StringBuffer and appender method

Question

I have to make a program that add Strings to each other as many as argument in method says. Program is not working and i dont know why. Thanks for your time. The problem is in the main in ap.append(" ma kota", 3).append( " i psa", 2);. I cant make changes to the main.

public class Appender {
    private StringBuffer sb;

    public Appender(){
        sb = new StringBuffer();
    }

    public Appender(String s){
        sb = new StringBuffer(s);
    }

    public StringBuffer append(String str, int n) {
        while(n > 0){
            sb.append(str);
            n--;
        }
        return sb;
    }

    public String toString(){
        String s = sb.toString();
        return "" + s;
    }
}

And this is my main :

public class Main {
    public static void main(String[] args) {
        Appender ap = new Appender("Ala");
        ap.append(" ma kota", 3).append( " i psa", 2);
        System.out.println(ap);
        ap.append(" ojej", 3);
        System.out.println(ap);
    }
}

Answer 1

public StringBuffer append(String str, int n) {
    while(n > 0){
        sb.append(str);
        n--;
    }
    return sb;
}

should change to

public Appender append(String str, int n) {
    while(n > 0){
        sb.append(str);
        n--;
    }
    return this;
}

because StringBuffer does not have method append(String s, int n)