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Java Java Data Structures Exploring the Java Collection Framework Sets

Getting the authors

I don't see why it won't accept the getAuthor() method as a String because it returns a String. The error says it is going in as a collection.

com/example/BlogPost.java
package com.example;

import java.io.Serializable;
import java.util.ArrayList;
import java.util.TreeSet;
import java.util.Set;
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.TreeSet;
import java.util.Set;

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 blogpost : getPosts()) {
      allAuthors.addAll(blogpost.getAuthor());
    }
    return allAuthors;
  }

}

1 Answer

andren
andren
28,552 Points

No the error is not stating that it is going in as a collection. It is stating that the addAll method expects a collection, not a single string like you are providing it with.

addAll is a method for adding all the elements of a collection to the set you are calling it on, hence the name addAll. For adding single values you instead simply use a method named add.

If you replace addAll with add like this:

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

Then your code will work.

Ok I get the difference now. Thanks :)