问 一个 JAVA 编程问题 ？

public class Document {
private String text;

//用于获取text
public String getText() {
return text;
}

//用于设置text
public void setText(String text) {
this.text = text;
}


public String toString(){
return text;
}
}

public class Email extends Document {
private String sender;
private String recipient;
private String title;

public Email() {
super();
}
public Email(String sender, String recipient, String title) {
super();
this.sender = sender;
this.recipient = recipient;
this.title = title;
}

public String getSender() {
return sender;
}
public void setSender(String sender) {
this.sender = sender;
}
public String getRecipient() {
return recipient;
}
public void setRecipient(String recipient) {
this.recipient = recipient;
}
public String getTitle() {
return title;
}
public void setTitle(String title) {
this.title = title;
}

public String toString(){
String str = sender+"发送标题为"+title+"的Email给"+recipient;
return str;
}
}

public class File extends Document {
private String pathname;

public String getPathname() {
return pathname;
}

public void setPathname(String pathname) {
this.pathname = pathname;
}
}

public class Test {

public static void main(String[] args) {
Document email = new Email("枫叶","雨雪","你好");
Document file = new File();

String str = email.toString();
System.out.println(str);

file.setText("同学好！");
System.out.println(file.getText());
}

}

能不能说的具体点.