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Java Java Data Structures Exploring the Java Collection Framework Sets

Posted by Bogdan Siverchuk
Bogdan Siverchuk
Courses Plus Student 1,707 Points

I do not know what it should return! It says it should return java.util.Set of all authors, java.util.Set is a library!

i just don't get what it should return! How can it return library?

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

public class Blog {
  List<BlogPost> mPosts;

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

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

    Set<String> allAuthors = new TreeSet<String>();
    for(BlogPost author : getPosts()){
     return ????? ;
    }

  }
}

2 Answers

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

Hi There Bogdan

Maybe what you mean is java.util.Set is an interface and to be honest you already done a good job wrapping the Set interface into TreeSet class. You even already import both packages for Set and TreeSet. However, your code in the getAllAutors method need some refinements. Most important is the method declaration. The challenge states that you need to return a Set of String consist of all authors, thus you must declare your method as: Set<String> getAllAuthors() {

The code should be like this:

public Set<String> getAllAuthors() {//<- remember they asked for a Set of String from this method
    Set<String> allAuthors = new TreeSet<>();//<- wrapping up Set interface inside TreeSet class
    for(BlogPost post : mPosts)  {//<-- iterate each BlogPost post from the list of mPosts
      allAuthors.add(post.getAuthor());//<--This is how you add authors of the posts into the Set
//Please note: post.getAuthor() is a public method in BlogPost class to get the author data of the post
    }
    return allAuthors;//<- we return the set... and it is all done
  }

I hope this can help a little

Bogdan Siverchuk
PLUS
Bogdan Siverchuk
Courses Plus Student 1,707 Points
Bogdan Siverchuk
Bogdan Siverchuk
Courses Plus Student 1,707 Points

May I ask why you do not declare TreeSet<String>? You leave <empty>

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

Hi Bogdan..

In Java programming when You declare using <.....> it is called generics. It basically parameterized over types. Long story short in Java SE 7 and later, you can replace the type arguments required to invoke the constructor of a generic class with an empty set of type arguments (<>) as long as the compiler can determine, or infer, the type arguments from the context. This pair of angle brackets, <>, is informally called the diamond. For example:

Set<String> someSet = new TreeSet<String>();

Since the member variable of both sides in the new declaration is the same (<String>) you can just write:

Set<String> someSet = new TreeSet<>();

It means the same when you use map for instance:

Map<String, Video> someMap = new HashMap<String, Video>();

Since both sides have the same member variables types (String as key and Video as value) you can shorten the code:

Map<String, Video> someMap = new HashMap<>();

I hope this can help