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Java Java Data Structures - Retired Exploring the Java Collection Framework Sets

Posted by Conner Olsen
Conner Olsen
Courses Plus Student 1,670 Points

Java Sets

Ok so I've been busy and stopped doing treehouse for a while I just started doing it again and the first exercise I get lots of different errors. Could someone please tell me what I am doing wrong.

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.*;

public class Blog {
  List<BlogPost> mPosts;

  public set<String> AllAuthors = new treeSet<String>();
  AllAuthors.addAll(BlogPost.getAuthors());

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

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

  public getAllAuthors List<authors> {
    return Allauthors
  }

}
Samuel Ferree
Samuel Ferree
31,722 Points
Samuel Ferree
Samuel Ferree
31,722 Points

Your code to initialize all your authors and add the posts to it is not syntactically valid, nor is it in the correct place

//This Line
public set<String> AllAuthors = new treeSet<String>();
//Should look like this
public Set<String> AllAuthors = new TreeSet<String>(); //Note the capitalization

//This Line
AllAuthors.addAll(BlogPost.getAuthors()); 
//Is incorrect for a couple of reasons
// 1. The BlogPost class does not define a public static method "getAuthors()"
// 2. This is executable code, and needs to be declared in a method, probably the constructor.

#2 is good advice for the initialization of AllAuthors as well. Let's look at how fixing these errors might look

public class Blog {
  private List<BlogPost> mPosts;
  private Set<String> allAuthors;

  public Blog(List<BlogPost> posts) {
    mPosts = posts;
    //initialize allAuthors here
    allAuthors = new TreeSet<String>();
    //Let's iterate through our posts to extract the authors
    for(BlogPost post : posts) {
      allAuthors.add(post.getAuthor());
    }
  }

  //Excess Omitted
}

These are just the errors I could see at a quick glance. If you post the exact errors, I can help you fix those as well.

1 Answer

Yanuar Prakoso
Yanuar Prakoso
15,196 Points
Yanuar Prakoso
Yanuar Prakoso
15,196 Points

Hi Conner

You need to define the getAllAuthors method as public Set<String> getAllAuthors() because you will create a Set not a List of Strings. Set prohibits you to append two identical Stings in one Set. Then since the challenge requires you to sort the authors alphabetically you need to use TreeSet. Please do not forget to import both java.util.Set and java.util.TreeSet

I am attaching my code that I used to solve the particular challenge for your reference:

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

As you can see in the part of the code inside getAllAuthors() method I use for loops to extract each author from each blog post. Then I add it into the Set called authors. The Java will then make sure no identical or duplicate author name in the Set and since I am using TreeSet the Java program will ensure the author Names will be sorted alphabetically.

I hope this can help you a little.