如何從子類訪問私有成員

Question

嘗試在MessagePost類的printShortSummary方法中打印用戶名。 用戶名在Post類中是私有的。 這樣做是出於教育目的。 仍然收到用戶名在Post中為私人用戶的錯誤。

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     * Record that a user has withdrawn his/her 'Like' vote.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

您應該調用getUserName()但是顯然不能，因為調用它會返回void並導致Stack Overflow異常：

public void getUserName()
{
  getUserName();
}

這應該是

public String getUserName()
{
    return userName;
}

這就是從子類訪問用戶名的方式。

之后，您可以將此方法修改為：

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

您擁有Post.getUserName(); 沒有任何意義，它不是靜態方法，因此無法以這種方式引用。

Answer 2

如果您無權訪問Post類或不想更改Post類，則可以使用java Reflection API訪問任何類中的私有成員

例如：

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());
} 

注意：無論如何，這不是面向對象的方式