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Java Java Data Structures Exploring the Java Collection Framework Sets

Paramvir Singh
Paramvir Singh
1,517 Points
Posted by Paramvir Singh
Paramvir Singh
1,517 Points

TreeSet was not found even though the java.util.Set was imported

I imported all the required packages and correctly initialized the Set interface but still, it is giving me the error.

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 com.example.BlogPost;

public class Blog {
  List<BlogPost> mPosts;

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

  public Set<String> getAllAuthors () {
    Set<String> setOfAuthor = new TreeSet<String>();

    for ( BlogPost x : mPosts ) {
      setOfAuthor.add(x.getAuthor);
    }

    return setOfAuthor;
  }

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

}

2 Answers

Yanuar Prakoso
Yanuar Prakoso
15,196 Points
Yanuar Prakoso
Yanuar Prakoso
15,196 Points

Hi There

You need to revise two things:

  1. Add import java.util.TreeSet;
  2. in the x.getAuthor you need to add () since you are calling a method from other class it should be: x.getAuthor()

here is the code should looks like:

package com.example;

import java.util.List;
import java.util.Set;
//import com.example.BlogPost;
import java.util.TreeSet;//<--you need to import this 


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> setOfAuthor = new TreeSet<String>();

    for ( BlogPost x : mPosts ) {
      setOfAuthor.add(x.getAuthor());//<--you need to add () on the getAuthor method calling
    }

    return setOfAuthor;
  }
}

PS Set and TreeSet is two different objects TreeSet is a class and Set is an interface. Although TreeSet as a Class implements Interface Set you still needs to import them in separate import statements.

I hope this can help a little. Happy coding

Paramvir Singh
Paramvir Singh
1,517 Points
Paramvir Singh
Paramvir Singh
1,517 Points

Hi Yanuar,

With your help, I was able to solve the question.

TreeSet is a class which implements Set interface so why I just cannot write java.util.Set; to use TreeSet implementation. ?

Thanks a lot!!

Yanuar Prakoso
Yanuar Prakoso
15,196 Points
Yanuar Prakoso
Yanuar Prakoso
15,196 Points

Hi Paramvir

The simplest and easiest answer I can give you is because there are many class that implement Interface Set. Thus you must be really clear on which class you want to call to act as a wrapper for the Interface Set you call to instantiate Set<String> setOfAuthor. in your code.

Basically, you cannot use an interface directly since interface only contains fields and methods signatures (name, parameters, exceptions). To gain full implementation of an interface you need to wrap it into a class that implements that interface. Therefore you wrap your Set<String> setOfAuthor into new TreeSet<String>(). Since you call two objects one is interface Set and the other is a class TreeSet you need to import both packages.

If you want to know more about interface and class you can visit this link and this link from Oracle Java documentation. Moreover, you can google to find out more.

I hope this can help a little