我正在学习 Java 入门课程，并且正在学习继承。我必须在已经编写的超类代码中创建一个新方法（getUserName）以使私有用户名变量公开，并且它可以编译并且运行良好：
import java.util.ArrayList;
public class Post
private String username; // username of the post's author
private long timestamp;
private int likes;
private ArrayList<String> comments;
/**
* Constructor for objects of class Post.
*
* @param author The username of the author of this post.
*/
public Post(String author)
{
username = author;
timestamp = System.currentTimeMillis();
likes = 0;
comments = new ArrayList<String>();
}
/**
* Record one more 'Like' indication from a user.
*/
public void like()
{
likes++;
}
/**
* Record that a user has withdrawn his/her 'Like' vote.
*/
public void unlike()
{
if (likes > 0) {
likes--;
}
}
/**
* Add a comment to this post.
*
* @param text The new comment to add.
*/
public void addComment(String text)
{
comments.add(text);
}
/**
* Return the time of creation of this post.
*
* @return The post's creation time, as a system time value.
*/
public long getTimeStamp()
{
return timestamp;
}
/**
* Display the details of this post.
*
* (Currently: Print to the text terminal. This is simulating display
* in a web browser for now.)
*/
public void display()
{
System.out.println(username);
System.out.print(timeString(timestamp));
if(likes > 0) {
System.out.println(" - " + likes + " people like this.");
}
else {
System.out.println();
}
if(comments.isEmpty()) {
System.out.println(" No comments.");
}
else {
System.out.println(" " + comments.size() + " comment(s). Click here to view.");
}
}
public String getUserName()
{
return username;
}
/**
* Create a string describing a time point in the past in terms
* relative to current time, such as "30 seconds ago" or "7 minutes ago".
* Currently, only seconds and minutes are used for the string.
*
* @param time The time value to convert (in system milliseconds)
* @return A relative time string for the given time
*/
private String timeString(long time)
{
long current = System.currentTimeMillis();
long pastMillis = current - time; // time passed in milliseconds
long seconds = pastMillis/1000;
long minutes = seconds/60;
if(minutes > 0) {
return minutes + " minutes ago";
}
else {
return seconds + " seconds ago";
}
}
}
但是当我尝试在子类中调用这个方法（在 printShortSummary 方法中）时，它不会编译：
`enter code here`import java.util.ArrayList;
`enter code here`public class MessagePost extends Post
{
private String message; // an arbitrarily long, multi-line message
/**
* Constructor for objects of class MessagePost.
*
* @param author The username of the author of this post.
* @param text The text of this post.
*/
public MessagePost(String author, String text)
{
super(author);
message = text;
}
public String printShortSummary;
{
System.out.println("Message post from " + getUserName);
}
/**
* Return the text of this post.
*
* @return The post's message text.
*/
public String getText()
{
return message;
}
}