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

Demetris Christofi
Demetris Christofi
1,819 Points

Map interface does not working properly

Hi,

I really cannot understand what is the error in my code below.

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 java.util.Map;
import java.util.HashMap;

public class Blog {
  List<BlogPost> mPosts;

  public Blog(List<BlogPost> posts) {
    mPosts = posts;
  }

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

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

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

Hello

what is the error that you are seeing? also, include you Main class as the error could be originating from there.

Thanks

Demetris Christofi
Demetris Christofi
1,819 Points

Hi Mark,

Here is the error received:

error: for-each not applicable to expression type for(String category : blogPost.getCategory()){ ^ required: array or java.lang.Iterable

Both classes are from the JCF objective exercise.

2 Answers

Seth Kroger
Seth Kroger
56,413 Points

getCategory() returns a single String, not an array or collection. This means that BlogPosts only have one category each and there is no need to iterate over an collection of categories. If you replace the 2nd for loop with String category = blogPost.getCategory() it should work perfectly.

Demetris Christofi
Demetris Christofi
1,819 Points

Thank you Seth. I really didn't think about not iterating over a collection and just grab directly the category by calling the method.

It works perfectly. Cheers.

The problem is here

 for (BlogPost blogPost : mPosts) {
   for(String category : blogPost.getCategory()){
     Integer count = categoryCounts.get(category);
     if (count==null){
       count = 0;
     }
     count++;
     categoryCounts.put(category,count);
   }
 }

Note this: for(String category : blogPost.getCategory()){

blogPost.getCategory() is a String

But is should be a collection

Please see the method signature for getCategory ... it returns a String

public String getCategory() { return mCategory; }

mCategory is a String

you may want to rethink your logic

Thanks