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

Posted by Christopher Judas
Christopher Judas
5,499 Points

java.lang.reflect.InvocationTargetException?

I think I have the Map figured out, and the preview/editor isn't coming up with any errors, but if I try to check my work the compiler throws a "java.lang.reflect.InvocationTargetException".

Does anyone know what might be causing this, or have a solution?

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.HashMap;
import java.util.List;
import java.util.Map;
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> authors = new TreeSet<>();
    for (BlogPost post: mPosts) {
      authors.add(post.getAuthor());
    }
    return authors;
  }

   public Map<String, Integer> getCategoryCounts() {
   Map<String, Integer> categoryCounts = new HashMap<String, Integer>();
   for (BlogPost post : mPosts) {
     String  category = post.getCategory();
     Integer count = categoryCounts.get(category);
      if (category == null) {
       count = 0; 
      }
      count++;
      categoryCounts.put(category, count);
    }
   return categoryCounts;
  }
}

2 Answers

jcorum
jcorum
71,830 Points

What caused the editor to flip (Yikes!) was one wrong word. You have:

if (category == null) {

It should be:

if (count == null) {
Christopher Judas
Christopher Judas
5,499 Points

Yeah, that did it...

Thanks!

Kourosh Raeen
Kourosh Raeen
23,733 Points
Kourosh Raeen
Kourosh Raeen
23,733 Points

I'm not quite sure what the cause of the exception is, but I made some changes in your code that you can see below. I used int instead of Integer and I define it only once outside the for loop. Then using the containsKey() method we can see if the current category is in the HashMap or not. If not we set count to 0, otherwise we get its value and assign it to count. Finally, add one to count and put the key-value pair in the HashMap:

public Map<String, Integer> getCategoryCounts() {
   Map<String, Integer> categoryCounts = new HashMap<String, Integer>();
   int count;
   for (BlogPost post : mPosts) {
      String  category = post.getCategory();
      if (categoryCounts.containsKey(category)) {
          count = categoryCounts.get(category);
      } else {
          count = 0;  
      }
      count++;
      categoryCounts.put(category, count);
    }
   return categoryCounts;
  }