error: no enclosing instance of type BPN is in scope {
<pre><code>class BPN {
public class BackpropagationNet extends NeuralNet
{
    Vector neuronLayerVector;
    NeuronLayer[] neuronLayerArray;
    //...
    public BackpropagationNet()
    {
        this.learningCycle = 0;
        this.maxLearningCycles = -1;
        //....
        resetTime();
    }
    //some functions
    void addNeuronLayer(int paramInt)
    {//....     }
    void connectLayers()
    {//....}

}
abstract class NeuralNet
{   
    final int PATTERN_LENGTH = 100;
    final int PATTERN_VALUE = 101;
    final int PATTERNFILE_LENGTH = 102;
    final int GENERAL_IO = 104;
    //....

}
static BackpropagationNet bpn;  

public static void main ( String[] args ) {
   // some logic...
    bpn = new BackpropagationNet();
        //...
    }
}
</code></pre>

<p>Well, this is short program which should demonstrate the problem.
When i try to compile i receive this error:
no enclosing instance of type BPN is in scope {(line 9)</p>
