Why do I have incompatible Types? Why does it say cannot find symbol for a function that does exist?

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;
  }
}

package com.example;
import java.util.Map;
import java.util.HashMap;
import java.util.ArrayList;
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<List<BlogPost>, Integer> getCategoryCounts()//add a new method called getCategoryCounts.
  {
    Map<List<BlogPost>, Integer> categoryCounts = new HashMap<List<BlogPost>, Integer>();
    for(BlogPost post : mPosts)
    {
      List<BlogPost> categories = categoryCounts.get(post.getPosts());
      if(categories == null)
      {
        categories = new ArrayList<BlogPost>(); 
        categories.add(post);
      }
      Integer count = categoryCounts.get(post);
      if(count == null)
      { 
        count = 0;
      }
      count++;
      categoryCounts.put(post, count);
    }
    return categoryCounts;//It should return a Map of category to count calculated by looping over all the posts
  }
}