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Java Java Data Structures Exploring the Java Collection Framework Sets

Cant get this one right

help me where I am wrong

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> allAuthors = new TreeSet<String>();
    for( Blog bp : posts ){
    return allAuthors;

4 Answers

Hassaan Ahmad
Hassaan Ahmad
1,979 Points

Hey Aditya, I see a couple of mistakes in your getAllAuthors() method.

  • To get the posts in your for loop, you need to call the getPosts() method defined above. +The variable 'bp' that you used has to be of the type BlogPost for the loop to work as it'll run through a list of Blog Posts. Makes sense? Hope that helps

That fine , but I never created an object in the blogPost class. If you see in the tutorial they have created objects and then called.

public Set<String> getAllAuthors(){ Set<String> allAuthors = new TreeSet<String>(); for( BlogPost bp : getPosts() ){ allAuthors.add(bp.getPosts()); } return allAuthors; }

can yo help me with this part of code

Hassaan Ahmad
Hassaan Ahmad
1,979 Points

You don't have to create a new instance of BlogPost object as the getPosts() method returns a list of BlogPost objects. As they are already of type BlogPost, you can use on them the methods offered by the BlogPost class.

Coming to your second question, as I see, getAllAuthors() method returns the Set of all authors.

  • The first line instantiates a new Set which is to be filled by the authors.
  • The statement of for loop goes like for each bp of type BlogPost in the List returned by getPosts(), get the author and add it to the allAuthors Set.
  • After the loop ends, return the Set of allAuthors.

In your code, I see that in the for loop, you are adding bp.getPosts() instead of bp.getAuthor() to the Set. Remember that you are supposed to add the authors in the Set.