FFT Java

I am no expert on this stuff but since no one else has answered, I'll give it a shot.

Try this (I haven't tested it but it should be close):

First, change "int x[] = new int[n];" to "double x[] = new double[n];"

Then, replace the last box with the following:

DoubleFFT_1D f = new DoubleFFT_1D(n/2);
f.realForwardFull(x);
for (int j = 0; j < n; j += 2) {
	double amplitude = 2 * x[j]/n;
	double frequency =(j/2) * h / T * sample_rate;
	System.out.println("frequency = "+frequency+", amp = "+amplitude);
}

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Be sure to check these results against your existing algorithm to determine if this method is correct.

Also, since the transform only uses the first half of x, you can save time in the loop populating x by changing "for (int i = 0; i < n*2; i+=2) {" to "for (int i = 0; i < n; i+=2) {".  There's no sense populating values that you're not using.

There is a complete package, including Java source code at
http://sites.google.com/site/piotrwendykier/software/jtransforms

It covers 1D, 2D and 3D. You should select what algorithm is the right for your needs, but most common is DFT.  The author claims it as the fastest FFT around and a benchmark is shown, to compare it against other packages.

It was developed in Unix environment, but compiles without problems in Windows or Linux, providing the right Java libraries.

Jose