Hello everyone,there's an urgent problem for me now.
My teacher offered me an exercise to make a interactive scene,where I got a rectangle at the top by default,it flashes when being placed on by a cursor and give birth of another rectangle when clicked,and the new born rectangle have the same interactive property like its mother object.
I see there would be a tree structure guiding behind,and how can I accomplish that?Can you give a simple example?Thank you in advance.

I'm terribly sorry,but I mean no insult,I'm somehow upset.
I shouldn't occupy the chance of others to be helped.
I'm regretful,forgive me.

To expand upon PhiLo's suggestion:

Yes, make a rectangle class.  It should hold the (x,y) location of the top-left corner of the rectangle, as well as the width and height of the rectangle.  It should have a method which, when given a pair of mouse coordinates, returns true or false if the mouse coordinates lie inside the rectangle:

boolean hitTest(float x, float y) {
 // it's your classwork, you fill in this part
}

It should also have a method that creates a new rectangle:

Rectangle makeNewRectangle() {
 // you fill in this part
}

The main part of your program will have an ArrayList to hold all of the rectangles.  You will implement the mousePressed() routine in such a way that when the user clicks the mouse, you check each Rectangle in the ArrayList to see whether the mouse was within that rectangle.  If it was, then you can call that rectangle's makeNewRectangle() method.  It will return a new rectangle, which you add to the ArrayList.

Your draw() routine will simply iterate over the ArrayList and call each Rectangle object's draw() method (which you should also implement, probably in terms of Processing's native rect() function).  In your draw() routine, you can also use the hitTestmethod on each rectangle to see whether the mouse is presently inside that rectangle, so as to draw it "plain" if the mouse isn't inside, or "flashing" if it is.