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Java Java Data Structures Organizing Data Comparable

Casey Huckel
PLUS
Casey Huckel
Courses Plus Student 4,257 Points
Posted by Casey Huckel
Casey Huckel
Courses Plus Student 4,257 Points

Thought this would pass. Can someone explain why it doesn't?

Code:

package com.example;

import java.util.Date;

public class BlogPost implements Comparable{
  private String mAuthor;
  private String mTitle;
  private String mBody;
  private String mCategory;
  private Date mCreationDate;

  public BlogPost(String author, String title, String body, String category, Date creationDate) {
    mAuthor = author;
    mTitle = title;
    mBody = body;
    mCategory = category;
    mCreationDate = creationDate;
  }

  public String[] getWords() {
    return mBody.split("\\s+");
  }

  public String getAuthor() {
    return mAuthor;
  }

  public String getTitle() {
    return mTitle;
  }

@Override
  public int compareTo(Object obj) {
    if(equals(obj)) {
      return 0;
    }
    int compareDate = mCreationDate.compareTo(obj.mCreationDate);
      return compareDate;   
  }

  public String getBody() {
    return mBody;
  }

  public String getCategory() {
    return mCategory;
  }

  public Date getCreationDate() {
    return mCreationDate;
  }
}

[MOD: added ```java markdown formatting -cf]

6 Answers

Chris Freeman
MOD
Chris Freeman
Treehouse Moderator 68,423 Points
Chris Freeman
Chris Freeman
Treehouse Moderator 68,423 Points

You are missing the casting of obj to BlogPost so it can be properly compared:

  public int compareTo(Object obj) {
    // Task 2 of 3
    // In this step, cast the Object passed to the compareTo method
    // to a com.example.BlogPost. Check for equality first, if they
    // are equal return 0. Leave the default return of 1 in place
    // for this step.
    BlogPost blogpost = (BlogPost) obj;
    if (equals(blogpost)) {
      return 0;
    }
    // Task 3 of 3
    // Alright, now let's order the blog post's creation date
    // chronologically, oldest first.
    // NOTE: The app where this class is being used ensures that no
    // two blog posts are posted at the same time, so you do not need
    // to worry about any edge cases.
    return mCreationDate.compareTo(blogpost.mCreationDate);
  }
Casey Huckel
PLUS
Casey Huckel
Courses Plus Student 4,257 Points
Casey Huckel
Casey Huckel
Courses Plus Student 4,257 Points

I added BlogPost blogpost = (BlogPost) obj; and it passed. Thanks Chris

Grigorij Schleifer
Grigorij Schleifer
10,365 Points
Grigorij Schleifer
Grigorij Schleifer
10,365 Points

Hi Casey,

inside your compareTo() method obj need to be downcasted to String or BlogPost. After that you need to compare the class itself with a String or a BlogPost using equals.

Grigorij

Casey Huckel
PLUS
Casey Huckel
Courses Plus Student 4,257 Points
Casey Huckel
Casey Huckel
Courses Plus Student 4,257 Points

Grigorij, Can you show me what you mean?

Chris, I've tried this but I get this error:

./com/example/BlogPost.java:38: error: cannot find symbol
      return mCreationDate.compareTo(blogpost.mCreationDate);
                                                                ^
  symbol:   variable blogpost
  location: class BlogPost
1 error

[MOD: added ```bash markdown formatting -cf]

Chris Freeman
Chris Freeman
Treehouse Moderator 68,423 Points
Chris Freeman
Chris Freeman
Treehouse Moderator 68,423 Points

blogpost should be defined if you've added the line:

    BlogPost blogpost = (BlogPost) obj;

Can you post your current full compareTo method?

Mehmet Arabaci
Mehmet Arabaci
4,432 Points
Mehmet Arabaci
Mehmet Arabaci
4,432 Points

Hi could someone explain why the code below does not work

   @Override
    public int compareTo(Object obj){
        BlogPost bp = (BlogPost) obj;
        if (bp.equals(obj)) {
            return 0;
        }
        return 1;
    }

but this code does

 @Override
    public int compareTo(Object obj){
        BlogPost bp = (BlogPost) obj;
        if (equals(obj)) {
            return 0;
        }
        return 1;
    }

I cant seem to understand what 

 if (equals(obj))

means. To me the code

if (bp.equals(obj))

makes more sense, clearly comparing bp to obj. but what is equals(obj) comparing to? Where is the first half of that statement.