java program to Replace a Word in String

Write a java program to Replace a word in String :

package String;
import java.util.Scanner;
public class ReplaceString {
public static void main(String[] args) {
Scanner in=new Scanner(System.in);
String str="Hello <<UserName>>, How are you?";
String uname=null;
do {
System.out.println("Enter user name :");
uname=in.nextLine();
if(uname.length()>3 && uname.length()<11)
break;
else {
System.out.println("Invalid user please ReEnter : ");
continue;
}

}while(uname.length()<3 || uname.length()>10);
System.out.println(uname);
str=str.replaceAll("<<UserName>>", uname);
System.out.println(str);
}

}