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Java Java Data Structures - Retired Organizing Data Comparable

andrew nelson
andrew nelson
3,416 Points
Posted by andrew nelson
andrew nelson
3,416 Points

I dont understand what: if(equals(obj)) is doing..

what is obj being compared against? if what equals obj??

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 int compareTo(Object obj){
    BlogPost ex = (BlogPost) obj;
    if (equals(obj)){
      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

Alexander Nikiforov
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.a{fill-rule:evenodd;}techdegree seal-36
Alexander Nikiforov
Java Web Development Techdegree Graduate 22,175 Points
Alexander Nikiforov
.a{fill-rule:evenodd;}techdegree seal-36
Alexander Nikiforov
Java Web Development Techdegree Graduate 22,175 Points

obj is being compared against the class where compareTo is executed.

One can add this keyword for clarity:

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

Also make sure that you put "ex" in "if" and not "obj", looking at your code ...

So when in some class you execute blog1.compareTo(blog2) then, blog1 will be compared to blog2:

    public static void main(String[] args) {
      BlogPost blog1 = new BlogPost(); // imagine that this constructor exists 
      BlogPost blog2 = new BlogPost();
      // compare blog1 to blog 2
      blog1.compareTo(blog2);
      // this means than in your if will be
      // if (blog1.equalsTo(blog2))
    }

Quote from docs:

Method compareTo ... Compares this object with the specified object for order. Returns a negative integer, zero, or a positive integer as this object is less than, equal to, or greater than the specified object.

https://docs.oracle.com/javase/7/docs/api/java/lang/Comparable.html