How to generate random string and put it in HashTable?

I want to generate a random unique String and put in hashtable. I did like below code but it is not unique and it is just One Character .

    while(k<4){
    for(int j=1 ; j<=13 ;j++){

    Hashtable<Integer, String> Deck = new Hashtable();  
         int myChar = (int) (Math.random()*str.length());
           StringBuilder sb = new StringBuilder();
            sb.append(str.charAt(myChar)); 

     int i = (int) (Math.random() *52) ;    
    Deck.put(i, sb.toString());
    System.out.print(Deck);
    }

    System.out.println();
    k++;
    }

output is this

{30=d}{9=e}{11=b}{10=d}{43=g}....

I want to be

{30=dea}{9=egt}{11=brf}{10=dgrtg}{43=grrh}{14=gwrt}.....