[Android] String problems seek help

String str = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789 @. + - * / _";

If the string above , I would like on the inside eight randomly selected to form a new string , how to achieve ?

passing under the guidance of the great God .
------ Solution ---------------------------------------- ----
here did not consider repeated

	public void testString()
  	{
  	    Random random = new Random();
  	    String str = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789@.+-*/_";
  	    
  	    int str_len = str.length();
  	    StringBuffer stringBuffer = new StringBuffer();
  	    for(int i=0; i<8; i++)
  	    {
  	        int temp_radom =  random.nextInt(str_len-1)%(str_len);
  	        stringBuffer.append(str.substring(temp_radom, temp_radom+1));
  	    }
  	    System.out.println(stringBuffer.toString());
  	}