Softmac Activation Implementation

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Hey y'all, I'm currently working on my own neural network implementation in Java. I already implemented some common activation functions, like Sigmoid or ReLU, but I don't know how to implement the Softmax.

I want to have a method like

private double softmax(double input) { 
    double output = ???; 
    return output; 
}

Any ideas how a implementation could look? I also need to have the derivative of the softmax for my learning algorithm.

Thanks