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

Charles Harpke
Charles Harpke
33,986 Points
Posted by Charles Harpke
Charles Harpke
33,986 Points

In Blog.java add a new method called getCategoryCounts. It should return a Map of category to count calculated by loopin

I get the following error: ./com/example/BlogPost.java:28: error: cannot find symbol for (BlogPost post : mPosts) { ^ symbol: variable mPosts location: class BlogPost Note: JavaTester.java uses unchecked or unsafe operations. Note: Recompile with -Xlint:unchecked for details. 1 error

Here is my code:

package com.example;

import java.io.Serializable;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;
import java.util.Map;
import java.util.HashMap;


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 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 (count == null) {
          count = 0;
        }
        count++;
        categoryCounts.put(category, count);
    }
    return categoryCounts;
  }

  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/BlogPost.java 
package com.example;

import java.io.Serializable;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;
import java.util.Map;
import java.util.HashMap;


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 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 (count == null) {
          count = 0;
        }
        count++;
        categoryCounts.put(category, count);
    }
    return categoryCounts;
  }

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

4 Answers

Seth Kroger
Seth Kroger
56,413 Points
Seth Kroger
Seth Kroger
56,413 Points

The getCategoryCounts() method is in the wrong class. Since you want the counts across all posts of an entire blog, and not an individual post it should go in Blog, not BlogPost.

Charles Harpke
Charles Harpke
33,986 Points
Charles Harpke
Charles Harpke
33,986 Points

Sorry about that....

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 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 (count == null) {
          count = 0;
        }
        count++;
        categoryCounts.put(category, count);
    }
    return categoryCounts;
  }
}

thanks for your help!
Charles Harpke
Charles Harpke
33,986 Points
Charles Harpke
Charles Harpke
33,986 Points

2 errors: ./com/example/Blog.java:30: error: cannot find symbol for (BlogPosts post : mBlogPosts) { ^ symbol: variable mBlogPosts location: class Blog ./com/example/Blog.java:30: error: cannot find symbol for (BlogPosts post : mBlogPosts) { ^ symbol: class BlogPosts location: class Blog Note: JavaTester.java uses unchecked or unsafe operations. Note: Recompile with -Xlint:unchecked for details.

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 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 (BlogPosts post : mBlogPosts) {
        String category = post.getCategory();
        Integer count = categoryCounts.get(category);
        if (count == null) {
          count = 0;
        }
        count++;
        categoryCounts.put(category, count);
    }
    return categoryCounts;
  }
}
Seth Kroger
Seth Kroger
56,413 Points
Seth Kroger
Seth Kroger
56,413 Points

You have BlogPost*s* but the class is BlogPost.

Araz kuberin
Araz kuberin
6,199 Points
Araz kuberin
Araz kuberin
6,199 Points

what does this do ??

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

if a constructor then how does it work why we need that ?? since we do not instantiate of the Blog class!