When using an array of objects, the data returned from the function must be cast to the object array's data type. For example: SomeClass[] items = (SomeClass[]) append(originalArray, element).

1. void setup() {
2.   size(rows * cellsize, cols * cellsize);
3.   String[] a1 = loadStrings("sync.txt");
4.   String[] b1 = {
5.     ""
6.   } 
7.   ;
8.   for (int i = 0; i < a1.length; i++) {
9.     if (a1[i].contains("-->")) {
10.       b1 =  (String [])append(b1, a1[i]);
11.     } // if
12.   }
13.   for (int i = 0; i < b1.length; i++) {
14.     println(b1[i]);
15.   }
16. ....

1. boolean [] ballsFalling = new boolean[0];

1. ballsFalling = (boolean[])append(ballsFalling,true);

1. ballsFalling = append(ballsFalling,5);