Well, long itself is a long integer ... In java, 3 is an integer and 3L is a long integer. This is to distinguish an integer from a long integer, with length l = 3；; //This is also possible, because 3 is an integer, and the length range is less than a long integer, so you can assign a value, and the system will add Llong l = 3L； by default; //This is the most formal assignment of long integer data ... Remember: in java, 3 stands for integer, 3L stands for long integer, and L stands for long integer.