Not exactly sure of what is being asked in Map challenge. The instructions are:

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.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 getCategoryCounts() {
    Map<String, Integer> categoryCounts = new HashMap<String, Integer>();
    for (BlogPost post: mPosts) {
      for (String category : post.getCategory()) {
        Integer count = categoryCounts.get(category);
        if(count == null) {
          count = 0;
        }
        count++;
        categoryCounts.put(category, count);
      }
    }
    return categoryCounts();
}
  private List<String> getCategory(String) {//always want return list interface,return a list of strings
    List<String> results = new ArrayList<String>(); //make a new list called results, its a new arraylist
    for (String word : getWords()) {// loop through array, check each word in the treet,to see if it has hashtag
      if (word.startsWith(prefix)) {//startsWith is special method of strings
        results.add(word);
      }
    }
    return results;


}