My program has a runtime error: exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 27

This is the full error message I received:

exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 27
        at java.lang.String.charAt(String.java:646)
        at Main.main(Main.java:268)
        at Ideone.assertRegex(Main.java:110)
        at Ideone.test(Main.java:42)
        at Ideone.main(Main.java:28)

I wrote my program in DrJava for a class assignment and it works, but when I enter my code into their code runner I get the runtime error. My code is as follows:

import java.io.*;
import static java.lang.System.*;

import java.util.Scanner;

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 length = tweet.length();
       if(length > 140){
         System.out.println("Excess Characters: " + (length - 140));}
       else{
       System.out.println("Length Correct"); 
       
       int countera = 0;
for( int i=0; i<tweet.length(); i++ ) {
    if( tweet.charAt(i) == '#' ) {
      i++;
      if (( tweet.charAt(i) != ' ' ) && ( tweet.charAt(i) != '\t' ))
        countera++;
    } 
}
       int counterb = 0;
for( int i=0; i<tweet.length(); i++ ) {
    if( tweet.charAt(i) == '@' ) {
      i++;
      if (( tweet.charAt(i) != ' ' ) && ( tweet.charAt(i) != '\t' ))
        counterb++;
    } 
}

int counterc = 0 ;
String search = "http://";

for ( int i = -1 ; ( i = tweet.indexOf( search, i + 1 ) ) != -1 ; counterc ++ );


       System.out.println("Number of Hashtags: " + countera);
       System.out.println("Number of Attributions: " + counterb);
       System.out.println("Number of Links: " + counterc);
       }
     }
}