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) {
    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))

[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]( However, the conditions that would favor using such an approach do not seem to be present.]