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Java Java Data Structures Exploring the Java Collection Framework Sets

Posted by Chris Gauthier
Chris Gauthier
Courses Plus Student 6,434 Points

Sorting a set with TreeSet Datastructures

Im not sure why this challenge is not passing.

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> authors = new TreeSet<String>();
    for ( List<BlogPost> post  : getPosts() ) {
        String newPost = (String) post;  
        authors.add(newPost); 
     }
    return authors;

  }


}

3 Answers

Chris Gauthier
Chris Gauthier
Courses Plus Student 6,434 Points

Great, that worked, thanks a lot!

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

Hi There

You are making a mistake in your for loop and your way to add the author name to the Set needs some improvement. 

       Set<String> authors = new TreeSet<String>();
    for ( List<BlogPost> post  : getPosts() ) { //<-- You need to get each BlogPost (let's name it post) from List mPosts
        String newPost = (String) post;  //<-- this is wrong
        authors.add(newPost); 
     }
    return authors;

Here is how I propose the solution to finish the challenge:

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

I hope this can help a little

Chris Gauthier
PLUS
Chris Gauthier
Courses Plus Student 6,434 Points
Chris Gauthier
Chris Gauthier
Courses Plus Student 6,434 Points

Hm..that code didn't pass the compiler.

I suspect its because the mPosts variable is out of scope, but I still don't see the solution.

Here's the error set:

./com/example/Blog.java:19: error: illegal start of type
    for(BlogPost post : mPosts){
    ^
./com/example/Blog.java:19: error: ')' expected
    for(BlogPost post : mPosts){
                     ^
./com/example/Blog.java:19: error:  expected
    for(BlogPost post : mPosts){
                              ^
./com/example/Blog.java:22: error: illegal start of type
    return allAuthors;
    ^
./com/example/Blog.java:22: error: ';' expected
    return allAuthors;
          ^
./com/example/Blog.java:19: error: cannot find symbol
    for(BlogPost post : mPosts){
                        ^
  symbol:   class mPosts
  location: class Blog
./com/example/Blog.java:20: error: cannot find symbol
        allAuthors.add(post.getAuthor());
                       ^
  symbol:   variable post
  location: class Blog
Note: JavaTester.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
7 errors
Yanuar Prakoso
Yanuar Prakoso
15,196 Points
Yanuar Prakoso
Yanuar Prakoso
15,196 Points

Hmmm... do your final answer looks like this?

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> allAuthors = new TreeSet<> ();
    for(BlogPost post : mPosts){
        allAuthors.add(post.getAuthor());
    }
    return allAuthors;

  }

}

Your error usually because you miss one ')' or '}'