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Java Java Data Structures Exploring the Java Collection Framework Maps

Posted by William J. Terrell
William J. Terrell
17,403 Points

Exploring the Java Collection Framework - Maps Challenge

I can't quite figure out what I need to do here. The question is as follows:

"In Blog.java add a new method called getCategoryCounts. It should return a Map of category to count calculated by looping over all the posts."

I've been using the code from the video as reference:

Map<String, Integer> hashTagCounts = new HashMap<String, Integer>();
    for (Treet treet : treets) {
      for (String hashTag : treet.getHashTags()) {
        Integer count = hashTagCounts.get(hashTag);
        if (count == null) {
          count = 0;
        }
        count++;
        hashTagCounts.put(hashTag, count);
      }
    }
    System.out.printf("Hash tag count: %s %n", hashTagCounts);

And the code from the challenge is below.

It says that we have to add a method called "getCategoryCounts". Does that mean that I need to do something like this:

public Set<String, Integer> getCategoryCounts() {

    // And then put my code from the challenge in here?                   

}

Any assistance or help explaining the concept would be appreciated :)

Thanks!

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;
import java.util.Map;       // Added this...
import java.util.HashMap;   //...and this...

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;
  }

  // And here's the rest...
  Map<String, Integer> categoryCounts = new HashMap<String, Integer>(); {
    for (BlogPost post : mPosts) {
      for (String category : post.getCategory()) {
        if (count == null) {
         count = 0; 
        }
        count++;
        categoryCounts.put(category, count);
      }
    }
    System.out.printf("Category count: %s %n ", categoryCounts);
  }


}

1 Answer

Seth Kroger
Seth Kroger
56,413 Points
Seth Kroger
Seth Kroger
56,413 Points

  1. You're missing both the method declaration and and the return statement, so yes you do need to enclose the code with what you put down (but with "Map" instead of "Set") and return the Map at the end.

  2. You need to get count from the Map before checking or incrementing.

  3. Since the BlogPost's category is a single string, not a Set or other collection, you don't need the inner for loop.

William J. Terrell
William J. Terrell
17,403 Points

So here's what I have now...

  // And here's the rest...
  public Map<String, Integer> getCategoryCounts() {
  Map<String, Integer> categoryCounts = new HashMap<String, Integer>(); 
    for (BlogPost post : mPosts) {
        if (count == null) {
         count = 0; 
        }
        count++;
        categoryCounts.put(category, count);
      }    
    System.out.printf("Category count: %s %n ", categoryCounts);


}

I assume you mean that I need to "get the count from the Map" before that if statement (that checks to see if count == null), but how do I do that? And how do I "return the Map at the end"?

Thanks!