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Java Java Data Structures Exploring the Java Collection Framework Maps

Ronit Mankad
Ronit Mankad
12,166 Points

Did not understand the question

I did not understand the question properly. A bit confused...please help by explaining the answer code . Thanks in advance.

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")) {
    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.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) {
    return authors;


1 Answer

Ronit, first you need to import java.util.Map and jave.util.HashMap. You do this in Blog.java. Then you need to create the method:

  public Map<String, Integer> getCategoryCounts() {
    Map<String, Integer> categoryCounts = new HashMap<>();

    for (BlogPost post : mPosts) {
      String category = post.getCategory();
      Integer count = categoryCounts.get(category);
         if (count == null) {
            count = 0;
         count ++;
         categoryCounts.put(category, count);
    return categoryCounts;

The method needs to return a Map<String, Integer> and create a new HashMap of that type. The String part will be the name of the category, and the Integer part the count. Note that you can't have primitives in Maps so you need to use Integer rather than int. Java will box and unbox automatically.

Then you loop over the posts in mPosts, get the category and the count and put them in the Map.