Convert a List to List in Java (or vice versa!)

Let us convert between List<Long> and List<Integer> in Java.

If we’re working with the following lists:

List<Integer> ints;
List<Long> longs;

We’ll be using Long::intValue and Integer::longValue to convert between the two types.

1. Using Java 8 streams

You can convert a List<Long> to a List<Integer> using map(Long::intValue) in a stream.

ints = longs.stream()
        .map(Long::intValue)
        .collec‌t(Collectors.toList(‌​))

And of course, you can convert a List<Integer> to a List<Long> using map(Integer::longValue).

longs = ints.stream()
        .map(Integer::longValue)
        .collec‌t(Collectors.toList(‌​))

2. Using a for loop

You can also use a traditional for loop to convert between the two types.

You’ll use the same intValue() method to convert from long to int.

for (Long num: longs) {
  ints.add(num.intValue());
}

Then, we’ll use longValue() to convert from int to long.

for (Integer num: ints) {
  longs.add(num.longValue());
}