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

Kemish Alejandro Grajeda
Kemish Alejandro Grajeda
923 Points

Mismatch from String to Collection

There is a problem in line 19 of the Blog class, the posts can't be added to the allAuthors Set. What is causing this problem?

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 com.example.BlogPost;

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 (BlogPost posts: mPosts){ 
      allAuthors.addAll(posts.getAuthor());
      } 
      return allAuthors;
    }
  // String cannot be converted to Collection
}

1 Answer

Kevin Faust
Kevin Faust
15,353 Points

Hi Kemish,

on this line:

allAuthors.addAll(posts.getAuthor());

it should be .add and not .addAll. the reason is because everytime we loop, we only take one blogpost object from our mPosts list and put into the set. addAll is used if you want to put something with many things inside it such as if you want to add an entire array of several things to a set.