General Error: null in iReport

[Sakura Hana]

Dear everyone!!!
I am new to Jaspersoft.
I was trying create bean datasource in iReport, but I get message like this:



This is my java code:
 

package com.source.myclass;public class barchart {    private String Name;    private double Number;       public barchart(){};       public void setName(String n){this.Name=n;}       public void setNumber(double n){this.Number=n;}       public String getName(){return this.Name;}       public double getNumber(){return this.Number;}}package com.source.main;import java.util.Collection;import java.util.HashSet;import java.util.Vector;import org.rosuda.JRI.REXP;import org.rosuda.JRI.Rengine;import com.source.myclass.barchart;public class RJAVA4 {       public RJAVA4(){};       public static Collection<barchart> getBarchartData() throws Exception{        Vector<barchart> bars = new Vector<barchart>();        try{            Rengine engine = new Rengine(new String[]{"--no-save"}, false, null);            REXP NameR = engine.eval("substring('ABCDEFGHIJ',1:10,1:10)");            REXP NumberR = engine.eval("rnorm(10)+100");            engine.end();                       String[] STRING_NAME = NameR.asStringArray();            double[] NUMBER = NumberR.asDoubleArray();                       barchart bar = null;            for(int i=0;i<10;i++){                bar = new barchart();                bar.setName(STRING_NAME[i]);                bar.setNumber(NUMBER[i]);                bars.add(bar);            }        }               catch(Exception e){            e.printStackTrace();        }        return bars;    }       public static void main(String[] args) throws Exception {        Collection<barchart> myCollection = new HashSet<barchart>();        myCollection = getBarchartData();        for(barchart element: myCollection){            System.out.println(element.getName() + "|||"+element.getNumber());        }    }}[/code]

Any helps? really appreciate.