String method Append() : StringBuilder vs StringBuffer

Question

With this code:

  public static void main(String[] args) {

    String s = \"Java\";
    StringBuilder buffer = new StringBuilder(s);
    change(buffer);

               


        

Answer 1

You might have imported a wrong StringBuilder class instead of java.lang.StringBuilder which does have a StringBuilder(String) constructor and charAt(int) method.

Could you check your import. You should have this one

import java.lang.StringBuilder;

Answer 2

Make sure that you are not defining your class name as StringBuilder

For Example: Even if you import it correctly

 import java.lang.StringBuilder;

But if you write your class as

 public class StringBuilder { //If class name matches below Object Creation
 public static void main(String[] args) {

String s = "Java";
StringBuilder buffer = new StringBuilder(s);  //Object creation
change(buffer);

   System.out.println("What's strBuf.charAt(5)? " + strBuf.charAt(3));
   System.out.println(buffer);
}

 private static void change(StringBuilder buffer) {

      buffer.append(" and HTML"); //you will get this error at append
  //The method append(String) is undefined for the type StringBuilder
} 
}

Suggestions

Rename your class name to something else but not StringBuilder

Answer 3

StringBuilder does have a constructor accepting a String as an argument, and does have a .charAt() method (which it it must implement since it implements CharSequence).

Conclusion: this is a mishap from the part of your IDE, which did not import the correct StringBuilder. You use another library which has the unfortunate "property" of having implemented a class by the same name -- but not in the same package.

Go see at the top of your file if the import line is:

import java.lang.StringBuilder;