Java 8 toMap IllegalStateException Duplicate Key

The pramodh’s answer is good if you want to map your value to 1. But in case you don’t want to always map to a constant, the use of the “merge-function” might help:

Map<Integer, Integer> map1 = Files.lines(Paths.get(inputFile))
                .map(line::trim())
                .map(Integer::valueOf)
                .collect(Collectors.toMap(x -> x, x -> 1, (x1, x2) -> x1));

The above code is almost the same as posted in the question. But if it encounters a duplicate key, instead of throwing an exception, it will solve it by applying the merge function, by taking the first value.