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

Jef Davis
Jef Davis
29,162 Points
Posted by Jef Davis
Jef Davis
29,162 Points

difficulty with getAllAuthors

I'm getting the following error when trying to create the method getAllAuthors(): ./com/example/Blog.java:21: error: cannot find symbol allAuthors.addAll(post.getPosts()); ^ symbol: method getPosts() location: variable post of type BlogPost Note: JavaTester.java uses unchecked or unsafe operations. Note: Recompile with -Xlint:unchecked for details. 1 error

Can anyone shed some light on where I'm going wrong?

Thanks.

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; 

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 post : mPosts) {
      allAuthors.addAll(post.getPosts()); 
    }
    return allAuthors; 
  }
}

1 Answer

Allan Clark
Allan Clark
10,810 Points
Allan Clark
Allan Clark
10,810 Points 
  public Set<String> getAllAuthors() {
    Set<String> allAuthors = new TreeSet<String>(); 
    for (BlogPost post : mPosts) {
      allAuthors.addAll(post.getPosts()); 
    }
    return allAuthors; 
  }

Inside the for-each loop, the post variable is a place holder for each BlogPost Java iterated through in the mPosts member List. The getPosts() method is a method on the Blog class not BlogPost. So Java doesn't know what to do when calling that method on a BlogPost object. We are looking to get the author of each post and add it to the Set. The getAuthor() method will be useful here. Also since you are adding only a single author through each iteration, you will want to use add() instead of addAll(). Try this:

allAuthors.add(post.getAuthor());
Jef Davis
Jef Davis
29,162 Points
Jef Davis
Jef Davis
29,162 Points

Thanks! I was on the same track there, but just wasn't quite thinking clearly about it. That worked exactly as it should have. Thanks again!