我在Java程序上遇到问题。线程“主”中的异常

java.lang.NullPointerException
at twoten.TwoTenB.<init>(TwoTenB.java:29)
at javapractice.JavaPractice.main(JavaPractice.java:32)
Java Result: 1

是我得到的错误。我真的可以使用一些帮助，因为我在这个地方呆了几个小时…

package twoten;

import java.util.Scanner;

public class TwoTenB {

public TwoTenB() {
    double percentage;
    double a[] = null;
    double total = 0;
    double var;
    System.out.print("\tRESULT\n\n");
    Scanner scan = new Scanner(System.in);
    //double[] mark = new double[7];
    for (int i = 0; i < 7; i++) {

        System.out.print("\nMarks in subject " + (i + 1) + "\t:\t");
        var = scan.nextDouble();

        a[i] = var;

        total = total + a[i];
       //percentage = first * second * third * fourth * fifth * sixth * seventh * 100 / 700;
    }

    percentage = total * 100 / 700;

    if (a[0] > 35 && a[1] > 35 && a[2] > 35 && a[3] > 35 && a[4] > 35 && a[5] > 35 && a[6] > 35 && percentage > 35) {
        if (percentage >= 60) {
            System.out.print("\nCongratulation!!! you've got FIRST dividion\n");
        } else if (percentage >= 45 && percentage < 60) {
            System.out.print("\nCongratulation!!! you've got SECOND dividion\n");
        } else if (percentage >= 35 && percentage < 45) {
            System.out.print("\nCongratulation!!! you've got THIRD dividion\n");
        }
    } else {
        System.out.print("\nSORRY!!! you've FAILED\n");
    }
    }
}