here...

1. ArrayList balls;
2. int ballWidth = 48;
3. void setup() {
4.   size(700, 700);
5.   smooth();
6.   noStroke();
7.   balls = new ArrayList();
8.   balls.add(new Ball(width/2, 0, ballWidth));
9. }
10. void draw() {
11.   background(255);
12.   for (int i = balls.size()-1; i >= 0; i--) {
13.     Ball ball = (Ball) balls.get(i);
14.     ball.display();
15.   }
16.   println( balls.size());
17. }
18. void mousePressed() {
19.   balls.add(new Ball(mouseX, mouseY, ballWidth));
20. }
21. class Ball {
22.   float x;
23.   float y;
24.   float w;
25.   final float fx=0;
26.   final float fy=0;
27.   Ball(float tempX, float tempY, float tempW) {
28.     x = tempX;
29.     y = tempY;
30.     w = tempW;
31.   }
32.   void fix() {
33.     if (keyPressed) {
34.       //      fx=this.x;  // commented these two lines out
    
35.       //      fy=this.y;
36.       this.x=fx;
37.       this.y=fy;
38.     }        //CRASH
39.   }
40.   void display() {
41.     fill(0);
42.     // rect(x, y, mouseX, mouseY);
43.     rect(x, y, w, w);  // used w here
    
44.   }
45. } 

void fix(){
  if(keyPressed){fx=this.x;fy=this.y; this.x=fx; this.y=fy;}        //CRASH
}
 
This crashes because fx and fy have been declared as final so they cannot have values assigned to something else.

I assume the fix method is simply a way to make x and y constant values. If that is true the method is not required.

If you change the class to the one below the attributes x and y are initialised in the constructor and cannot be changed later.

1. class Ball {
2.   final float x;
3.   final float y;
4.   float w;
5. 
   
6.   Ball(float tempX, float tempY, float tempW) {
7.     x = tempX;
8.     y = tempY;
9.     w = tempW;
10.   }
11.  
12.   void display() {
13.     fill(0);
14.     rect(x,y,mouseX,mouseY);
15.   }
16. } 

does exist a way to convert an int to a final int?

You cannot change a vraiable into a final variable.


final int x;

must be initialised immediately e.g.

final int x = 42;

or initialised inside a constructor.

A varable such as

int y;

cannot be converted into a final (constatnt) value later.

The general rule is that an identifier e.g. x or y is either a varaible OR a constant and cannot be changed to the other at runtime.

maybe this helps you.
It doesn't answer any of your questions about final, but maybe does what you want: Draw a rect by dragging (holding) the mouse.


usage:
you paint rects when you press and hold the mouse. Only the outline of the rect is shown.

On releasing mouse (or key press) the rect is stored and becomes solid black.

1. ArrayList balls;
2. int ballWidth = 48;
3. //
4. // mouse dragging
5. boolean mouseHold=false;
6. int startX, startY;
7. //
8. void setup() {
9.   size(700, 700);
10.   smooth();
11.   noStroke();
12.   balls = new ArrayList();
13.   // balls.add(new Ball(width/2, 0, ballWidth, ballWidth));
14. }
15. void draw() {
16.   background(255);
17.   for (int i = balls.size()-1; i >= 0; i--) {
18.     Ball ball = (Ball) balls.get(i);
19.     ball.display();
20.   }
21.   println( balls.size());
22.   // this is during dragging
23.   if (mouseHold) {
24.     stroke(30);
25.     noFill();
26.     rect(startX, startY, mouseX-startX, mouseY-startY);
27.   }
28. }
29. void mousePressed() {
30.   // starts dragging
31.   mouseHold=true;
32.   startX=mouseX;
33.   startY=mouseY;
34. }
35. void mouseReleased() {
36.   // ends dragging
37.   balls.add(new Ball(startX, startY, mouseX-startX, mouseY-startY));
38.   mouseHold=false;
39. }
40. void keyPressed() {
41.   // ends dragging
42.   balls.add(new Ball(startX, startY, mouseX-startX, mouseY-startY));
43.   mouseHold=false;
44. }
45. // ==================================================================
46. class Ball {
47.   float x;
48.   float y;
49.   float w;
50.   float h;
51.   final float fx=0;
52.   final float fy=0;
53.   Ball(float tempX, float tempY, float tempW, float tempH) {
54.     x = tempX;
55.     y = tempY;
56.     w = tempW;
57.     h = tempH;
58.   }
59.   //  void fix() {
60.   //    if (keyPressed) {
61.   //      //      fx=this.x;  // commented these two lines out
62.   //      //      fy=this.y;
63.   //      this.x=fx;
64.   //      this.y=fy;
65.   //    }       
    
66.   //  }
67.   void display() {
68.     fill(0);
69.     noStroke();
70.     // rect(x, y, mouseX, mouseY);
71.     rect(x, y, w, h);  // used w and h here
72.   }
73.   //
74. }  // class
75. //

new version:

• you can abort the current rect by pressing q
• you can undo rects you've drawn with backspace.
• you got better text output to explain wht you can do

1. ArrayList balls;
2. int ballWidth = 48;
3. //
4. // mouse dragging
5. boolean mouseHold=false;
6. int startX, startY;
7. //
8. void setup() {
9.   size(700, 700);
10.   smooth();
11.   // noStroke();
12.   balls = new ArrayList();
13.   // balls.add(new Ball(width/2, 0, ballWidth, ballWidth));
14. }
15. void draw() {
16.   background(255);
17.   for (int i = balls.size()-1; i >= 0; i--) {
18.     Ball ball = (Ball) balls.get(i);
19.     ball.display();
20.   }
21.   println( balls.size());
22.   // this is during dragging
23.   if (mouseHold) {
24.     fill(0);
25.     text("Release mouse to draw, press 'q' to abort rect. Rect is from "
26.       +startX+", "
27.       +startY+" to "
28.       +mouseX+", "
29.       +mouseY+".", 20, 20);
30.     stroke(30);
31.     noFill();
32.     rect(startX, startY, mouseX-startX, mouseY-startY);
33.   }
34.   else
35.   {
36.     fill(0);
37.     text("Backspace to undo. Click and hold mouse to draw. Mouse is at "
38.       +mouseX+", "
39.       +mouseY
40.       +".", 20, 20);
41.   }
42. }
43. void mousePressed() {
44.   // starts dragging
45.   mouseHold=true;
46.   startX=mouseX;
47.   startY=mouseY;
48. }
49. void mouseReleased() {
50.   // ends dragging
51.   if (mouseHold) {
52.     balls.add(new Ball(startX, startY, mouseX-startX, mouseY-startY));
53.     mouseHold=false;
54.   }
55. }
56. void keyPressed() {
57.   // ends dragging
58.   if (key=='q') {
59.     mouseHold=false;
60.   }
61.   if (key==BACKSPACE) {
62.     mouseHold=false;
63.     if (balls.size()>0)
64.       balls.remove(balls.size()-1);
65.   }
66.   else
67.   {
68.     balls.add(new Ball(startX, startY, mouseX-startX, mouseY-startY));
69.     mouseHold=false;
70.   }
71. }
72. // ==================================================================
73. class Ball {
74.   float x;
75.   float y;
76.   float w;
77.   float h;
78.   final float fx=0;
79.   final float fy=0;
80.   Ball(float tempX, float tempY, float tempW, float tempH) {
81.     x = tempX;
82.     y = tempY;
83.     w = tempW;
84.     h = tempH;
85.   }
86.   //  void fix() {
87.   //    if (keyPressed) {
88.   //      //      fx=this.x;  // commented these two lines out
89.   //      //      fy=this.y;
90.   //      this.x=fx;
91.   //      this.y=fy;
92.   //    }       
    
93.   //  }
94.   void display() {
95.     fill(0);
96.     noStroke();
97.     // rect(x, y, mouseX, mouseY);
98.     rect(x, y, w, h);  // used w and h here
99.   }
100.   //
101. }  // class
102. //

many thans chrisir