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

Posted by Darrell Parrent
Darrell Parrent
Courses Plus Student 1,539 Points

I'm getting errors for the return statement. Why does the compiler not recognize "cat" variable?

If I understand HashMap correctly I should be able to use get on the key to return contents, but this is generating an error.

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.HashMap;
import java.util.HashSet;
import java.util.Set;
import java.util.Map;
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> catCounts = new HashMap<String, Integer>();
  for (BlogPost post : mPosts){
    String cat = post.getCategory();
    Integer count = catCounts.get(cat);
    count++;
    catCounts.put(cat, count);
     }
  return catCounts.get(cat);
  }
}
Darrell Parrent
Darrell Parrent
Courses Plus Student 1,539 Points

Thank you for your help. I still having trouble with the return value. I'm getting an error: /com/example/Blog.java:44: error: cannot find symbol return catCounts.get(cat); This seems to me that I'm using the wrong key, but I'm lost.

Dan Johnson
Dan Johnson
40,533 Points
Dan Johnson
Dan Johnson
40,533 Points

For your return value, your method should have the following signature:

public Map<String, Integer> getCategoryCounts()

Since that's the type catCounts is all you'll need to do is this at the end:

return catCounts;

Feel free to post your updated code if this didn't solve it.

1 Answer

Dan Johnson
Dan Johnson
40,533 Points
Dan Johnson
Dan Johnson
40,533 Points

Using get will give you the value, but for this challenge you're returning a map of category names to their occurrence (make sure to specify the generic types of the return in the method signature), so for that you'll just return catCounts as is.

One other thing you'll need to watch out for is NullPointerExceptions. When you use get with a key that doesn't exist yet, it'll return null. Since the first time you encounter a category it won't be in the map yet, count will get set to null, and calling ++ with throw a NullPointerException. Add a check to get around this:

if(catCounts.get(cat) != null) {
    // Increment the count like before.
}
else {
    //Start the count out at 1.
    catCounts.put(cat, 1); 
}
Darrell Parrent
Darrell Parrent
Courses Plus Student 1,539 Points

Dan thanks for the help, I see what was happening now. Again, thanks for your time.