问Java程序关于(“”)的重要性

EN

package js01;

import java.util.Scanner;

//search 1 
//user input of an alphabet and a sentence, return a count of that alphabet
public class J0306_search {
     public static void main(String[] args) {
         String str1;
         String ch;
         //must be a string type although the input will be a chracter because the user input is taken as a string 
         int count=0;
         int i;

         Scanner s=new Scanner (System.in);
         System.out.println("enter a sentence");
         str1=s.nextLine();
         System.out.println("enter an alphabet that u would like to count");
         ch=s.next();

         for (i=0;i<str1.length();i++) {

             if (ch.equals(""+str1.charAt(i))) {

                 //why is "" needed?
                 //
                 count++;
             }
         }
         System.out.println("count is:"+count);
         s.close();
     }

}