First of all, I never get to say this, but I see everybody else saying it all the time:
Welcome to Javaranch!
Dear all:
Can you help me?
How can I open *.db file ?
The database file is a binary file, so you'll need to read it with something that implements DataInput, and I suggest RandomAccessFile.
But did you just want to take a peek at its contents?
You should grab a hex editor point it to that file. Try something like
frhed (free hex editor).