After the connection between two objects is made, these objects cannot connect anymore to other nodes.

1.  if (drawing==true) {
2.     if ((a==0)&&(b==1)) lines[0].visible=true;
3.     if ((a==0)&&(b==3)) lines[1].visible=true;
4.     if ((a==0)&&(b==4)) lines[2].visible=true;
5.     if ((a==1)&&(b==2)) lines[3].visible=true;
6.     if ((a==1)&&(b==3)) lines[4].visible=true;
7.     if ((a==1)&&(b==4)) lines[5].visible=true;
8.     if ((a==2)&&(b==3)) lines[6].visible=true;
9.     if ((a==3)&&(b==4)) lines[7].visible=true;
10.   }

1. color dotColor = color(192);
2. color dotColorOver = color(255);
3. color dotColorClicked = color(255, 255, 0);
4. // arrays are used to store the dots and to store the lines
5. Dot[] dots = new Dot[5];
6. Line[] lines = new Line[8];
7. // 99 is just a number to indicate that no dot is selected
8. // it cannot be zero, because the first element in the array is zero
9. int selectedDot=99, startDot = 99, endDot = 99;
10. // if a dot is selected draw a line from that dot to the mouse position
11. boolean drawing;
12. boolean solved;
13. void setup() {
14.   size(200, 200);
15.   frameRate(15);
16.   drawing=false;
17.   solved=false;
18.   /* create each dot, the coordinates are random */
19.   dots[0] = new Dot((int) random(width), (int) random(height));
20.   dots[1] = new Dot((int) random(width), (int) random(height));
21.   dots[2] = new Dot((int) random(width), (int) random(height));
22.   dots[3] = new Dot((int) random(width), (int) random(height));
23.   dots[4] = new Dot((int) random(width), (int) random(height));
24.   /* set the color for each dot and draw the dot */
25.   for (int i=0; i<5; i++) {
26.     dots[i].c = dotColor;
27.     dots[i].display(dots[i].c);
28.   }
29.   lines[0] = new Line(0, 1);
30.   lines[1] = new Line(0, 3);
31.   lines[2] = new Line(0, 4);
32.   lines[3] = new Line(1, 2);
33.   lines[4] = new Line(1, 3);
34.   lines[5] = new Line(1, 4);
35.   lines[6] = new Line(2, 3);
36.   lines[7] = new Line(3, 4);
37. }
38. void draw() {
39.   background(205);
40.   /* draw the dots */
41.   for (int i=0; i<5; i++) {
42.     dots[i].display(dots[i].c);
43.   }
44.   /* draw the lines */
45.   for (int i=0; i<8; i++) {
46.     if (lines[i].visible==true) {
47.       lines[i].display();
48.     }
49.   }
50.   /* draw a line on mouse movement */
51.   if (drawing==true) {    
52.     line(dots[selectedDot].x, dots[selectedDot].y, mouseX, mouseY);
53.   }
54. }
55. void mouseReleased() {
56.   if (selectedDot!=99) {
57.     dots[selectedDot].c=dotColor;
58.   }
59.   /* find out which dot was selected */
60.   drawing=false; 
61.   for (int i=0; i<5; i++) {
62.     if ((dots[i].selected()==true) && (i!=startDot)) {
63.       drawing=true;
64.       dots[i].c=dotColorClicked;
65.       selectedDot=i;
66.       break;
67.     }
68.   }
69.   if (drawing==true) {
70.     if (startDot==99) {
71.       startDot=selectedDot;
72.     }
73.     else if (endDot==99) {
74.       endDot=selectedDot;
75.       if ((!dots[startDot].hasLine) && (!dots[endDot].hasLine)) {
76.         drawLine(startDot, endDot);
77.         dots[startDot].hasLine=true;
78.         dots[endDot].hasLine=true;
79.       }
80.       else {
81.         println("was already connected.");
82.       }
83.     }
84.     startDot=selectedDot;
85.     endDot=99;
86.   }
87.   else {
88.     startDot=99;
89.     endDot=99;
90.   }
91. }
92. void mouseMoved() {
93.   if (solved==false) {
94.     for (int i=0; i<5; i++) {
95.       if (selectedDot==i) {
96.         // do not change on mouseover if the dot is the selected dot
97.       }
98.       else
99.         if (dots[i].selected()==true) {
100.           dots[i].c=dotColorOver;
101.         }
102.         else {
103.           dots[i].c=dotColor;
104.         }
105.     }
106.   }
107. }
108. /* mouseDrag is the same as mouseMove */
109. void mouseDragged() {
110.   mouseMoved();
111. }
112. class Dot {
113.   int x; // dot x position
114.   int y; // dot y position
115.   int s; // dot size
116.   color c; // dot color
117.   boolean selected;
118.   boolean hasLine=false;
119.   Dot(int localX, int localY) {
120.     x = localX;
121.     y = localY;
122.     s = 20;
123.     c = color(192);
124.   }
125.   void display(int dotColor) {
126.     fill(dotColor);
127.     ellipse(x, y, s, s);
128.   }
129.   boolean selected() {
130.     /* this code makes a square over the circle */
131.     /*   if (mouseX >= x-(s/2) && mouseX <= x+(s/2) &&
132.      mouseY >= y-(s/2) && mouseY <= y+(s/2)) {   */
133.     /* code for circle taken from Examples -> Topics -> GUI -> Button */
134.     float disX = x - mouseX;
135.     float disY = y - mouseY;
136.     if (sqrt(sq(disX) + sq(disY)) < s/2 ) {
137.       return true;
138.     }
139.     else {
140.       return false;
141.     }
142.   }
143. }
144. class Line {
145.   int startX, startY;
146.   int endX, endY;
147.   boolean visible;
148.   Line(int startDot, int endDot) {
149.     startX=dots[startDot].x;
150.     startY=dots[startDot].y;
151.     endX=dots[endDot].x;
152.     endY=dots[endDot].y;
153.     visible=false;
154.   }
155.   void display() {
156.     line(startX, startY, endX, endY);
157.   }
158. }
159. void drawLine(int startDot, int endDot) {
160.   /* set a and b to something different than from 0 to 4 */
161.   int a = 9;
162.   int b = 9;
163.   /* change the order of the selected dots, so that the
164.    * dot with the lowest number comes first
165.    * then check which line fits to the selected dot combination
166.    */
167.   if (startDot < endDot) {
168.     a = startDot;
169.     b = endDot;
170.   }
171.   else if (startDot > endDot) {
172.     a = endDot;
173.     b = startDot;
174.   }
175.   else {
176.     drawing=false;
177.   }
178.   if (drawing==true) {
179.     if ((a==0)&&(b==1)) lines[0].visible=true;
180.     if ((a==0)&&(b==3)) lines[1].visible=true;
181.     if ((a==0)&&(b==4)) lines[2].visible=true;
182.     if ((a==1)&&(b==2)) lines[3].visible=true;
183.     if ((a==1)&&(b==3)) lines[4].visible=true;
184.     if ((a==1)&&(b==4)) lines[5].visible=true;
185.     if ((a==2)&&(b==3)) lines[6].visible=true;
186.     if ((a==3)&&(b==4)) lines[7].visible=true;
187.   }
188. }