Dealing with ArrayList items in a static method

[Deena Lee]

Friends,
I am having problems with ArrayList items in a method that must be static. The code picks two random mice in the mice ArrayList. It then is suppose to compare the two mice to see if they are of different sex and both over the age of 1.

Here is my code

public static void mate (ArrayList mice, int index)
	{
		//randomly choose two mice
		Random random = new Random();
		int a = (int) (Math.random() * mice.size());
		int b = (int) (Math.random() * mice.size());

		if(((isMale(mice, a) == true && isMale(mice, b) == false) || ((isMale(mice, a) == false) && (isMale(mice, b) == true)))
			&& ((getAge(mice, a) >1) && (getAge(mice, b) >1)))
		{
			//randomly create 0-4 mice and append to arrayList
			int n = random.nextInt(5);
			for (int i=n; i>=0; i--)
			{
			      mice.add(new Mouse());
			}

		}
	} // end mate method    

The code as is generates an error message like this for all my metods in the if loop ...
non-static method getAge(java.util.ArrayList,int) cannot be referenced from a static context
&& ((getAge(mice, a) >1) && (getAge(mice, b) >1)))

I have tried to change the code (and the methods) to not accept parameters, to something similar to this ...

if((mice(a).isMale() == true && mice(b).isMale() == false)    

but, I then get this error message:
cannot find symbol
symbol : method mice(int)
location: class Mouse ... I have imported the Java.util.ArrayList class into the Mouse class. The mice is the ArrayList and the (int) should be the index of the item in the list.

Any help on this is appreciated.
Thanks!

[Gregg Bolinger]

getAge() will need to be a static method in order to be referenced from a static method. There's nothing you can do to make it work other than making it static.

[Deena Lee]

Gregg Bolinger wrote:getAge() will need to be a static method in order to be referenced from a static method. There's nothing you can do to make it work other than making it static.

Ok, that's understandable, but when I changed the getAge code so it wasn't static, I got a cannor find mice(int) method
mice(a).getAge()

What should I be looking for to make it work?

[Ram Narayan.M]

mice ArrayList contains list of "Mouse" objects..right?

to get the object stored in the arraylist and if your Mouse class comprises getAge() method , use

Mouse obj= (Mouse)mice.get(a);
obj.getAge() ;

[Campbell Ritchie]

== true???

Dreadful. Never write == true. You just leave that bit out. Never write == false. You use the ! (bang operator) to negate the Boolean value.
Not

if(((isMale(mice, a) == true && isMale(mice, b) == false) || ((isMale(mice, a) == false) && (isMale(mice, b) == true))) 

but

if(((isMale(mice, a) && !isMale(mice, b) || ((!isMale(mice, a)) && (isMale(mice, b)))) 

Or better, if you simply want two mice of different sex, try

if (isMale(mice, a) != isMale(mice, b)) . . .

[Deena Lee]

Thank you Narayan and Ritchie,

I've got the code in better form and compiling thanks to both of your comments.

However, the program isn't returning the results it should be. So, I'll keep at it.

[David Newton]

Or better yet, "areMateable(mousie, otherMousie)" or "mouse.canMateWith(anotherMouse)"

After all, a male mouse may be a castrato, and we must be sensitive to their needs.

[Wouter Oet]

I've got a few remarks about your code that hopefully will improve your code:

If your ArrayList only contains mouses or subclasses of mouse then you can declare your ArrayList to be generic:
ArrayList<Mouse> mice = new ArrayList<Mouse>();
Or even better, program against an interface instead of an implementation:
List<Mouse> mice = new ArrayList<Mouse>();

The generic part will provide compile time type safety and will remove casts from your code.
The interface part will allow you to change the list implementation without massively changing your code
and then there is no need to know the implementation details because you only need the specification on the interface.

And finally the method isMale() sounds like a prime candidate to be a property of mice.

[Deena Lee]

Thanks everyone! I've got it working!

[Campbell Ritchie]

You're welcome . As an alternative to the != operator, you could try the XOR operator (caret = ^).