How to perform nested 'if' statements using Java 8/lambda?

The essential observation here is that your problem involves a non-isomorphic transformation: a single input element may map to zero, one, or two output elements. Whenever you notice this, you should immediately start looking for a solution which involves flatMap instead of map because that's the only way to achieve such a general transformation. In your particular case you can first apply filter for a one-to-zero element mapping, then flatMap for one-to-two mapping:

List<Integer> result =
    IntStream.rangeClosed(1, 10)
             .filter(i -> 10 % i == 0)
             .flatMap(i -> i == 5 ? IntStream.of(i) : IntStream.of(i, 10 / i))
             .boxed()
             .collect(toList());

_{(assuming import static java.util.stream.Collectors.toList)}

You could declare a body for a lambda. For example:

Runnable run = () -> System.out.println("Hey");

Could be

Runnable run = () -> {
    System.out.println("Hey");
};

Within that body, you can create nested statements:

Runnable run = () -> {
    int num = 5;

    if(num == 5) {
        System.out.println("Hey");
    }
};

Use flatMap as you are trying to add elements into the pipeline or a 1-to-many mapping. Map is a one to one mapping.

ArrayList<Integer> result = (ArrayList<Integer>) IntStream.rangeClosed(1, 10)
                .boxed()
                .filter(i -> 10 % i == 0)
                .flatMap((Integer i) -> {return i!=5 ? Stream.of(i, (10/i)):Stream.of(i);})
                .collect(Collectors.toList());

This results in the same list as

ArrayList<Integer> result2 = new ArrayList<Integer>();

        for (int i = 1; i <= 10; i++) {
            if (10 % i == 0) {
                result2.add(i);
                if (i != 5) {
                    result2.add(10 / i);
                }
            }
        }

---

In case your wondering which way is faster the loop method is ~3 times faster than using streams.

Benchmark                     Mode  Cnt      Score     Error  Units
testStreams.Bench.loops       avgt    5     75.221 ±   0.576  ns/op
testStreams.Bench.streams     avgt    5    257.713 ±  13.125  ns/op