1. SimpleVirus reproduce() {
2.     if (random(0,1) < (maxBirthProb * (1- popDensity))) {
3.         SimpleVirus newVirus = new SimpleVirus(maxBirthProb, clearProb);
4.         return newVirus;
5.     } else {
6.          print("NoChildException");
7.          return null;
   
8.     }
9. }

Since the method is returning an object of type SimpleVirus then change the methods return type to match. If a method has a return type then something must be returned, hence line 7

Like this...

1.   SimpleVirus reproduce() {
2.     if (random(1) < (maxBirthProb * (1- popDensity))) {
3.       return new SimpleVirus(maxBirthProb, clearProb);
4.     } else {
5.       return null;
6.     }
7.   }

Or even these more compact ones: 

SimpleVirus reproduce() {

    if ( random(1) < maxBirthProb * (1 - popDensity) )
        return new SimpleVirus(maxBirthProb, clearProb);

    return null;
}

boolean doesClear() {

    return random(1) < clearProb ? true : false;
}

What an awesome forum. Thanks for all the help people. Now, one other question. Let's say I create a virus called V1 and add something like this:

You use SimpleVirus like you did for V1.

1. SimpleVirus V2 = V1.reproduce();

However you probably want to look into an ArrayList since the number of virus instances is unkown and each reproduction uncertain. Then you can do things like add to the arrayList if the new virus is not null, go over all the viruses in the list and reproduce etc.

Perfect. Yes, I will be looking into arrays. In my python implementation I had a list of virus instances that I could work with. So an array is the next logical step. Thanks again for the help.

I believe you're not very used to static-typing languages like C, C++, D, C#, Java, etc...
You have to declare the data type of a variable before you can assign anything to it! 
Be it a primitive or an object.

Crap, one more question. I run into a null pointer exception when I do the following:

To GotoLoop: very true. My experience is all in Python but I'm learning to branch out...slowly.

Inside your class SimpleVirus , the variable popDensity is always = 0!
Thus, maxBirthProb * (1 - popDensity) always return maxBirthProb itself.
With maxBirthProb = .01f, the chances to get a non- null instance is absurdly null! 
Moreover, you have to test whether V2 is different than null before accessing anything within that.

P.S.: Gonna try learning Python + PyGame soon. Already know a little! HEHEHE

SimpleVirus V1, V2;

void setup() {

    float val, val2;

    V1 = new SimpleVirus(.78f, .14f);
    val = V1.getMaxBirthProb();

    SimpleVirus V2 = V1.reproduce();
    
    if (V2 != null)
        val2 = V2.getMaxBirthProb();
    else
        println("V2 is null!!!\n");

    println(V1.popDensity);

    exit();
}

Yeah, I caught that about popDensity. Thanks. I actually want to run the method inputting a value for popDensity. In python I could do something like:

 What's the syntax for that in Processing?

You can find a list of Processing functions & syntax here -> Processing Reference.

However, mostly what you're asking here is about Java, not exactly Processing exclusivity!

You know, Processing is an API framework for Java. And also for other languages like JavaScript, Ruby, Python, etc!
Much like PyGames is an SDL wrapper framework for Python, and Löve2D for Lua!

The corresponding keyword self in Java is this.
However, you won't be needing to use that in Java as much as in Python (or Lua).

Can you actually clarify the comment about APIs. If I understand you correctly, Processing offers a set of classes that allow access to parts of the larger Java toolkit? And that Processing also exists in a variant that accesses Python?

and when I call that method later simply plug in a popDensity value like SimpleVirus.reproduce(.5)

If you need to access a field variable from a class,
you can do so either directly or creating a method inside that class for it:

• V1.popDensity = .5f;      //  you can change any variables from a class. V1 is an instance reference of class SimpleVirus
  
• V1.changeDensity(.5f);   //  changeDensity() would be a method you create inside class SimpleVirus

Don't fret to ask!  

Processing sketches themselves are wrapped by a class named PApplet.
Which in turn offers all those neat graphics functions for easy graphics programming!
You can take a look at Processing's source code here -> Processing

Now about variants, the most famous and kicking is Processing.JS, which uses a JavaScript base running @ HTML5 in any modern browser!!!

Python & Ruby variants are more like a niche I believe:

• Python -> Processing.PY
• Ruby    -> Ruby-Processing

P.S.: a more complete Processing variant list -> Sister Projects.