ITI1121 2013 exam question 1, code implementation

Likeable.java

interface Likeable {
  void like();
  int getLikes();
}

Makefile

.PHONY: tags default
default:
	javac *.java
	java Test
tags:
	ctags -f .tags -R .

NewsFeed.java

public class NewsFeed {
  // privates
  //--------------------------------------------------------------------------------

  /**
   * The number of Posts in this.posts.
   */
  private int    n = 0;

  // privates to be initialized by constructor
  //--------------------------------------------------------------------------------

  private Post[] posts;
  private int    increment;

  public NewsFeed(int capacity, int increment){
    this.posts     = new Post[capacity];
    if (increment < 1){
      System.err.println("Invalid increment passed to NewsFeed(..) constructor.");
      System.exit(1);
    }
    this.increment = increment;
  }

  public int size() {
    return this.n;
  }

  public Post get(int i){
    if ( i >= this.n ){
      System.err.printf("Post index %d is out of bounds of integers on [0,%d)\n",i,n);
      System.exit(1);
    }
    return this.posts[i];
  }

  public void add(final Post post){
    if (this.posts.length <= this.n){
      // Here we must resize the array.
      Post[] newarray = new Post[this.posts.length+this.increment];
      System.arraycopy(posts, 0, newarray, 0, n);
      this.posts = newarray;
    }

    // Here we add the post the array
    this.posts[n++] = post;
  }
}

PhotoPost.java

public class PhotoPost extends Post {
  protected String filename, caption;

  public PhotoPost(String username, String filename, String caption){
    super(username);
    this.filename=filename;
    this.caption=caption;
  }
  
  public String toString(){
    return String.format(super.toString()+", %s, %s","PhotoPost",this.filename,this.caption);
  }
}

Post.java

import java.util.Date;

public abstract class Post implements Likeable {

  protected int    likes = 0;
  protected String username;
  protected Date   timestamp;

  public Post(String username) {
    this.username  = username;
    this.timestamp = java.util.Calendar.getInstance().getTime();
  }

  public void like() {
    ++this.likes;
  }

  public int getLikes(){
    return this.likes;
  }

  public String toString(){
    return String.format("%%s :\t%s, %s, likes = %d",timestamp.toString(),username,likes);
  }
}

Test.java

public class Test {
  public static void main(String[] args){

    NewsFeed messages;
    Post msg1, msg2;

    messages = new NewsFeed(100,10);

    msg1 = new PhotoPost("David","funny.png", "Birthday party");
    msg1.like();
    messages.add(msg1);

    msg2 = new TextPost("David", "Dinner at your place with Alexe");
    msg2.like();
    msg2.like();

    messages.add(msg2);
    messages.add(new TextPost("Nancy","Okay"));

    for (int i=0; i < messages.size(); ++i){
      System.out.println(messages.get(i));
    }
  }
}

TextPost.java

public class TextPost extends Post {
  protected String message;
  public TextPost(String username, String message){
    super(username);
    this.message=message;
  }

  public String toString(){
    return String.format(super.toString()+", %s","TextPost",this.message);
  }
}