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Java Java Data Structures Exploring the Java Collection Framework ArrayLists

Posted by William J. Terrell
William J. Terrell
17,403 Points

Exploring the Java Collection Framework - Array Lists Challenge

The question is as follows:

"Let's add a method that returns a list of all links in the body of the document. All words that start with http should be considered links. Name it getExternalLinks.

NOTE: Remember to import the needed classes from java.util. HINT: We already made the getWords method, we should use it!"

I feel like I should be close, but I get the following when trying to run the code below:

./com/example/BlogPost.java:38: error: cannot find symbol
    return getWordsPrefixedWith("http");
           ^
  symbol:   method getWordsPrefixedWith(String)
  location: class BlogPost
./com/example/BlogPost.java:45: error: cannot find symbol
      if (word.startsWith(prefix)) {
          ^
  symbol:   variable word
  location: class BlogPost
./com/example/BlogPost.java:46: error: cannot find symbol
        results.add(word);
                    ^
  symbol:   variable word
  location: class BlogPost
Note: JavaTester.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
3 errors

On top of that, I'm still not entirely sure how this all works... (Am I making it more complicated than it really is?)

Any assistance would be greatly appreciated. I've left my "steps" in comments in the code below.

Thanks!

com/example/BlogPost.java 
package com.example;

import java.io.Serializable;
import java.util.ArrayList;     // Added this...
import java.util.Arrays;       // ...and this...
import java.util.Date;
import java.util.List;        //..and this...

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 List<String> getWords() {          // Changed this from String[] to List<String>...
    String[] words = mBody.split("\\s+");   //Added this using the code that was on the next line...
    return Arrays.asList(words);            // And added this...
  }

  // Added this...
  public List<String> getLinks() {
    return getWordsPrefixedWith("http");
  }

  // Then here's the rest...
  public List<String> getExternalLinks(String prefix) {
   List<String> results = new ArrayList<String>();
    for (String words : getWords()) {
      if (word.startsWith(prefix)) {
        results.add(word);
      }
    }
    return results;
  }

  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;
  }
}

2 Answers

William J. Terrell
William J. Terrell
17,403 Points

Aha! This got it! :)

package com.example;

import java.io.Serializable;
import java.util.ArrayList;     // Added this...
import java.util.Arrays;        // This...
import java.util.Date;
import java.util.List;        // And this...

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);
  }

  // Changed this, using the example from the video...
  public List<String> getWords() {
    String[] words = mBody.split("\\s+");
    return Arrays.asList(words);
  }

  // Added this...
  public List<String> getExternalLinks() {
   return getWordsPrefixedWith("http"); 
  }

  // And here's the rest...
  public List<String> getWordsPrefixedWith(String prefix) {
    List<String> results = new ArrayList<String>();
    for (String word : getWords()) {
      if (word.startsWith(prefix)) {
       results.add(word); 
      }
    }
    return results;
  }

  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;
  }
}

Though I still don't feel like I fully understand how it works... I kind of feel like I'm staring into an engine, and I can see that this part turns that part but...it's like I'm just zomb-ing out in front of the machine... :\

By the way, is it considered gauche for one to select his or her own answer as the "Best Answer"?

Cory Carlson
Cory Carlson
5,275 Points
Cory Carlson
Cory Carlson
5,275 Points

import java.util.List; // And this...

Thanks, I didnt have that part imported and was getting the same error.

Nirbhay Agarwal
Nirbhay Agarwal
2,852 Points
Nirbhay Agarwal
Nirbhay Agarwal
2,852 Points

package com.example;

import java.io.Serializable; import java.util.Date; import java.util.Arrays; import java.util.ArrayList; 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 List<String> getWords() { String[] words = mBody.split("\s+"); return Arrays.asList(words); } public List<String> getExternalLinks() { List<String> docBody = new ArrayList <String>(); for(String words : getWords()) { if(words.startsWith("http")) { docBody.add(words); } } return docBody; }

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; } }