Hey, 

also you need to cast the appended array like this

ah quarks beat me to it ;)

Using underscores to differentiate between parameters and class attributes is not a Java convention - so if you are happy using this then keep using it.

Thank you to both quarks and tomschofield. It works.

What I don't understand though is when I'm doing the same thing to an integer array, why am I not facing the same problem? Why am I getting away with not casting?

If by integer array you mean int[] then it is not an array of objects because int is a primitive data type, and it means that Processing uses a different append method even though the synatx looks the same.

Java also provides a wrapper class of int called Integer - now if you used that type then you would need to cast to Integer[]

Gotcha, thanks quark :)
I'm learning new something every day. It's kinda awesome ^.^

Internally, there are 6 separate implementations of the append() method.  The type of array you pass in determines which one is called.  If you pass in an array of bytes, chars, floats, ints, or Strings, you will get back an array of the same type.  If you pass in an array of any other object, you get back an Object array that must be cast to the appropriate type.

I've never used append() until today! 
In my opinion, if some1 needs to grow or shorten the size of an Array, he/she should be using a List instead!
Or at least, add up a boolean variable to a class to flag whether an object is active or "dead" there! 

Here's an online example where I've used a fixed-size Array for Firework class.
But there's an isInactive boolean to denote it's in "stand-by" mode: 

http://studio.processingtogether.com/sp/pad/export/ro.9Q6oRai8-41WJ/latest

Of course, I can't have more than MAX_FIRES fires at the same time on screen. 

The problem with List is that it's not portable across Processing platforms.  If you want the same sketch to work in both Java and JavaScript, you're stuck with arrays.  However, these append methods are terribly inefficient.  Every time you call append() to add an element to your array, you are creating a whole new array, copying the elements from the old to the new, and throwing away the old array.

The problem with List is that it's not portable across Processing platforms.  If you want the same sketch to work in both Java and JavaScript, you're stuck with arrays.

Of course, the only Java List officially supported in Processing is ArrayList.
Another common Java Collection is HashMap, which is derived from Map instead of List.