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Java Java Data Structures - Retired Exploring the Java Collection Framework Upgrade Comparable to use Generics

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

What's needed here?

I'll attach my code:

package com.example;

import java.io.Serializable;
import java.util.Date;

public class BlogPost implements Comparable, Serializable {
  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(BlogPost other) {
    if (equals(other)) {
      return 0;
    }
    return mCreationDate.compareTo(other.mCreationDate);
  }

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

  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;
  }
}
com/example/BlogPost.java
package com.example;

import java.io.Serializable;
import java.util.Date;

public class BlogPost implements Comparable, Serializable {
  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(BlogPost other) {
    if (equals(other)) {
      return 0;
    }
    return mCreationDate.compareTo(other.mCreationDate);
  }

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

  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;
  }
}

3 Answers

Ken Alger
STAFF
Ken Alger
Treehouse Teacher

Casey;

Let's have a look:

Task 1

Now that we know about Generics, let's upgrade our Comparable interface defined on the BlogPost class to use it. Remember to change the parameter in the compareTo method to be of type BlogPost.


I'm going to work backwards here a bit and hopefully by the end you will see why. Our given compareTo() method is:

  public int compareTo(Object obj) {
    BlogPost other = (BlogPost) obj;
    if (equals(other)) {
      return 0;
    }
    return mCreationDate.compareTo(other.mCreationDate);
  }

We're asked to "upgrade" this to use a BlogPost instead of an Object here and in doing so remove the casting statement inside our method.

step1.java
  public int compareTo(BlogPost other) {
    if (equals(other)) {
      return 0;
    }
    return mCreationDate.compareTo(other.mCreationDate);
  }

Great. Almost there. If we run that code, we get back an error:

BlogPost is not abstract and does not override abstract method compareTo(Object) in Comparable

Great. What does that mean? Well, if we think for a second, during our upgrade process we forgot to "upgrade" our class interface in addition to our code. We need to let our class know that we want to utilize Comparable<BlogPost> to be able to compare BlogPost objects. Therefore we need to add that up top where we define our class:

snippet.java
public class BlogPost implements Comparable<BlogPost>, Serializable {
// other stuff here

Take a look at the Comparable documentation for more information.

Post back if you are still stuck or have further questions.

Happy coding,
Ken

Chris Jones
seal-mask
.a{fill-rule:evenodd;}techdegree seal-36
Chris Jones
Java Web Development Techdegree Graduate 23,933 Points

Thanks, Ken!

Just FYI to anyone else reading this - if you comment out the casting line in the compareTo method like below, the code challenge doesn't realize it's commented out. You actually have to delete this line to pass the challenge.

Just wanted to save some other people some frustration!

//BlogPost other = (BlogPost) obj;

Thank you Ken, I didn't realize I had to change my generic in the call to compareTo and was starting to bang my head against the monitor. I go through so many monitors that way.

public class BlogPost implements Comparable<BlogPost>, Serializable { private String mAuthor; private String mTitle; private String mBody; private String mCategory; private Date mCreationDate;

public int compareTo(BlogPost other) { //BlogPost other = (BlogPost) obj; if (equals(other)) { return 0; } return mCreationDate.compareTo(other.mCreationDate);

But im still getting errors pliz help!!!