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Java Java Data Structures Exploring the Java Collection Framework Sets

Posted by John Harles
John Harles
20,614 Points

Java Data Structures, Set challenge. What does <> mean when no type is specified.

I have completed the Set challenge and it to works in two different ways. This line of code Set<String> authors = new TreeSet<String>(); completed the challenge. This line of code Set<String> authors = new TreeSet<>();, I left off the type String, and it still works. My question is what does <> mean when using generics.

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;

public class Blog {
  List<BlogPost> mPosts;

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

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

    public Set<String> getAllAuthors() {
      //used the code below and completed the challenge
      //Set<String> authors = new TreeSet<String>();
      //The code below also works.
      Set<String> authors = new TreeSet<>();
      for (BlogPost post: mPosts) {
       authors.add(post.getAuthor());
      }
     return authors;
  }


}

1 Answer

Simon Coates
Simon Coates
28,694 Points
Simon Coates
Simon Coates
28,694 Points

https://docs.oracle.com/javase/tutorial/java/generics/genTypeInference.html I think the relevant quote is "You can replace the type arguments required to invoke the constructor of a generic class with an empty set of type parameters (<>) as long as the compiler can infer the type arguments from the context. This pair of angle brackets is informally called the diamond."

John Harles
John Harles
20,614 Points

Sorry for the late response. Thanks for finding information on the empty generic glass. I was surprised to see it since I don't believe the instructor talked about the empty type arguments for the generic class.

John