Head first programme

[Rob Strange]

Hi,

I am working my way through the Head first Java book and can't work out why the following code won't compile, is it a typo in the book?

public class Puzzle4 {

	public static void main(String [] args) {

		Puzzle4b[] obs= new Puzzle4b[6];
		int y = 1;
		int x = 0;
		int result = 0;
		
		while (x < 6) {
			obs [x] = new Puzzle4b();
			obs[x].ivar = y;
			
			y = y * 10;
			x = x + 1;
			
		}
		x = 6;
		while (x > 0) {
			x = x + 1;
			result = result + obs[x].doStuff(x);
		}
		
		System.out.println("result " + result);
			
		
		}
			
		
	}


class Puzzle4b {
	int ivar;
	public int doStuff (int factor) {
		if (ivar > 100) {
			return ivar*factor;
		}
		else {
			return ivar*(5 - factor);
		}
	}
	
}

I am using Eclipse.

Many thanks

[Edit - added code tags - MB]

[Matthew Brown]

TellTheDetails - what error do you get?

Edit - at second glance (now I've formatted the code) it looks like there's too many curly brackets. Look at line 27.

[Rob Strange]

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 7
at Puzzle4.main(Puzzle4.java:22)


I have tried removing the curly bracket and it throws another error,

eclipse has no crosses in the code so I am at a loss.

thank you for your help.

[Matthew Brown]

That's not a compiler error - it's a run-time error. So we can probably assume that it does compile correctly. Sorry about misleading you with the {}s - the indentation isn't quite right, which threw me.

OK, now let's look at that exception. It says "ArrayIndexOutOfBoundsException" - which means you're trying to access an element of an array that doesn't exist. It tells you it's on line 22. I'm guessing that corresponds to line 21 of the code listed.

So, what's the value of x on that line? If you can't work it out, put a print statement on the line before to print the value. You set x to 6 on line 18, and then keep increasing it. So when it hits line 21 for the first time the value is 7 (also mentioned in the error message). But you've created obs with a length of 6 - obs[7] doesn't exist. Which throws the exception.

Actually, that whole loop looks suspect. You start at 6, and keep increasing x until it's no longer negative. Which means that without the exception it will never end (actually, in practice is will end once x gets too big to hold in an int variable, but that's probably not what you intended).

[Rob Strange]

I will have another look and try to work it out, I think the biggest problem is I don't know exactly what the code is doing as you have just explained very clearly, many thanks.

[Thomas Kennedy]

That second while loop is really odd:

x = 6;  // OK...
while (x > 0) { // OK, it looks like we will be decrementing x...   
   x = x + 1; // Huh?
   result = result + obs[x].doStuff(x);   
}

Arrays are tricky to handle, so go for maximum readability to avoid screwups. And avoid duplication. Try something like this:

package puzzle;

class Puzzle4 {
	
	public static void main(String [] args) {   
		  
        Puzzle4b[] obs= new Puzzle4b[6];   
        int y = 1;   
        int x = 0;   
        int result = 0;   
        
        for( ; x<obs.length; x++){ // Don't repeat the literal 6 here, use the array's length property
        	obs[x] = new Puzzle4b();   
            obs[x].ivar = y;
            y *= 10; // same as y=y*10 but easier to read
        }
       
        x=0; // reset our counter variable before using it

       // Your code goes through the loop from length-1 to 0 (that is, backwards). Here I go from 0 to length-1, which is easier to follow.

        for( ; x><obs.length; x++){ // er, this is previewing as ><, not sure why. it's the same as the first loop anyway.
        	result += obs[x].doStuff(x); //
        }   
           
        System.out.println("result " + result);
    }
}

package puzzle; // that is, this goes in a separate file. Not absolutely necessary but keeps things neat.

class Puzzle4b {   
    int ivar; // you should make this private use a setter method, or pass the int to the constructor
    
    public int doStuff (int factor) {   
        if (ivar > 100) {
            return ivar*factor;
        }
        else {   
            return ivar*(5 - factor);   
        }   
    }  
}