You can see what is goind wrong if you step through your code

1. void keyReleased()
2. {
3.   int number1 = int(key) - int('0');
4.   text(number1 + "+", 270, 20);
5.   if (number1 != '0') {
6.     int number2 = int(key) - int('0');
7.     text(number2, 293, 20);
8.     int sum = number1 + number2;
9.     text("=", 307, 20);
10.     text(sum, 322, 20);
11.   }
12.     loop();
13. }

Now imagine you have pressed and released the key <2> then your code will be executed
Line 3 - will store the numeric value 2 in number1
Line 4 - will display first number and the plus sign
Line 5 - compares number1 to 48 (the ASCII value for key <0>) for inequality (this will always be true!!!)
Line 6...10 - will always be executed since the test in line 5 will always be true. so you get 2+2=4

Also your program does not check for valid keys - try a non-numeric key

The code below solves these problems.

1. int number1 = -1;
2. int number2 = -1;
3. int sum;
4. 
   
5. void setup()
6. {
7.   size(500, 200);
8.   background(255);
9.   fill(0);
10.   PFont f = createFont("Verdana", 16);
11.   textFont(f);
12. }
13. 
    
14. void draw()
15. {
16.   background(255);
17.   text("Type a number to be summed:", 10, 20);
18.   if (number1 >= 0) {
19.     text(number1 + "+", 270, 20);
20.   }
21.   if (number2 >=0) {
22.     text(number2, 293, 20);
23.     text("=", 307, 20);
24.     text(sum, 322, 20);
25.   }
26. }
27. 
    
28. void keyReleased()
29. {
30.   int n = int(key) - int('0');
31.   if (n >= 0 && n <= 9)
32.     if (number1 < 0) {
33.       number1 = n;
34.     }
35.     else if (number2 <0) {
36.       number2 = n;
37.       sum = number1 + number2;
38.     }
39. }

Wrong forum section too. You posted 9 threads, perhaps you can start and read the section descriptions, now? Thanks.
(Moved from Processing Implementations.)

Note: if you don't understand these descriptions, perhaps because you don't master English, or because the descriptions are not precise enough, don't hesitate to ask questions on them (eg. in General Discussions section), so we can improve them. Thanks.

Hi quarks,

Thanks for your indepth reply. I don't understand line

32.   if (number1 < 0) {   and 35. either.

Is this checking that number1 is negative and why would that be necessary?

Thanks,

Shane

I am not a native English speaker, and I still make errors, indeed. I actually can appreciate useful remarks on my usage. (Although I would have written "because you haven't master ed English", so I suppose I am too thick to write correct English now.)

And there are lot of people not mastering English here, hence my genuine question: I don't want to judge somebody because they struggled with English. I can't guess the origin / native language of somebody by looking at their pseudonym, so I take general caution.
So, my tone wasn't ironic or depreciative or something, but truly trying not to hurt the feelings of somebody having difficulties with English.

And, yes, my remark was about this specific thread, I meant that since you are not new to the forum, you should start and get familiar with the topic of each section. Processing Implementations is about non-Java languages reproducing the Processing API, and your message seemed to be about regular Processing.
I moved routinely dozens of messages around in the forum, because of such sloppiness (or overlooking, again, I don't want to judge hastily), and sometime I post a message about such error, in the hope some people will read them and take actions to avoid such errors.
If my tone can be perceived as displeasing, it might be because I am clumsy with my usage of English, and well, honestly, because sometime I am tired of repeating such remarks because people don't read https://forum.processing.org/allforums (but, again, these descriptions can probably improved, hence my remark).

Last remark: I don't kick off people from this forum because they displease me (and I am quite tolerant, anyway), only when they spam the forum (ie. posting links to commercial sites, etc.).

To end on an on-topic message, quarks initializes the variables with -1, so if they are still negative, it means they are not initialized yet. This allows to assign the first typed value to number1, then the second one to number2.

I understand now, thanks

No problem: I took no offense, people often write without knowing everything about the person they talk too (I too, but I try to be cautious...), and having the right tone in the written medium is hard, particularly for those not at ease with writing (or with a foreign language). Sometime I am ironic (trying to be light), sometime I am slightly upset (but I should be more patient), but my remarks are always aiming to be educative, ie. as an attempt to reduce the number of errors.

And, again, I don't delete threads or messages because they could offend me (except if language is excessive, perhaps).

Back to Processing, and have fun (and spread fun) teaching it!