How to read input from a serial port and then transfer it to a txt file?

Hii I would like to know how I shall read the input from a serial port and then transfer it to txt file using java

COM port access is made through the javax.comm api.
The latest version of the comm api does not support Windows, so if you are on that platform your options are listed on our Java IO FAQ.
Have a look at the Java Tutorial for how to write to a file.