如何从子类访问私有成员

Question

Trying to print the username in the method printShortSummary in the MessagePost class. The username is private in the Post class. Doing this for educational purposes. Still getting error that the username is private in Post.

import java.util.ArrayList;

/**
 * This class stores information about a news feed post in a 
 * social network. Posts can be stored and displayed. This class
 * serves as a superclass for more specific post types.
 * 
 * @author Michael Kölling and David J. Barnes
 * @version 0.2
 */
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<>();
    }

    public void getUserName()
    {

      getUserName();

    }
    /**
     * 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     * Record that a user has withdrawn his/her 'Like' vote.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.");
        }
    }

    /**
     * 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";
        }
    }
}









import java.util.ArrayList;

/**
 * This class stores information about a post in a social network news feed. 
 * The main part of the post consists of a (possibly multi-line)
 * text message. Other data, such as author and time, are also stored.
 * 
 * @author Michael Kölling and David J. Barnes
 * @version 0.2
 */
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 static void printShortSummary()
    {
      Post.getUserName();
      System.out.print ("Message postfrom" + username);

    }

    /**
     * Return the text of this post.
     * 
     * @return The post's message text.
     */
    public String getText()
    {
        return message;
    }
}

Answer 1

You should be calling getUserName() but obviously cannot because it returns void and causes a Stack Overflow exception if called:

public void getUserName()
{
  getUserName();
}

This should be

public String getUserName()
{
    return userName;
}

And then that's how you access the user name from the subclass.

After that you would modify this method to be:

public static void printShortSummary()
{
    //Post.getUserName();
    System.out.print ("Message postfrom" + getUserName());
}

The portion where you have Post.getUserName(); doesn't make any sense, it's not a static method and can't be referenced in that manner.

Answer 2

If you don't have access to Post class or you don't wanna change Post class, you can use java Reflection API to access private members in any class

Ex:

public void printShortSummary() throws NoSuchFieldException, IllegalArgumentException, IllegalAccessException {
    Field field = Post.class.getDeclaredField("username");
    field.setAccessible(true);
    System.out.print("Message post from " + field.get(this).toString());
} 

Note : Anyway this not the object oriented way