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

Posted by Chris Gauthier
Chris Gauthier
Courses Plus Student 6,434 Points

Hash Map 7

Hello, ok...imported the map command and that got me through that error. Its now complaining that I need to return a map instead of a hashmap, even though Ive declared that the method is to return a hashmap.

Any thoughts?

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 HashMap<String,Integer> getCategoryCounts() {
    Map<String,Integer> hashCounts = new Map<>();
    Integer count = 0;
    for (BlogPost post : mPosts) {
            String author = post.getAuthor();
                        if (hashCounts.get(author) == null) {
                 count = 0;
            }
                else {
                count++;
            }
                    hashCounts.put(author,count);               
    }

        return hashCounts;
  }

}
Linda de Haan
Linda de Haan
12,413 Points
Linda de Haan
Linda de Haan
12,413 Points

You declared the return type of the method as HashMap, but it's actually returning a Map, as HashCounts is of type Map, not HashMap.

Chris Gauthier
Chris Gauthier
Courses Plus Student 6,434 Points

So Im now wondering why this won't pass the compiler?!

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 Map<String,Integer> getCategoryCounts() {
    Map<String,Integer> hashCounts = new HashMap<>();
    Integer count = 0;
    for (BlogPost post : mPosts) {
            String author = post.getAuthor();
                        if (hashCounts.get(author) == null) {
                 count++
            }
                else {
                count++;
            }
                    hashCounts.put(author,count);               
    }

        return hashCounts;
  }

}

2 Answers

Grigorij Schleifer
Grigorij Schleifer
10,365 Points
Grigorij Schleifer
Grigorij Schleifer
10,365 Points

Hi Chris,

you need to search for category inside the for loop and use the getCategory() method instead of using getAuthor().

My suggestion would be something like this:

public Map<String,Integer> getCategoryCounts(){
    //  intialize the map as Hashmap
    Map<String,Integer> hashCounts = new HashMap<>();
    // iterate over all post in mPosts to obtain the categories for every post
    for(BlogPost post : mPosts){
        // fetch the category
      String category = post.getCategory();
      //For every new category that has not been counted yet, initialize new counter
      if(hashCounts.get(category) == null){
       // create counter 
       int counter = 0;
        // map the category to the newly initialized counter
        hashCounts.put(category, counter);
      }
      // if the category alredy has a counter, we just fetch it from the map
      int counter = hashCounts.get(category);
      // increment the category's counter
      counter++;
      // put it back into your map
      hashCounts.put(category, counter);
    }
    // when done iterating all of mPosts return
    return hashCounts;
  }

I hope that helps ... let us know if not!

Chris Gauthier
PLUS
Chris Gauthier
Courses Plus Student 6,434 Points
Chris Gauthier
Chris Gauthier
Courses Plus Student 6,434 Points

yep, that did it. Thanks bunches!!