关于#java#的问题，如何解决？

Question

4、String类练习：程序填空
要求调用String类的函数完成,提交Filename类文件。
// FilenameDemo.java
public class FilenameDemo {
public static void main(String[] args) {
Filename myHomePage = new Filename("/home/mem/index.html",'/', '.');
System.out.println("Extension = " + myHomePage.extension());
System.out.println("Filename = " + myHomePage.filename());
System.out.println("Path = " + myHomePage.path());
}
}
// Filename.java
public class Filename {
private String fullPath; //全路径名
private char pathSeparator; //路径符
private char extensionSeparator; //扩展名符
public Filename(String str, char sep, char ext) {
fullPath = str;
pathSeparator = sep;
extensionSeparator = ext;
}
public String extension() { //取得扩展名

}
public String filename() { //取得文件名
                 
}

public String path() { //取得路径

}

}
程序运行结果为：
Extension = html
Filename = index
Path = /index.html

Answer 1

public String extension() { //取得扩展名
    return fullPath.substring(fullPath.indexOf(extensionSeparator) + 1);
}
public String filename() { //取得文件名
    return fullPath.substring(fullPath.lastIndexOf(pathSeparator) + 1, fullPath.indexOf(extensionSeparator));
}
public String path() { //取得路径
    return fullPath.substring(fullPath.lastIndexOf(pathSeparator));
}