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

Chris Vukin
Chris Vukin
17,787 Points
Posted by Chris Vukin
Chris Vukin
17,787 Points

Map exercise

What am I doing wrong here?? Hints 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 java.util.List;
import java.util.Set;
import java.util.TreeSet;
import java.util.HashMap;
import java.util.Map;

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

4 Answers

David Axelrod
David Axelrod
36,073 Points
David Axelrod
David Axelrod
36,073 Points 
  public Map<String, Integer> getCategoryCounts(){

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

//end of for loop
    //return hashmap
    return count;

  }

There are a few things that were causing bugs in your code. #1 is that when you were defining a method, you were actually trying to define the map. The HashMap has to be created after you create the method.

Next is that this algo only needed to iterate through the posts, not the get categories. For each post you get the category and then you use the counting algorithm to add to the HashMap.

Then just return the count and you're done!

Mod Edit: Fixed grammar.

Chris Vukin
Chris Vukin
17,787 Points
Chris Vukin
Chris Vukin
17,787 Points

Interesting, thank you! So the HashMap is a data type that I am setting the variable count = to?

David Axelrod
David Axelrod
36,073 Points
David Axelrod
David Axelrod
36,073 Points

Not a problem!

count is a hashmap but a hashmap is just a type of map. map is the data type i think hashmap<> count = new hashmap<> isn't technically wrong but its not the best way to set up the hashmap

Chris Vukin
Chris Vukin
17,787 Points
Chris Vukin
Chris Vukin
17,787 Points

Awesome! Thanks again!

Dennis Addo
PLUS
Dennis Addo
Courses Plus Student 2,943 Points
Dennis Addo
Dennis Addo
Courses Plus Student 2,943 Points

If you want a shorter version or a more clean and simple one you can do it like this but all the same i thin better to make it simple to understand.

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

}