为什么我得到运行时错误：StringIndexOutOfBounds？

Question

Doing my AP Computer Science homework right now but I am stuck with run-time errors. Does anyone know what is wrong with my code? The program was working fine on Dr.Java but it shows run-time error on my website tester in edhesive...

class Main{

 public static void main (String str[]) throws IOException {
   Scanner scan = new Scanner(System.in);

   System.out.println("Please enter a tweet:");
   String tweet = scan.nextLine();

   int hash = 0;
   int attr = 0;
   int link = 0;
   int ch = 0;
   if(tweet.length()>140)
   {
   System.out.println("Excess Characters: " + (tweet.length() - 140 ));
   }

   else
   {
   tweet=tweet.toLowerCase();
   System.out.println("Length Correct");


   for(ch=0; ch<tweet.length(); ch++)
   {
   if(tweet.charAt(ch) == '#' && ((ch++)<=(tweet.length())) && (tweet.charAt(ch++)!=' ' && tweet.charAt(ch++)!='\t'))
   {
   hash++;
   }
   if(tweet.charAt(ch) == '@' && ((ch++)<=(tweet.length())) && (tweet.charAt(ch++)!=' ' && tweet.charAt(ch++)!='\t'))
   {
   attr++;
   }
   if(tweet.charAt(ch) == 'h' && ((ch + 7)<=(tweet.length())))
   {
   String a = new String("http://");
   String sub = new String(tweet.substring(ch, ch + 7)); 
     if (sub.equals(a))
     {link++;}
   }



   }

   System.out.println("Number of Hashtags: " + hash);
   System.out.println("Number of Attributions: " + attr);
   System.out.println("Number of Links: " + link);

   }

}
}

Answer 1

Because of ch++ the value of ch is getting incremented after checking this condition (ch++)<=(tweet.length()) .

Explanation :

if(tweet.charAt(ch) == '#' && ((ch++)<=(tweet.length())) && (tweet.charAt(ch++)!=' ' && tweet.charAt(ch++)!='\t'))
   {
   hash++;
   }

For above code there are 4 conditions (for i=0):

1. tweet.charAt(ch) ch = 0
2. ((ch++)<=(tweet.length())) ch = 0, but ch++ so the value will be increamented after the condition check.
3. (tweet.charAt(ch++) ch=1 (becuase of Point No. 2)
4. tweet.charAt(ch++) ch = 2 (for the same reason).

Try this:

class Main{

 public static void main (String str[]) throws IOException {
   Scanner scan = new Scanner(System.in);

   System.out.println("Please enter a tweet:");
   String tweet = scan.nextLine();

   int hash = 0;
   int attr = 0;
   int link = 0;
   int ch = 0;
   if(tweet.length()>140)
   {
   System.out.println("Excess Characters: " + (tweet.length() - 140 ));
   }

   else
   {
   tweet=tweet.toLowerCase();
   System.out.println("Length Correct");


   for(ch=0; ch<tweet.length(); ch++)
   {
   if(tweet.charAt(ch) == '#' && ((ch+1)<(tweet.length())) && (tweet.charAt(ch+1)!=' ' && tweet.charAt(ch+1)!='\t'))
   {
   hash++;
   }
   if(tweet.charAt(ch) == '@' && ((ch+1)<(tweet.length())) && (tweet.charAt(ch+1)!=' ' && tweet.charAt(ch+1)!='\t'))
   {
   attr++;
   }
   if(tweet.charAt(ch) == 'h' && ((ch + 7)<(tweet.length())))
   {
   String a = new String("http://");
   String sub = new String(tweet.substring(ch, ch + 7)); 
     if (sub.equals(a))
     {link++;}
   }



   }

   System.out.println("Number of Hashtags: " + hash);
   System.out.println("Number of Attributions: " + attr);
   System.out.println("Number of Links: " + link);

   }

}
}