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

Gleb Ermolaev
Gleb Ermolaev
1,029 Points

I cannot solve this challenge

Still cannot figure out where the mistake is. Compiler prints "For category Food expected 2, but recieved null".

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;
import java.util.HashMap;

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> map = new HashMap<String, Integer>();

    for (BlogPost post : mPosts) {
      if (map.get(post.getCategory()) != null) {
        map.put(post.getCategory(), map.get(post.getCategory() + 1));
      } else {
        map.put(post.getCategory(), 1);
      }
    }

    return map;
  }
}

1 Answer

andren
andren
28,558 Points

The issue is this line:

map.put(post.getCategory(), map.get(post.getCategory() + 1));

The + 1 is placed within the wrong parenthesis. With the way the code is written you ask ask map to get the value post.getCategory() + 1 rather than asking it to get post.getCategory() and then adding 1 to the value that is returned.

If you move the + 1 outside the parenthesis pair it is in like this:

map.put(post.getCategory(), map.get(post.getCategory()) +1);

Then your code will function as intended.