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Java Java Data Structures - Retired Organizing Data Comparable

Posted by Chris Shearon
Chris Shearon
Courses Plus Student 17,330 Points

Stuck again. Order the creation date, oldest first?

In step 3 of the challenge, do I add an arrays.sort to the obj? Create a new method? I'm lost here. I've watched the video several times over and either I'm just not getting it or it's not being explained very well.

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;
  }

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

  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;
  }
}

1 Answer

Dan Johnson
Dan Johnson
40,533 Points
Dan Johnson
Dan Johnson
40,533 Points

You'll be modifying the compareTo method in order to compare the Date properties between the two.

You can leverage the compareTo method of Date to determine the order.

Chris Shearon
Chris Shearon
Courses Plus Student 17,330 Points

Thanks for explaining it. I passed the challenge but I still don't completely understand.

Dan Johnson
Dan Johnson
40,533 Points
Dan Johnson
Dan Johnson
40,533 Points

If you're still wondering about compareTo, here's how it works:

When implementing the Comparable interface you can define which type you want this object to be comparable with using generics (Or keep it as Object and just cast).

Then you determine what makes the object you're comparing against less than, equal to, or greater than and assign the following values:

If the object is less than the calling object, return -1:

// Return -1
myComparableObject.compareTo(lessThanMyComparable);

If the object is equivalent to the calling object, return 0:

// Return 0
myComparableObject.compareTo(equivalentToMyComparable);

Or if the object is greater than, return 1:

// Return 1
myComparableObject.compareTo(greaterThanMyComparable);

For a more concrete example, here's a sample application thrown together in Workspaces using Date:

import java.util.*;
import java.text.*;


public class Application {

   public static void main(String... args) {
      // Parse the Dates in Month/Day/Year format.
      SimpleDateFormat format = new SimpleDateFormat("MM-dd-yyyy"); 

      // Arbitrary Dates for comparison
      // Note that you can get the current date
      // just by creating a new Date object.
      String thePast = "02-10-2002";
      String theFuture = "09-20-2075";
      String today = "05-15-2015";

      try {
         Date pastDate = format.parse(thePast);
         Date futureDate = format.parse(theFuture);
         Date todaysDate = format.parse(today);

         // The past will be before the future (-1 returned)
         System.out.printf(
           "%s compared to %s returns %d", 
           thePast, theFuture, 
           pastDate.compareTo(futureDate));
         System.out.println();

         // The future will be after the current date (1 returned)
         System.out.printf(
           "%s compared to %s returns %d", 
           theFuture, today, 
           futureDate.compareTo(todaysDate));
         System.out.println();

         // Equivalent dates (0 returned)
         System.out.printf(
           "%s compared to %s returns %d", 
           thePast, thePast, 
           pastDate.compareTo(pastDate));
         System.out.println();
      }
      catch(ParseException e) {
         e.printStackTrace();   
      }

   }

}

If it was something else you weren't sure about just leave another comment and I'll see if I can explain it.