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Java Java Data Structures Exploring the Java Collection Framework Maps

Maps Challenge

I get this error in my preview and don't know what it is or how to fix it:

JavaTester.java:107: 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 JavaTester.java:110: error: cannot find symbol for (BlogPost bp : blog.getPosts()) { ^ symbol: method getPosts() location: variable blog of type Blog Note: JavaTester.java uses unchecked or unsafe operations. Note: Recompile with -Xlint:unchecked for details. 2 errors

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")) {
    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;
package com.example;
import java.util.Map;
import java.util.HashMap;
import java.util.List;
import java.util.Set;
import java.util.TreeSet;

public class Blog {
  List<BlogPost> mPosts;

  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;
        categoryCounts.put(category, count);
    return categoryCounts;

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

1 Answer

Damian Adams
Damian Adams
21,351 Points

You didn't post the code from your JavaTester class, but from the looks of it it seems like you have two errors going on in your JavaTester.java class:

  1. You're trying to create a Blog item using a specific constructor which isn't defined in your Blog class. By default, Java provides a parameterless constructor. If you want to use a different one you will have to write it yourself.
  2. You're calling getPosts() on your Blog object, which is a method that is not yet defined in your Blog class.

Try going over the code in your JavaTester class to see if you can spot the lines (hint: lines 107 and 110 are good places to take a look) that are causing these compiler errors.