Confusion with extensions and fields

Basically I am meant to move the getTimestamp accessor method and timestamp field from MessagePost to Post, but when I do that I get a "has private access in" error. How can I fix this? I know I could make the field protected but I don't think it's good practise and I've tried using getters and setters but I get an unexpected type error.

public class MessagePost extends Post
{
    private String username;  // username of the post's author
    private String message;   // an arbitrarily long, multi-line message
    
    private int likes;
    private ArrayList<String> comments;

 
    public MessagePost(String author, String text)
    {
        username = author;
        message = text;
        timestamp = System.currentTimeMillis(); //where im getting the errors
        likes = 0;
        comments = new ArrayList<>();
    }

public class Post
{
    private long timestamp;
  
    public Post(){
        this.timestamp=timestamp;

    }
    public long getTimeStamp()
    {
        return this.timestamp;
    }

}

private members are not accessible outside of the class in which they're defined. To access members from an inheriting class, you're looking for the protected access modifier:

protected long timestamp;

Or perhaps keep the member private and make a protected setter:

private long timestamp;

protected void setTimeStamp (long time) {
    this.timestamp = time;
}

<hr />

Side note... This looks like a bug:

public Post(){
    this.timestamp=timestamp;
}

It's setting the field to itself. Perhaps you meant to include a parameter on that constructor:

public Post(long timestamp){
    this.timestamp=timestamp;
}

In this case you might not need any setter or protected member at all, as the MessagePost constructor would need to pass along the timestamp value to the Post constructor. Perhaps something like this:

public MessagePost(String author, String text)
{
    super(System.currentTimeMillis());
    username = author;
    message = text;
    likes = 0;
    comments = new ArrayList<>();
}

The MessagePost constructor shouldn't initialize timestamp. Since it belongs to Post, its constructor is in charge of initializing it:

// In the Post class
public Post() {
    this.timestamp = System.currentTimeMillis();
}

// In the MessagePost class
public MessagePost(String author, String text) {
    // Post() is implicitly called here, and timestamp is initiazlied
    username = author;
    message = text;
    likes = 0;
    comments = new ArrayList<>();
}

Just move this line:

timestamp = System.currentTimeMillis(); //where im getting the errors

from MessagePost's constructor to Post's constructor, replacing the line there that doesn't quite make sense (per @David's side note). You have to initialize timestamp to something after all. What makes more sense than to set it to the current time initially.

If you want to set timestamp to something else later, add a setter to the Post class for it.