There are multiple ways to count the number of 1's or set bits in an integer number in Java. You can use a bitwise and bit shift operator on your own, or, you can use Java API to count the number of set bits. Java 1.5 added two utility methods called bitCount(int i) which returns a number of 1's in your integer number, java.lang.Long class has a similar bitCount(long number) for long primitives. As I have said earlier, Coding questions are an important part of any Java interview, and from that recursion and bitwise operations are most popular.