需要解释FFT类的参数

Question

The transform method produces a NullPointerException:

FastFourierTransformer transformer = new FastFourierTransformer(null);
    try {
        Complex[] complx = transformer.transform(sineValue, null);

I suspect the issue is the 'null' parameters, but I don't understand what the documentation is requiring for "DftNormalization" and for " TransformType".

For instance, if I type in the word STANDARD for DftNormalization or the word FORWARD for TransformType, Eclipse shows an error. Drilling down in the documentation didn't provide any help, just lists the words FORWARD and STANDARD.

I need a forward transform with standard normalization for a real array.

I generate the test array:

sineValue = new double[4096];

    for (int i = 0; i < 4096; i++) {
        sineValue[i] = Math.sin(i * Math.PI * 2 / MIN_RATE) + 0.0
                * Math.sin(i * Math.PI * 4 / MIN_RATE) + 0.0
                * Math.sin(i * Math.PI * 15.3 / MIN_RATE);
        System.out.println("line " + i + "value: " + sineValue[i]);

MIN_RATE is 256.

Answer 1

When passing the second argument you need to prefix the value by the type name:

FastFourierTransformer transformer = new FastFourierTransformer(null);
try {
    Complex[] complx = transformer.transform(sineValue, TransformType.FORWARD);

Alternatively use can do a static import:

import static org.apache.commons.math3.transform.TransformType.*;

Then you can omit the type name:

FastFourierTransformer transformer = new FastFourierTransformer(null);
try {
    Complex[] complx = transformer.transform(sineValue, FORWARD);

The same is applicable to the constructor call:

FastFourierTransformer transformer = new FastFourierTransformer(DftNormalization.STANDARD);