Example of multithreading from a reddit question

Entropy.java

import java.util.ArrayList;
import java.util.List;
import java.util.Collections;


public class Entropy {

	public static void main(String[] args) {
		

	}
	
	private double log(double num, int base){
	    return Math.log(num)/Math.log(base);
	}

	public double entropy(List<String> data){
		
		double entropy = 0;
	    
	    ArrayList<EntropyCalculator> calcs = new ArrayList<EntropyCalculator>();
	    int numProcessors = Runtime.getRuntime().availableProcessors();
	    for (int i = 0; i < numProcessors; i++)
	    {
	    	calcs.add(new EntropyCalculator());
	    }
	    
	    int j = 0;
	    for (int i = 0; i < data.size(); i++) {
	    	calcs.get(j).addData(data.get(i));
	    	if (j == calcs.size() - 1) {
	    		j = 0;
	    	} else {
	    		j++;
	    	}
	    }
	    
	    for (int i = 0; i < calcs.size(); i++) {
	    	calcs.get(i).run();
	    }
	    
	    //we just wait for one of them because the rest shouldn't be far behind.
	    while (calcs.get(0).isRunning()) {
	    	try {
	    		//100 ms sleep while we wait, this could be optimized
				Thread.sleep(100);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
	    }
	    
	    for (int i = 0; i < calcs.size(); i++) {
	    	//check my math, you may want to use another way to combine results
	    	entropy += calcs.get(i).getResult();
	    }
	    
	    return entropy;
	}
	
	private class EntropyCalculator implements Runnable {
		
		private double result = 0;
		
		private List<String> data = Collections.emptyList();
		
		private boolean running = false;
		
		public void run() {
			running = true;
		    double entropy = 0.0;
		    double prob = 0.0;

		    if(this.iFrequency.getKeys().length==0){
		        this.setInterestedFrequency(data);
		    }


		    String[] keys = iFrequency.getKeys();

		    for(int i=0;i<keys.length;i++){

		        prob = iFrequency.getPct(keys[i]);
		        entropy = entropy - prob * log(prob,2);
		    }

		    iFrequency.clear();
		    
		    result = entropy;
			running = false;
		}
		
		
		public double getResult() {
			return result;
		}


		public void addData(String data) {
			this.data.add(data);
		}
		
		public boolean isRunning() {
			return running;
		}
	}

}