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Java Java Data Structures Exploring the Java Collection Framework Sets

pratham daswani
pratham daswani
979 Points
Posted by pratham daswani
pratham daswani
979 Points

Sets and Sorting

what's the bug in this program?

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

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


public class BlogPost implements Comparable<BlogPost>, 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;
  }

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

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

  public List<String> getExternalLinks() {
    List<String> links = new ArrayList<String>();
    for (String word : getWords()) {
      if (word.startsWith("http")) {
        links.add(word);
      }
    }
    return links;
  }

  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/Blog.java 
package com.example;

import java.util.List;
import java.util.Set;
import java.util.TreeSet;

public class Blog {
  List<BlogPost> mPosts;

  public Blog(List<BlogPost> posts) {
    mPosts = posts;
  }

  public List<BlogPost> getPosts() {
    return mPosts;
  }

  public Set<String> getAllAuthors()
  {
    Set<String> answer = new TreeSet<String>();
    for (BlogPost post: mPosts)
    {
      answer.addAll(post.getAuthors());
    }
    return answer;
  }
}

2 Answers

Kourosh Raeen
Kourosh Raeen
23,733 Points
Kourosh Raeen
Kourosh Raeen
23,733 Points

You are calling a method called getAuthors() on post, but that method does not exist. Instead call the getAuthor() method and then use the add() method to add it to the set.

pratham daswani
pratham daswani
979 Points
pratham daswani
pratham daswani
979 Points

but isn't post the object of blogpost? and entitled to use getAuthors() ? Can you share some code?

Kourosh Raeen
Kourosh Raeen
23,733 Points
Kourosh Raeen
Kourosh Raeen
23,733 Points

Yes, post is an object of type BlogPost, but the BlogPost class doesn't have a method called getAuthors(). You just need to change getAuthors() to getAuthor() and addAll() to add().

Simon Coates
Simon Coates
28,694 Points
Simon Coates
Simon Coates
28,694 Points

This seems to work:

 public Set<String> getAllAuthors()
  {
    Set<String> answer = new TreeSet<String>();
    for (BlogPost post: mPosts)
    {
      answer.add(post.getAuthor());
    }
    return answer;
  }

fyi, Kourosh Raeen is right, the problem was that you were attempting to call a method that didn't exist (look at the BlogPost class definition). The method that does exist returns a single value, requiring the use of the set class add method rather than addAll.