and under mouseMoved I have this:

public void mouseMoved(MouseEvent e)
{
  if((e.getX()>=0 && e.getX()<=100) && (e.getY()>=0 && e.getY()<=100))
  {
    robot.mouseMove(mainOutput.getX()+200,mainOutput.getY()+200);
  }
}

What this does is that if the user tries to move towards the first 100x100 pixels of the frame, the pointer will translated. What I want to do however is recreate an "impassable wall".

Basically when the user attempts to go in the region it cannot pass the end points of the region. What I want to know is how would I go about doing this?

Answer 1

I think you just need to clarify to yourself what the behaviour of a mouse hitting a wall would be, in terms of coordinates.
Hopefully this terrible image helps;

Whenever the mouse moves, you want to check if it's in the forbidden region (the region beyond the wall; the no man's land). To do this, just check if the x coordinate (assuming a horizontal wall in this example) of the mouse is beyond its limit (the x coordinate of the wall).
If it is, move the mouse back to the wall, preserving its y value (set its x coordinate to that of the wall)

FOR A CAGE:

The case of having a surrounding, square wall is a little bit more complicated, in terms of where to place the mouse when breaching the wall.
Observe that there are 8 different regions to consider where the mouse could enter the 'forbidden zone', and each boundary should translate the mouse differently.

The coordinate of the boundary corners are in red (and consist of 4 values; xL, xR, yT, yB for left, right, top, bottom respectively)).
The green text in each region describes the conditions that must all be true for the mouse to lie in that region (they're how you detect the mouse must be in that region) where x and y are the mouse coordinates.

You can see that the four 'side' regions involve a simple translation; just altering one of the coordinates of the mouse (the x coordinate for a vertical wall, y for a horizontal wall) to match that of the wall.
The four 'corner' regions can entirely change the mouse coordinate (to their corner coordinate!)

Answer 2

Assuming the impassable area is a JPanel, you can add a mouse listener to it that will respond whenever the mouse enters the area, and then do the relocation stuff that you already have.

   JPanel pnlArea = new JPanel();
   pnlArea .addMouseListener(new MouseAdapter() {
      @Override public void mouseEntered(MouseEvent arg0) {
         //execute some codes if the mouse pointer has enterd the area.
      }
   });

Answer 3

Unfortunately, this is a bit more difficult than it seems. Let me first illustrate the problems with a simple move-to-outside-of-boundary approach.

As you can see, in this case the boundary approach will detect the mouse inside the boundary, and move it to the blue point in the corner. Let me emphasize this, it detects the location of the mouse. What we want is to capture the movement of the cursor, and have it end at the red point. There are also other problems with this method that may not be immediately apparent.

So how do we capture the movement of the mouse? We need to capture the mouse displacement (black arrow) as a vector by keeping track of the previous location as well. I assume you can do this. So how do we calculate the new location? Well, we can perform line intersection of the displacement vector with the lines that make up the edges of the box. As you're only dealing with horizontal and vertical lines, it is greatly simplified and can be done with just a bit of thinking. If you're lazy, copy a generalized line intersection algorithm.

You may think that this approach is too rigorous, but it is the most robust way. I can already think of two additional issues with the simpler approach. Also, you're actually doing 2D hitbox detection. This is quite a useful thing to know.