How to convert a Field class instance to Generic Instance

    import java.lang.reflect.Field;
import java.lang.reflect.Type;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Scanner;

public class calculation {
    static double pm10,pm2dot5,no2,o3,co,so2,nh3,pb;
    static HashMap<String, Double> concentration;
    static HashMap<String, Double> subIndex;
    static Scanner sc ;
    public static HashMap<Integer, Integer> _pm10,_pm2dot5,_no2,_o3,_so2,_nh3;
    public static HashMap<Integer, Double> _co,_pb;
    public static void main(String[] args) {
        // TODO Auto-generated method stub
                indexCalculation();
    }
    static void indexCalculation () throws NoSuchFieldException, SecurityException
    {
        Iterator<String> gases = concentration.keySet().iterator();
        while(gases.hasNext())
        {
            String gas = gases.next();
            double value = concentration.get(gas);
            String str = "_"+gas;//this will create string instance like _so2,_nh3 etc..
            Class obj = calculation.class;
            Field field = obj.getField(str);
            Iterator<Integer> breakpoint = field.keySet().iterator();//unable to do..
        }
    }
}

• How to convert field instance to HashMap instance ,so thay i can call keySet() method on it.
• Or their any other to call an instance at run time.**