When I do floating point operation, why some unwanted result comes

Q. When I do floating point operation, why some unwanted result comes
out?
// Example, why the red extra comes out?
//*******************************
double x = 27.475;
double y = 7.22;
System.out.println(x - y); // 20.255000000000003
//*******************************
A:
That is a famous floating point phenomenon. Your number is represented in decimal, and your computer is represented them in binary, some discrepancy will happen with that conversion back-and-forth. It is normal since Machine Language / Assembly / Fortran time. If you go to an interview, someone ask you this question, you don't know the answer, unexperienced programmer situation will be exposed to the interviewer immediately.
When you output, format it to what you desired. How the precision changed? This is no easy job. You need understand what is radix.
Radix for decimal is 10
Radix for binary is 2
29.3 in decimal is actually
2 * 101 + 9 * 100 + 3 * 10 -1
In binary
11101.010011001.............
Do the same thing, but replace 10 to 2. However, the number will never end. You will find out something is short in decimal may become infinite long in binary. And the opposite will not be infinite long, but possible longer then the computer system allowed. Computer cannot allow infinite long number, it must be truncated to fit the precision of the floating point number in the system.Therefore, the value of some floating point number changes in the conversion from one radix to another radix back-and-forth. If 29.125 in decimal converts to binary, then no problems. It will be 11101.001 in binary precisely, since 0.125 is 2-3 or 1/8