Need help with an array in Space Invaders program

[Mostpured]

Okay so I have this code.

	
	public synchronized boolean isHit (FilledRect missile)
	{
		for (int col = 0; col < aliens[0].length; col++)
		{
			for (int row = 0; row < aliens.length; row++)
			{
				if (aliens[row][col]!= null && aliens[row][col].overlaps (missile))
				{
					aliens[row][col] = null;

					return true;
				}
				return false;
			}
		}
	}

And I get an error saying that it cannot find symbol method overlaps. So is there something wrong that I am doing?

[jim829]

Your aliens array should be holding an object of some type. That type must have a method called overlaps that takes a FilledRect object as an argument. Apparently, that is not the case (or the method is not publicly accessible).

Regards,
Jim

[Mostpured]

Okay, I am doing this part incorrectly. I basically copied this piece of code from another class that was working with a visible image but for this class I need it to work with the array. The object the aliens are holding is of type Alien. Here is the direction for this step my teacher gave me.

Create a synchronized method isHit() to determine if any of the living aliens have been hit by the missile and if hit, set the alien to null.

[pbrockway2]

As Jim says there must be an accessible method overlaps() in the class of whatever the aliens array is supposed to contain.

There's not enough context in what you posted to see what is causing the problem. Where is the aliens array declared? And what is the definition of the class of thing it is supposed to contain?

[Mostpured]

im using objectDraw.

objectdraw.ActiveObject;
objectdraw.DrawingCanvas;
objectdraw.VisibleImage;
objectdraw.FilledRect;

The array is declared within the Invaders class. The Invaders class also contains the synchronized method I need to write. The Alien class contains all the information relevant for the alien including the actions the alien will perform and the images associated with the alien.

[Mostpured]

Also, I don't really understand synchronized methods. I had this method just to check something out.

        public synchronized boolean isHit ()
	{
		for (int row = 0; row < aliens.length; row++)
		{
			for (int col = 0; col < aliens[0].length; col++)
			{
				if (aliens[row][col] != null)
				{
					System.out.println("hi");
					aliens[row][col] = null;

					return true;
				}
			}
		}
        }

Wouldn't all the aliens become null as soon as I run the program? Shouldn't I get the message on the console? Nothing happens though?

[Norm]

Nothing happens

Try debugging the code by adding some println() statements to show that the method is called and that each of the for loops executes.

[Mostpured]

Okay now I have this code

	
        public synchronized boolean isHit (FilledRect missile)
	{
		for (int row = 0; row < aliens.length; row++)
		{
			for (int col = 0; col < aliens[0].length; col++)
			{
				if (aliens[row][col].isHit(theMissile))
				{
					System.out.println("hi");
					aliens[row][col] = null;

					return true;
				}
			}
		}

		return false;
	}

It compiles, however, nothing is happening when I run the program.

[Norm]

nothing is happening when I run the program.

I don't see any println() statements for debugging.

How do you know if any code is executing? Add lots of println() statements to show what code is being executed.

[jim829]

Your last code snippet looks like it might be doing recursive calls. Are you familiar with recursion and its subtleties?

Regards,
Jim

[Mostpured]

THANK YOU!! I figured out the problem using a whole bunch of println() statements and found out I had to add an if statement to make sure the missile stops moving when it overlaps the alien!

[Mostpured]

Okay, now another problem. I have this boolean method.

	/**
	 * Determine if the alien has other aliens below it
	 *
	 * @param row the row of the current alien
	 * @param col the column of the current alien
	 * @return true or false depending whether or not the alien is blocked
	 */

	public boolean isBlocked (int row, int col)
	{
		if (aliens[row - 1][col] == null)
		{
			return true;
		}
		return false;
	}

I wrote this method to make sure the alien can drop the bomb only if there is no alien below it. As you can see, I need the column number and row number, however, im not sure how to get them when I use the method. I wrote this to use isBlocked()

if (!aliens[row][col].isBlocked(aliens[row], aliens[col]))

[jim829]

Just look at what your method takes as argument and look what you are actually passing. Just think about it.

Regards,
Jim

[Mostpured]

Okay thanks I found it