Java bútor

package butorgyar;


public class butorlap {
    private int hossz;
    private int szel;
    private boolean tipus;

    public static final int tartoLap = 5000;
    public static final int hatLap = 500;

    public butorlap(int hossz, int szel, boolean tipus){
        this.hossz = hossz/100;
        this.szel = szel/100;
        this.tipus = tipus;
    }

    public int arSzamol(){
        return this.hossz*this.szel*((this.tipus == true)?butorlap.tartoLap:butorlap.hatLap);
    }

}
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public class butor {
    private int lapokSzama;
    private butorlap[] butor;

    public butor(){
        this.lapokSzama = 0;
        this.butor = new butorlap[100];
    }

    public void setLapokSzama(int lapokSzama){
        this.lapokSzama = lapokSzama;
    }

    public void hozzaAd(butorlap b){
        butorlap[] ujButor = new butorlap[butor.length+1];
        for (int i=0; i < butor.length;i++){
            ujButor[i] = butor[i];
        }
        ujButor[ujButor.length-1] = b;
        butor = ujButor;
    }

    public int arSzamol(){
        int sum = 0;
        for(int i=0; i < butor.length;i++){
            sum = sum + butor[i].arSzamol();
        }

        return sum;
    }
}
------------------
import butorgyar.*;

public class foo {
    public static void main(String[] args){
        butor b1 = new butor();
        butorlap bl1 = new butorlap(200,300,false);
        b1.setLapokSzama(10);
        b1.hozzaAd(bl1);
        System.out.println(b1.arSzamol());
    }
}
------------------
Exception in thread "main" java.lang.NullPointerException
    at butorgyar.butor.arSzamol(butor.java:34)
    at foo.main(foo.java:10)
Java Result: 1