I can't quite understand what you mean by

the last entry is replaced with the previous one.

and

but it is not a good method when you have other stuff rather than just the number...since everything will disappear.

Nonetheless, I did a more compact version of which accepts buffMax entries into buff.
Perhaps it can help you out a bit. Check it out:
P.S.: Set buffMax = 1 to restore previous original version behavior.

final static byte  leftMargin = 10;
final static byte  buffMax = 5;
final static byte  FPS = 30;
final static color BG  = 0xFF, FG = 0;

String buff = "";
boolean isCellFull;

void setup() {

    size(300, 100);
    frameRate(FPS);

    //textFont(loadFont("AngsanaNew-Bold-25.vlw"), 45);
}

void draw() {

    background(BG);
    stroke( frameCount % FPS < FPS>>2 ? BG : FG );

    int rPos = (int) textWidth(buff) + leftMargin; 
    line(rPos, 9, rPos, 40);

    pushMatrix();

    fill(FG);
    translate(rPos, 35);

    for (int i = buff.length(); i != 0;) {
        char k = buff.charAt(--i);
        translate(-textWidth(k), 0);
        text(k, 0, 0);
    }

    popMatrix();
}

void keyPressed() {

    char k  = key;
    int len = buff.length();
    boolean isValidInput = k>='1' && k<='4';

    if (isValidInput)
        if (isCellFull)    buff = buff.substring(0, len-1) + k;
        else               buff += k;

    else if (k == BACKSPACE && len > 0) {
        buff = buff.substring(0, len-1);
        isCellFull = false;
    }

    if (buff.length() == buffMax)    isCellFull = true;
}

Thanks a lot GoToLoop for your replay and for the more compact version. I'm sorry for not explaining my problem very well. What I really want to know is if there is a way (i'm sure there is hehe) to execute the following code but only the first random number that is generated in the draw method is displayed in the sketch.

ok, the program must wait until user has entered 5 numbers.


in draw() do nothing until isCellFull is true.

1. if (isCellFull)  {
2.       // all you have in draw at the moment
3. }

you set isCellFull to true in keypressed  - good!



Global var myRandomNumber

Then have another global var myRandomNumber.

Set it once in keypressed  where you set isCellFull to true.

in draw() only use myRandomNumber, especially here:

1.   text((int)random(1, 4), 150, 50); //I want my program to only print the number that had been generated first. <========================

Greetings, Chrisir   

like this...........

1. final static byte  leftMargin = 10;
2. final static byte  buffMax = 5;
3. final static byte  FPS = 30;
4. final static color BG  = 255, FG = 0;
5. String buff = "";
6. boolean isCellFull=false;
7. int myRandomNumber=-1;
8. 
   
9. void setup() {
10.   size(300, 100);
11.   background(BG);
12.   frameRate(FPS);
13.   //textFont(loadFont("AngsanaNew-Bold-25.vlw"), 45);
14. }
15. void draw() {
16.   if (isCellFull) {
17.     background(BG);
18.     stroke(0);
19.     fill(10);
20.     if (myRandomNumber>-1) {
21.       //I want my program to only print the number that had been generated first. <========================
22.       text(buff.substring(myRandomNumber, myRandomNumber+1), 150, 50);
23.     }
24.   }
25.   else {
26.     background(BG);
27.     //    for (int i = buff.length(); i != 0;) {
28.     //    char k = buff.charAt(--i);
29.     //    translate(-textWidth(k), 0);
30.     //    text(k, 0, 0);
31.     fill(FG);
32.     text(buff, 10, 20);
33.     stroke( frameCount % FPS < FPS>>2 ? BG : FG );
34.     int rPos = (int) textWidth(buff) + leftMargin;
35.     line(rPos, 9, rPos, 40);
36.   }
37. }
38. 
    
39. void keyPressed() {
40.   char k  = key;
41.   int len = buff.length();
42.   boolean isValidInput = k>='1' && k<='4';
43.   if (!isCellFull) {
44.     if (isValidInput) {
45.       if (isCellFull)   
46.         buff = buff.substring(0, len-1) + k;
47.       else              
48.         buff += k;
49.     }
50.     else
51.       if (k == BACKSPACE && len > 0)
52.     {
53.       buff = buff.substring(0, len-1);
54.       isCellFull = false;
55.     }
56.     if (buff.length() == buffMax) {  
57.       isCellFull = true;
58.       myRandomNumber=int(random(0, buffMax));
59.       println("random number is: "
60.         + myRandomNumber+".");
61.     }
62.   }
63. } // func
64. //