This should print the list of positions without the -1 at the end that Peter Lawrey's solution ~~has~~ had.

int index = word.indexOf(guess);
while (index >= 0) {
    System.out.println(index);
    index = word.indexOf(guess, index + 1);
}

It can also be done as a for loop:

for (int index = word.indexOf(guess);
     index >= 0;
     index = word.indexOf(guess, index + 1))
{
    System.out.println(index);
}

[Note: if guess can be longer than a single character, then it is possible, by analyzing the guess string, to loop through word faster than the above loops do. The benchmark for such an approach is the [Boyer-Moore algorithm](https://en.wikipedia.org/wiki/Boyer%E2%80%93Moorestringsearch_algorithm). However, the conditions that would favor using such an approach do not seem to be present.]