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Java Java Data Structures Organizing Data Comparable

Bjorn Van de Weyngaert
Bjorn Van de Weyngaert
1,297 Points

Interfaces: can't wrap my head around this line of code

I'm looking into the subject of interfaces. In the exercise I've written this line of code:

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

Question 1: So first we place the value for "obj" into "other" and then we ask if "obj" equals "other". Correct? But I read this code as if we made them equal in the line before that (as we stored "obj" in "other").

Question 2: In the prior video we had to write the same line of code for the "Treets" class. In Example.java we didn't use the compareTo method, but we did use the sort method for arrays. Does the sort method automatically calls the compareTo method? Otherwise, how can it know to sort by creationDate if we didn't call the compareTo method there?

I'm sure I'm overlooking something and as you see I'm pretty confused on this topic :-) Could someone please explain? Thanks.

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;
  }

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

}

1 Answer

andren
andren
28,521 Points

So first we place the value for "obj" into "other" and then we ask if "obj" equals "other". Correct?

The first part is pretty much correct. You take obj and cast it down to a BlogPost object and store it in other. But you are misunderstanding the equals line.

The equals method belongs to the object that we are writing this method in. And it compares that object against the (other) object that was passed in. That is the default behavior of the equals method, which is a method that all object have since they inherit it from the Object class.

So let's say we had code that looked like this: blogPost1.compareTo(blogPost2). In that case the equals method would belong to and be comparing blogPost1, and it would be comparing it against blogPost2, which is what would be stored in the other variable.

Does the sort method automatically calls the compareTo method?

Yes, it does. sort only works on arrays of objects that implement the comparable interface. And the reason for that is that it is depended on the objects having a compareTo method for it to evaluate how it should sort the objects in the array.