This should be in red, blinking letters in the first page of the Processing tutorial (unless it is there already): unless you make a specific, cumulative kind of sketch, always start your draw() function with a background() call (or similar).
Otherwise, everything you draw on each frame will just be drawn over the previous frames.
Note: frameRate() and textSize() calls can be safely moved to setup(): no need to repeatedly set them.

the behavour i see is like a new line being printed with the text appearing under the last text line, not over the top of each other?

I dont't have serial but this is the idea when you want the last entry on top of a list:

you store the serial data in temp (line 16) and after the while loop you place temp with a line break \n in front of val (line 20).

Greetings, Chrisir   

1. String val="pwwe";
2. int j=0;
3. void setup()
4. {
5.   size(500, 500);
6.   frameRate(1);
7.   textSize(32);
8. }
9. void draw()
10. {
11.   background(111);
12.   //myPort.write("temp?");
13.   int  i=0;
14.   String temp="";
15.   while ( i < 10) {
16.     temp = temp+char(i+j+65) ;
17.     i++;
18.   }
19.   j++;
20.   val = temp+"\n"+val;
21.   println(val);
22.   if (val != null && val.length() > 0) {
23.     text(val, 50, 100);
24.   }
25. }

doesn't processing allow some kind of separation of the formatting and code though? why do i need a loop to create new text in the same place as old text?

Hm...
Maybe look at libs like g4p or controlp5.....

" the behavour i see is like a new line being printed with the text appearing under the last text line, not over the top of each other?"
Uh? It looks like you are always displaying the text at the same place, though.
I don't have a serial device too, so I cannot experiment.

Unless you are meaning that's the behavior you look for. In this case, Chrisir is showing the right way.

When you only want to see the last line / temp var (in my code), show it via text() and don't use val.

That works together with background().

You can also choose a font and a fontsize and a color for the text.

text() also allows to define a rect that will not be exceeded even by too long text.

Hope this helps!

Greetings, Chrisir



like this...

1. //String val="pwwe";
2. int j=0;
3. void setup()
4. {
5.   size(500, 500);
6.   frameRate(1);
7.   textSize(32);
8. }
9. void draw()
10. {
11.   background(111);
12.   //myPort.write("temp?");
13.   int  i=0;
14.   String temp="";
15.   while ( i < 10) {
16.     temp = temp+char(i+j+65) ;
17.     i++;
18.   }
19.   j++;
20.   // val = temp+"\n"+val;
21.   //println(val);
22.   if (temp != null && temp.length() > 0) {
23.     text(temp, 50, 100, 110, 200);
24.   }
25. }

I know why my example wasn't working, I declared val as a global and each iteration of draw() was appending to the variable.