Java Java Data Structures Exploring the Java Collection Framework Maps

yas abi
yas abi
58 Points

Do not know what is wrong with my answer

Hi Anyone know why this code is wrong?

package com.example;

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")) {
    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.List;
import java.util.Set;
import java.util.Map;
import java.util.TreeSet;
import java.util.HashMap;

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> allPosts = new HashMap<String, Integer>();
      for( BlogPost Post : mPosts){
        String category = Post.getCategory();
         Integer count = allPosts.get(category);
          if(count == null){
            count = 0;

  }  return allPosts;

  public Set<String> getAllAuthors() {
    Set<String> authors = new TreeSet<>();
    for (BlogPost post: mPosts) {
    return authors;

1 Answer

Grigorij Schleifer
Grigorij Schleifer
10,354 Points

Hi yas, you have so many thing right and just a couple of snippents can be improved :smile:

Check the code below and let me know if we need to go over it together.

public Map<String,Integer> getCategoryCounts(){
    //  intialize the map as Hashmap
    // no need to specify the Hashmap parameters again
    Map<String,Integer> allPosts = new HashMap<>();
    // iterate over all post in mPosts to obtain the categories for every post
    // for every mPost you will check if there is a category available and store it in category
    for(BlogPost Post : mPosts){ // I would choose a lower case name for the variables, it a convention thing ...
        // fetch the category like you did
      String category = Post.getCategory();
      // if category is not in the HashMap, condition is true
      if(allPosts.get(category) == null){
       // create counter inside the loop and not before the loop creation
       int counter = 0;
        // map the category to the newly initialized counter
        allPosts.put(category, counter);
      // if the category alredy has a counter, we just fetch it from the map
      int counter = allPosts.get(category);
      // increment the category's counter inside the if statement
      // put it back into your map
      allPosts.put(category, counter);
    // when done iterating all of mPosts return
    return categoryCounter;