Java空指针异常NullPointerException

package varycode;

import java.util.ArrayList;
import java.util.Random;

public class VaryCodeDemo {

    public static void main(String[] args) {
        // TODO 自动生成的方法存根
        VaryCode test1=new VaryCode(null);
        Random r2=new Random(2);
        int choice ;
        char c[] = {'a','b','c','d','e','f'};
        
        
        for(int i=0;i<6;i++) {
            choice = r2.nextInt(1)*7;
        switch(choice) {
        case 1:
            c[0]=test1.downC();
            break;
        case 2:
            c[1]=test1.downC();
            break;
        case 3:
            c[2]=test1.downC();
            break;
        case 4:
            c[3]=(char) test1.num();
            break;
        case 5:
            c[4]=(char) test1.num();
            break;
        case 6:
            c[5]=test1.downC();
            break;
            }
        }
        for(int i=0;i<6;i++) {
               System.out.print(c[i]);
            }
        }
    }

package varycode;

import java.util.ArrayList;
import java.util.Random;

public class VaryCodeDemo {

    public static void main(String[] args) {
        // TODO 自动生成的方法存根
        VaryCode test1=new VaryCode(null);
        Random r2=new Random(2);
        int choice ;
        char c[] = {'a','b','c','d','e','f'};
        
        
        for(int i=0;i<6;i++) {
            choice = r2.nextInt(1)*7;
        switch(choice) {
        case 1:
            c[0]=test1.downC();
            break;
        case 2:
            c[1]=test1.downC();
            break;
        case 3:
            c[2]=test1.downC();
            break;
        case 4:
            c[3]=(char) test1.num();
            break;
        case 5:
            c[4]=(char) test1.num();
            break;
        case 6:
            c[5]=test1.downC();
            break;
            }
        }
        for(int i=0;i<6;i++) {
               System.out.print(c[i]);
            }
        }
    }

 

遇到的问题：

无法赋值

空指针异常