Why is method repeated use of method returning null? [closed]

I am producing a Rubik's Cube scramble generator which creates a string of 17-18 moves/members. Basically, I'm putting a for loop for getting one move each time; 18 times. In the second function (static String printMove()), I have used java.util.Random to give a new move every time. However, the first move is random, after which all the other 17 members/moves are null. I rechecked it and can't understand where I am going wrong.

Here's my code, if anyone could help. I really needed help on this since I thought of creating a GUI based on this simple system later but can't get the basics itself.

import java.util.*;
class Gen
{
    public static void main()
    {
        String scramble[]=new String[18];
        for(int x=0;x<=17;x++)
        {
            scramble[0]=printMove();
        }
        String sc=Arrays.toString(scramble);
        System.out.println(sc);
    }
    public static String printMove()
    {
        String moves[]={"","","R","R\'","R2","U","U\'","U2","F","F\'","F2","L","L\'","L2","D","D\'","D2","B","B\'","B2"};
        String move="";
        Random r=new Random();
        int ch=r.nextInt(19)+1;
        switch(ch)
        {
            case 1:
                move=moves[ch];
                break;
            case 2:
                move=moves[ch];
                break;
            case 3:
                move=moves[ch];
                break;
            case 4:
                move=moves[ch];
                break;
            case 5:
                move=moves[ch];
                break;
            case 6:
                move=moves[ch];
                break;
            case 7: 
                move=moves[ch];
                break;
            case 8:
                move=moves[ch];
                break;
            case 9:
                move=moves[ch];
                break;
            case 10:
                move=moves[ch];
                break;
            case 11:
                move=moves[ch];
                break;
            case 12:
                move=moves[ch];
                break;
            case 13:
                move=moves[ch];
                break;
            case 14:
                move=moves[ch];
                break;
            case 15:
                move=moves[ch];
                break;
            case 16:
                move=moves[ch];
                break;
            case 17:
                move=moves[ch];
                break;
            case 18:
                move=moves[ch];
                break;
            case 19:
                move=moves[ch];
                break;
        }
        return move;
    }
}