When creating custom handlers the first parameter should be of the same type (class) as the control so you should use

public void handleSensorAtomTextEvent(GTextField text, GEvent event) {   }

which explains why it was not using your custom handler.

The code below shows how you can use a single handler for an array of controls AND detect which of the controls is affected.

1. import g4p_controls.*;
2. GTextField[] sensorsList;
3. GLabel lblOuput;
4. int nbr = 3;
5. public void setup() {
6.   size(300, 300);
7.   lblOuput = new GLabel(this, 10, 10, 280, 16);
8.   lblOuput.setTextAlign(GAlign.LEFT, null);
9.   sensorsList = new GTextField[nbr];
10.   for (int i = 0; i < nbr; i++) {
11.     sensorsList[i] = new GTextField(this, 10, 40 + i * 25, 200, 16);
12.     sensorsList[i].addEventHandler(this, "handleSensorListtEvents");
13.     sensorsList[i].tagNo = i;
14.     sensorsList[i].tag = "sensor " + i;
15.   }
16. }
17. 
    
18. public void handleSensorListtEvents(GTextField textcontrol, GEvent event) {
19.   int idx = -1;
20.   // Which GTextField is it?
21.   for (int i = 0; i < nbr; i++) {
22.     if (textcontrol == sensorsList[i])
23.       idx = i;
24.   }
25.   if (idx >= 0) {
26.     switch(event) {
27.     case CHANGED:
28.       lblOuput.setText("Sensor " + idx + " text changed");
29.       break;
30.     case SELECTION_CHANGED:
31.       lblOuput.setText("Sensor " + idx + " selected '" + textcontrol.getSelectedText() + "'");
32.       break;
33.     case ENTERED:
34.       lblOuput.setText("Sensor " + idx + " enter / return key typed");
35.       break;
36.     }
37.   }
38. }
39. public void draw() {
40.   background(200,200,255);
41. }
42. 
    

Every control has two public members tag (String) and tagNo (int) which are not used by G4P but can be used for anything the programmer wants, notice that I set these when I create the text fields. So the event handler code could be simplified further to

1. public void handleSensorListtEvents(GTextField textcontrol, GEvent event) {
2.   switch(event) {
3.   case CHANGED:
4.     lblOuput.setText("Sensor " + textcontrol.tagNo + " text changed");
5.     break;
6.   case SELECTION_CHANGED:
7.     lblOuput.setText("Sensor " + textcontrol.tagNo + " selected '" + textcontrol.getSelectedText() + "'");
8.     break;
9.   case ENTERED:
10.     lblOuput.setText("Sensor " + textcontrol.tagNo + " enter / return key typed");
11.     break;
12.   }
13. }

HTH