String problems seek help

string of random string

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

As the above string, I think inside eight randomly selected to form a new string, how?

Passing under the guidance of the great God.

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Reply:
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 ()); 
}