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Java Java Data Structures Exploring the Java Collection Framework Maps

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

Hash Map tutorials, revision 2

Hello, there! I tried this myself again and thought this might be a little closer to the proper solution.

Can somebody help point me in the right direction?

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<>();
    for (BlogPost post: mPosts) {
      authors.add(post.getAuthor());
    }
    return authors;
  }

  public HashMap<String,Integer> getCategoryCounts() { 
    Map<String,Integer> authorMap = new HashMap<String,Integer>();
    Integer count = 0;
    String author;

    for (BlogPost post: mPosts) {
        author = post.getAuthor();  
      count++;

    }
    authorMap = put(author,count);

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

Oh, sorry, i saw that as an easy mistake. Lets try this again:

This:

 public HashMap<String,Integer> getCategoryCounts() { 
    Map<String,Integer> authorMap = new HashMap<String,Integer>();
    Integer count = 0;
    String author;

    for (BlogPost post: mPosts) {
        author = post.getAuthor();  
      count++;

    }
    authorMap .(author,count);

  }
}

Doesn't work.

Can you explain why?

Chris

4 Answers

Craig Dennis
STAFF
Craig Dennis
Treehouse Teacher
Craig Dennis
Craig Dennis
Treehouse Teacher

Hi Chris Gauthier !

Does this video here at this point help:

https://teamtreehouse.com/library/maps-2?t=510

There isn't a nested loop but the surrounding logic is similar.

Let me know if that helps to get you unstuck.

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

That didn't quite do it for me, do you have something else?

Craig Dennis
STAFF
Craig Dennis
Treehouse Teacher
Craig Dennis
Craig Dennis
Treehouse Teacher

This line in particular:

authorMap .(author,count);

Is missing a method.

Also remember you need to pull the count out of the Map as well. If it is null, set it to 0.

That help?

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

One question, tho :

How do i get the count out of the map without actually filling in the map first?

ie) this code loops through the list posts and gets the author of each post:

public HashMap<String,Integer> getCategoryCounts() {
    Map<String><Integer> hashCounts = new HashMap<String><Integer>();
    for (BlogPost post : mPosts) {
            String author = post.getAuthor();

    }

However, I don't have the map yet, so i can't start looping over the map and incrementing the count of how many instances exist.

That might get me unstuck.

Thanks Chris

Craig Dennis
Craig Dennis
Treehouse Teacher
Craig Dennis
Craig Dennis
Treehouse Teacher

Small typo, but this code creates a new hash map.

Map<String, Integer> hashCounts = new HashMap<>();

If the key doesn't exist it will return a null when you hashCounts.get(author). In the beginning nothing will exist. You are looping through the BlogPosts, and populating the map.

That make sense?