Java For Kids Tutorial 8

Java Mouse Capture

Methods

Java Mouse Move

 	// initialise variables
 	// start an infinite loop
 	// check if mouse move returns true
 	// get the location of the mouse
 	// print the location of the mouse
 	// restart infinite loop

Lets do some code to flesh out the above algorithm

	// initialise variables
	int mx = 0;
	int my = 0;
 	// start an infinite loop
 	while(true)
 	{
 	// check if mouse move returns true
 	if(getMouseEvent())
 	{
 	// get the location of the mouse
 	mx = getMouseX();
 	my = getMouseY();
 	// print the location of the mouse
 	printLine("x= "+ mx + "y = "+ my);
 	}
 	// restart infinite loop
 	}

Beautiful. Did you see all those numbers streaming down in the bottom textarea.. ?
That was all the positions of the mouse being printed..
Next up we want to draw a circle to follow where the mouse goes.. Instead of printing the locations of mouseX and mouseY we will use those numbers to draw the circle at the x and y of the mouse..

So now just change the line "printline("x = ..... etc) to

	setColor(red);
	fillCircle(mx,my, 20);

What happened ?... Do you only want one red ball to be drawn on the screen ..?
Well, we will have to clear the screen and set a delay. Remember the last lesson where we set a delay and cleared the screen for animation.. Same story..

Lets add ..

	delay(0.10);
	clearDrawing();
	

Yep it worked.. but it needs one minor adjustment.. The mouse pointer is located at the top right of the red ball and i want it in the center of the ball..
So we need to subtract what ?
Half the radius of the red ball in the x and y coordinates..

So change the fillCircle code ..

	fillCircle(mx-10, my-10, 20);
	

Here is the complete code ..

void main() {

// initialise variables
	int mx = 0;
	int my = 0;
 	// start an infinite loop
 	while(true)
 	{
 		// check if mouse move returns true
 		if(getMouseEvent())
 	{
 	// get the location of the mouse
 	mx = getMouseX();
 	my = getMouseY();
	// draw circle at mouse posn
 	setColor(red);
	fillCircle(mx-10, my-10, 20);
	// set animation delay and clear screen
	delay(0.10);
	clearDrawing();
 	}
 	// restart infinite loop
 	}
}

Well how did that go.. there are a few problems.. The animation is jerky. And when the mouse is not moving, there is no ball painted.. As an exercise, see if you can solve those problems..

Java Mouse Click

The method that will be used mainly for capturing mouse clicks will be getMouseButton(int buttonNumber) . This method returns true if the corresponding number for a mouse button is clicked.

• 1 - Left Mouse click
• 2 - Center Mouse button clicked
• 3 - Right Mouse clicked

Lets try it out !

void main() {
	int mx = 0 ;
	int my = 0 ;
	// start infinite loop
	while(true)
	{
	if(getMouseEvent())
	{
		// get mouse posns
		mx = getMouseX();
		my = getMouseY();
		// check if left button clicked
		if(getMouseButton(1))
		{
		printLine("Left mouse clicked at " + mx + " " + my);
		}
		// check if right button clicked
		if(getMouseButton(3))
		{
		printLine("right mouse clicked at " + mx + " " + my);
		}

	}
	}
}
}

Right! Now lets see if we can have the red ball be a defender in a game of Aliens Invasion. We need to move the red ball back and forth along the bottom whenver we move the mouse.

Lets write an alogorithm first ..

	// initialise x and y posn variables
	// start loop
	// check for mouse move or click
	// move red ball in tune with mouse
	// end loop

Copy the algorithm into our judo text editor and fill in the code.. Try it out... What did you get ? Anything like this..?

void main() {
	// initialise x and y posn variables
	int mx = 0 ;
	int my = 0 ;
	// start loop
	while(true) {
	// check for mouse move or click
	if(getMouseEvent())
	{
		mx = getMouseX();
		my = getMouseY();
	// move red ball in tune with mouse
	setColor(red);
	fillCircle(mx-10, getDrawingHeight()-20, 20);
	delay(0.025);
	clearDrawing();
	}
	// end loop
	}
}

Now we want the defender to fire a bullet or missile.. And we want the bullet to fire from the position that the defender is when the left mouse is clicked.
So we need 2 new variables and call them clickX and clickY .... and also we need an amount that the bullet goes each time the loop goes round ..
Dont forget we have a delay which is our pretend framerate..

Lets do some test code to see what we can do..

void main()
{
	int mx = 0;
	int my = 0;
	int clickx = 0 ;
	int clicky = getDrawingHeight() - 20;
	int speed = 5 ;   // 5 pixels per 0.025 seconds
	boolean clicked = false;
	// infinite loop
	while(true){
		if(getMouseEvent())	// check mouse event
		{
			mx = getMouseX();
			my = getMouseY();
			setColor(red);
	fillCircle(mx-10, getDrawingHeight()-20, 20);
			if(getMouseButton(1))	// left clik
			{
				clicked = true;
				clickx = getMouseX();

			}
		}
		if(clicked){			// set when left click
			setColor(green);
			fillCircle(clickx, clicky,10);
		if(clicky > 0 )
		{clicky = clicky - speed;}
		else {clicked = false;
				clicky=getDrawingHeight();}
			delay(0.025);
			clearDrawing();
		}
	}
}

We are getting somewhere but still lots of problems.. The red ball only gets erased when the button is pressed. The bullet only fires once. The clickx position is changing as we drag the mouse to left or right.. Try and have a go at solving these problems..

In the next lesson we will go on to designing our space invaders game and use the real Java compiler..

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