I'm trying to figure out how to create an object on-the-fly, meaning while a program is running, after setup() has already run.  In the sample code below, I'm trying to create a new "mrect" object whenever the mouse is clicked.  So, the program starts out with an mrect.length of only 1, but hopefully, upon each mouse click, that number is incremented.

I've used arrays of objects before, but it seems that creating a new object is not really as simple as appending a new value to the array.  Should it be?  

Copy and paste the following into a new sketch and it will run.  Then try uncommenting the two lines I have commented out in mousePressed(), one at a time.  Those are the two methods I have tried to append/expand the array.  What am I doing wrong?  What is the syntax for adding a new object to an object array and initializing it at the same time?

Thanks!

Code:

MRect[] mrect = new MRect[1];

void setup()
{
  size(500, 500);
  fill(255, 150);
  noStroke();
  mrect[0] = new MRect(1, 50, 0.25, 0.01*height);  //Initialize the first mrect
}

void draw()
{
  background(0);
  for (int i = 0; i < mrect.length; i++) {  //Loops through all mrect objects
    mrect[i].display();
  }
  for (int i = 0; i < mrect.length; i++) {  //Loops through all mrect objects
    mrect[i].move(mouseX, mouseY, 30);
  }
}

void mousePressed() {  
  MRect[] mrectNew = new MRect[1];
  mrectNew[0] = new MRect(5, 200, 50, 0.2*height);
  //mrect = append(mrect, mrectNew);  //Triggers "ype of the right sub-expression, "java.lang.Object", is not assignable to the variable" error
  //MRect[] mrect = (MRect[]) expand(mrect);  //Triggers "var may be accessed before having been definitely assigned a value" error
  println(mrect.length);
}

class MRect 
{
  int w; // single bar width
  float xpos; // rect xposition
  float h; // rect height
  float ypos ; // rect yposition
  float d; // single bar distance

  MRect(int iw, float ixp, float ih, float iyp) {
    w = iw;
    xpos = ixp;
    h = ih;
    ypos = iyp;
    println("created new object");
  }

  void move (float posX, float posY, float damping) {
    float dif = ypos - posY;
    if (abs(dif) > 1) {
	ypos -= dif/damping;
    }
    dif = xpos - posX;
    if (abs(dif) > 1) {
	xpos -= dif/damping;
    }
  }

  void display() {
    rect(xpos + (w*10), ypos, w, height*h);
  }
}
 

Awesome!  Thanks so much.  I had seen the documentation on append(), but guess I misinterpreted it.  Thank you for explaining so clearly why I need to include the "casting" step.

BONUS QUESTION:  Is it possible to destroy/delete objects in a similar fashion?

See:

http://processing.org/reference/shorten_.html

http://processing.org/reference/subset_.html

In general, see "Array Functions" here:

http://processing.org/reference/index.html

In Java (the underlying language of Processing), you cannot change the length of an array once you initialize it. A zero length array would not have much use. The Processing append() API takes an array and an element and returns you a *different* array one larger than the one you gave it with the contents of the array you passed plus the element in the last position of the new array.

Since you set your global mrect variable to this new array by setting it to the result of append(), the old array becomes "dereferenced" from your sketch, but it still exists for some time in memory until the Java Virtual Machine running your Processing sketch garbage collects it (clears it from memory to free up space for new objects).

Thanks, smith, for that uncommonly clear explanation of how Processing manages arrays.  Now it makes sense why you have to use the structure array = append(array, newArrayObjectToBeAppended).

I asked about having a zero length array because it would be useful for my project.  Rather than creating the array with one initial, "dummy" element, it would be easier to create a zero length array, and then only add the "real" data elements to it as needed.

Also, st33d, thanks for that example.  I think ArrayLists would be better for my purposes.  In one particular project, I don't want to have to have a fixed maximum number of objects.  (It's a data visualization project, where there could be either tens or thousands of data points, so I want to be able to add and remove them as needed, on-the-fly.)

Thanks again to you both for your help.