Java - Alter frequency using FFT

After searching for FFT code in java I have come across three classes that do the job, read the wav file, process the wav file and write out a new wav file. The only problem I am having with the code is how to change the frequency. A line in the code leaves a gap for "// change frequency, put code here for HW5" but I am at a loss as how to do that?

Heres the code:

public class fft_wav
{
public fft_wav(String file_in, String file_out)
{
double amin, amax, aavg;
int m; // next power of 2

read_wav R = new read_wav(file_in, file_out, false);
// wav_read(file_in);  done by constructor
int n开发者_开发百科 = R.outix;  // number of sound samples
for(m=64; m<2500000; m*=2) if(m>2*n) break;
System.out.println("power of 2 m=" + m);

System.out.println("have "+n+" points for FFT");
double A[] = new double[m];
double B[] = new double[m];
double B2[] = new double[m];
for(int i=0; i<2*n; i+=2) A[i] = (double)R.A[i/2]; // real
for(int i=1; i<2*n; i+=2) A[i] = 0.0;              // imag
amin = A[0];
amax = A[0];
aavg = 0.0;
for(int i=0; i<2*n; i+=2)
{
  amin = Math.min(amin, A[i]);
  amax = Math.max(amax, A[i]);
  aavg += A[i];
}
aavg = aavg/(double)n;
System.out.println("input min="+amin+", max="+amax+", avg="+aavg);
for(int i=2*n; i<m; i++) A[i] = 0.0; // zero to power of 2

B = Cxfft.fft(A,1.0);

// change frequency, put code here for HW5

A = Cxfft.fft(B,-1.0);
amin = A[0];
amax = A[0];
aavg = 0.0;
for(int i=0; i<2*n; i+=2)
{
  amin = Math.min(amin, A[i]);
  amax = Math.max(amax, A[i]);
  aavg += A[i];
}
aavg = aavg/(double)n;
System.out.println("output min="+amin+", max="+amax+", avg="+aavg);

for(int i=0; i<2*n; i+=2) R.A[i/2] = (int)(0.99*A[i]);
for(int i=0; i<n; i++)
{
  if(R.A[i]>127)  R.A[i]= 127;
  if(R.A[i]<-127) R.A[i]=-127;
}
R.wav_write(file_out);
}

public static void main (String[] args)
{
String file_in = "notify.wav";
if(args.length>0)
{
  file_in = args[0];
}
String file_out = "junk.wav";
if(args.length>1)
{
  file_out = args[1];
}
new fft_wav(file_in, file_out);
} // end main
} // end class fft_wav of fft_wav.java