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

Posted by John Cuddihy
John Cuddihy
5,493 Points

category count is too much for me

I don't know how to construct the category count method, working from craigs treets classes, but I am getting nowhere , help appreciated

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 com.example.BlogPost;
import java.util.HashMap;
import java.util.Map;
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 Map<String, Integer> getCategoryCounts(){
    Map<String, Integer> catCounts = new HashMap<String, Integer>();
    for(BlogPost category : mposts){
      for(String catTag : post.getCategory()){
        Integer count = catCounts.get(catTag);
        if(count == null){
          count = 0;
        }
        count ++;
        catCounts.put(catTag, count);
      } 

    }
    return catCounts;
  }
}
Jose Patria
Jose Patria
1,600 Points
Jose Patria
Jose Patria
1,600 Points

Maybe the second for is the mistake. Try this

public Map<String, Integer> getCategoryCounts() { Map<String, Integer> categoryCounts = new HashMap<String, Integer>(); for (BlogPost post : mPosts) { String category = post.getCategory(); Integer count = categoryCounts.get(category); if (count == null) { count = 0; } count++; categoryCounts.put(category, count); } return categoryCounts; }

1 Answer

samiff
samiff
31,206 Points
samiff
samiff
31,206 Points

Hi John,

You have two nested enhanced for loops, when the solution to this challenge only requires one. The Treet project required two for loops because for every Treet, there may be multiple hashtags for example we needed to iterate through. In this Blog project, each BlogPost will only have one category seen as private String mCategory;

Try stepping back with some pseudo-code (there are multiple approaches to solving this):

Create the map (you've done this just fine).
For every blogPost in mPosts.
Get the category for that blogPost.
Check if the map contains the category (the key for the created map).
If the map does not contain the category, add to the map the category (key) and value (1).
If the map did contain the category already, update the map with they key (category) and value of  the current key's value plus one.

You may see a different approach from another student here, and reference the official Map API here. Hope that helps clear things up!