Java//不知道哪里的错误

Question

package chap06;
public class StringBuf {
protected StringBuf(){}
public static void main (String []args){
StringBuffer buf=new StringBuffer("java");
buf.append("Guide Ver1/");
buf.append(3);
int index=5;
buf.insert(index, "Student");
index=23;
buf.setCharAt(index, ".");
int start=24;
int end=25;
buf.replace(start, end, "4");
String s=buf.toString();
System.out.println(s);
}
}
第十一行出错😥

Answer 1

单引号表示char，双引号是String
buf.setCharAt(index, '.');