我的程序只在打印文本文件时打印出一堆“null”

public class Song 
{
private String title;
private String artist;

public Song (String Title, String Artist)
{
    Title = title;
    Artist = artist;
}
public String toString()
{
    return (title +" "+ artist);
}
}
public class Song 
{
private String title;
priv