For an array of primitives, the simplest implementation speaks for itself:

public static int findIndexOfElement(int[] arr, int toFind) {

  for (int i = 0; i < arr.length; i++) {
    if (arr[i] == toFind) {
      return i;
    }
  }

  return -1;
}

Relying on Java 8 functional style, we can try to loop the array and filter the elements that match the given element. In the end, simply return the first found element:

public static int findIndexOfElement(int[] arr, int toFind) {

  return IntStream.range(0, arr.length)
    .filter(i -> toFind == arr[i])
    .findFirst()
    .orElse(-1);
}

For an array of Object, there are at least three approaches. In the first instance, we can rely on the equals() contract:

public static <T> int findIndexOfElementObject(T[] arr, T toFind) {

  for (int i = 0; i < arr.length; i++) {
    if (arr[i].equals(toFind)) {
      return i;
    }
  }

  return -1;
}

Secondly, we can rely on a Comparator:

public static <T> int findIndexOfElementObject(
    T[] arr, T toFind, Comparator<? super T> c) {

  for (int i = 0; i < arr.length; i++) {
    if (c.compare(arr[i], toFind) == 0) {
      return i;
    }
  }

  return -1;
}

And thirdly, we can rely on Java 8 functional style and a Comparator:

public static <T> int findIndexOfElementObject(
    T[] arr, T toFind, Comparator<? super T> c) {

  return IntStream.range(0, arr.length)
    .filter(i -> c.compare(toFind, arr[i]) == 0)
    .findFirst()
    .orElse(-1);
}