String str="1234567890qwertyuioplkjhgfdsazxcvbnmQWERTYUIOPLKJHGFDSAZXCVBNM    ";

                   int b[]=new int[4];

                  Random a =new Random();

                   for(int i=0;i<4;i++)

                   {

                      b[i]=a.nextInt(62);

                  System.out.print(str.substring((b[i]),(b[i]+1) )+" ");

                  }