Alright, I continued to work on this. Currently I have an editor where we can type text,

That's not an easy task, but apparently you advance nicely, which is good.
I don't know what is your ambition (just type & display a screenful of text or type a novel...) but for long text, a text component like Scintilla uses a special kind of document, with a hole in the middle. Actually, the "hole" moves where the caret (not the cursor, which is for the mouse) is, so when the user types, you only append chars in the hole, no need to move around all the text behind.
The theory behind this is explained in the Data Structures in a Bit-Mapped Text Editor paper. Of course, you still have to move chars around when the user moves the caret, but it is less frequent and possibly by a smaller amount.

Caret position should be computed from the amount of chars between the caret and the start of the text. Same for the other positions, like the anchor of a selection. So you have to be able to compute and map such position from an x, y coordinate in the window...
There is lot more to tell, the topic is vast...

Thank you very much for yourn reply, it has valuable information.

I need a texteditor for an android application. the Gui4Processing library is not working for me. I ended up writing a very (very) simple "texteditor" (its a euphamism) myself:

1. String input = "  ";
2. void setup(){
3. size (400,400);
4. }
5. void draw(){
6.   background(90,80,90);
7.   fill(255,600,60);
8.   text(input, 15, 20, 300, 300);
9.  
10. }
11. void keyPressed(){
12.     if (key == BACKSPACE){
13.       if(input.length()>0)  {
14.       input = input.substring(0,input.length()-1);
15.       }
16.     } else if (key == CODED) {
17.     if (keyCode == SHIFT){
18.     }
19.     }
20.    else {
21.     input+= key;
22.   println( input );
23.  
24.   //str = str.substring(0,str.length()-1)
25. }
26. }

If you have anything more elaborate, could you please share it?

@ Psysoul: I just jump in and would like to ask you to share your editor?

Cheers, chrisir