Flutter/Java Plot Mp3 Frequency intensity
The Discrete Fourier Transform (for which FFT is the most commonly used algorithm) is exactly what you need since it will map your time domain samples to the frequency domain. Assuming you have the sound samples over a period of time and the frequency those were sampled with, you will be able to achieve your goal, it doesn't really matters if it is real time or not.
The second step would be to process the results from your FFT, which would produce the amplitudes of the frequencies present in your sample.
Try Github for Java based FFT implementations https://github.com/search?q=java+fft and you are likely to find also some examples there.