Java Data Structures

Question

Here is the question and my code is listed below.

I've added a new class called Blog. It is initialized with a list of BlogPost objects. Create a method in the Blog class called getAllAuthors that loops over all the posts and returns a java.util.Set of all the authors, which are stored as Strings. They should be sorted alphabetically.

However, I am getting a strange error that I do not know how to approach to fix:

JavaTester.java:114: error: constructor Blog in class Blog cannot be applied to given types; Blog blog = new Blog(BlogPostFixture.posts); ^ required: no arguments found: List reason: actual and formal argument lists differ in length

Help? Explain?

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 Set<String> getAllAuthors() {
    Set <String> set = new TreeSet<>();
    for (BlogPost authorName : mPosts) {
     set.add(authorName.getAuthor()); 
    }
    return set;
  }

  public List<BlogPost> getPosts() {
    return mPosts;
  }
}

Answer 1 · 2017-02-27T15:03:30Z

February 27, 2017 3:03pm

THe description implies that the class should have a constructor, but doesn't seem to tell you to write it. The error disappears with:

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){
      this.mPosts = posts;
  }

  public Set<String> getAllAuthors() {
    Set<String> set = new TreeSet<>();
    for (BlogPost authorName : mPosts) {
     set.add(authorName.getAuthor()); 
    }
    return set;
  }

  public List<BlogPost> getPosts() {
    return mPosts;
  }
}

something is calling the code using Blog blog = new Blog(BlogPostFixture.posts); This expects there to be a constructor that receives a List of BlogPosts. There isn't one. When there is no explicit constructor provided on a class, a default constructor is created, which expects no parameters (ie. arguments). THe compiler is telling you there's a discrepancy between the available constructor (the automatic, invisible no-argument one) and a method call that expects to be able to pass a value in.

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Chris Stringer

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Java Data Structures

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