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Java Java Data Structures - Retired Organizing Data Comparable

Posted by Bogdan Rancichi
Bogdan Rancichi
4,464 Points

Challenge for Comparable interface error

When i create my code for resolving this challenge, it says that Task 1 is not being resolved, but in Task 1 it says that you have to always return 1, so now if a return -1 its an error, I don't understand whats the problem.

com/example/BlogPost.java 
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;
  }

  public String getBody() {
    return mBody;
  }

  public String getCategory() {
    return mCategory;
  }

  public Date getCreationDate() {
    return mCreationDate;
  }

  public int compareTo ( Object obj) {
    BlogPost b = (BlogPost) obj;
    if (b.equals(this)) {
      return 0;
    }
    if ( b.mCreationDate.compareTo(this.mCreationDate) <= 0) {
      return -1;
    }

    return 1;
  }
}

5 Answers

Steve Hunter
Steve Hunter
57,712 Points

Finito ... 

  public int compareTo(Object obj){
    BlogPost bp = (BlogPost) obj;
    if(this.equals(obj)){
      return 0; 
    }
    if(this.mCreationDate.before(bp.mCreationDate)){
       return -1; 
    }
    return 1;  
  }
Mark Buckingham
Mark Buckingham
5,574 Points
Mark Buckingham
Mark Buckingham
5,574 Points

I do not have a compiler at hand to check, but couldn't this be simplified by just returning the date.compareTo() result?

public int compareTo ( Object obj) {
    BlogPost b = (BlogPost) obj;
    if (b.equals(this)) {
      return 0;
    }
    return b.mCreationDate.compareTo(this.mCreationDate);
  }
Bogdan Rancichi
Bogdan Rancichi
4,464 Points

Yes it could be, but in some strange way it doesn't respect Task 1. I think this is an error from their validation.

Mark Buckingham
Mark Buckingham
5,574 Points
Mark Buckingham
Mark Buckingham
5,574 Points

Yes it seems the challenges want the answer written in a specific way, which is fair as trying to cover all the possible permutations would be quite a task.

Abirbhav Goswami
Abirbhav Goswami
15,450 Points

But I don't remember learning the 'this' keyword. Thanks though!

Steve Hunter
Steve Hunter
57,712 Points
Steve Hunter
Steve Hunter
57,712 Points

I had the same problem when trying to help someone else with this challenge. The post for the is here - there may be something useful in there for you.

One thing, you can't call the compareTo method inside its own definition.

I'll see if I can get through the challenge and add more help to this!

Steve.

Steve Hunter
Steve Hunter
57,712 Points
Steve Hunter
Steve Hunter
57,712 Points

Right - the following code passes the second part of the challenge without it reverting to failing task 1:

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

I don't think the new variable bp is required - I'll try with it deleted.

I'll keep going!

Steve.

Bogdan Rancichi
Bogdan Rancichi
4,464 Points
Bogdan Rancichi
Bogdan Rancichi
4,464 Points

Thank you, for your answer, but i still don't know why my solution was wrong.

Steve Hunter
Steve Hunter
57,712 Points

I'll be honest; I have no idea what was going on in that one!

However, in your solution, you were using compareTo within the definition of the compareTo method:

public int compareTo ( Object obj) {
    BlogPost b = (BlogPost) obj;
    if (b.equals(this)) {
      return 0;
    }
    if ( b.mCreationDate.compareTo(this.mCreationDate) <= 0) {  // <-- compareTo within compareTo?
      return -1;
    }

    return 1;
  }

So, if you'd got the correct comparator to test the order of the two dates, the rest was fine. I used after initially, but that failed, so I switched to before.

Steve.

Bogdan Rancichi
Bogdan Rancichi
4,464 Points

Using compareTo in compareTo is not a issue because i am implementing the compareTo for the BlogPost and then i use post.mCreationDate.compareTo , and the second compareTo is from the Date object already implemented in Java.

Steve Hunter
Steve Hunter
57,712 Points

Ah right - I didn't realise compareTo was already a method of the Date class.

The whole challenge could use the output of that within a switch statement. Switch the result of the compareTo for the three cases:

  • < 0 return -1
  • 0 return 0
  • >0 return 1

In your compareTo, did you get the greater/less than direction correct? I've been looking at this too long now, so can't get my head round the date differences.

Steve.

Bogdan Rancichi
Bogdan Rancichi
4,464 Points

Yes i did return -1 when the compareTo() < 0, but never mind it works with .before, thanks Steve.

Rui Peixoto
Rui Peixoto
22,820 Points
Rui Peixoto
Rui Peixoto
22,820 Points

public int compareTo ( Object obj) {

if (equals(((BlogPost) obj)) {
  return 0;
}
if ( mCreationDate.before(((BlogPost) obj).mCreationDate)) {  
  return -1;
}

return 1;

}