Error: com.teamtreehouse.Treet cannot be cast to java.lang.Comparable

package com.teamtreehouse;
import java.util.Date;

public class Treet implements Comparable{

  private final String author;
  private final String description;
  private final Date creationDate;

  public Treet(String author, String description, Date creationDate){
    this.author = author;
    this.description = description;
    this.creationDate = creationDate  
  }

  @Override
  public String toString(){
    return String.format ("Treet: \"%s\" by %s on %s", 
                          this.description,
                          this.author,
                          this.creationDate);
  }

  @Override
  public int compareTo(Object obj){
    Treet other = (Treet) obj;
    if (equals(other)){
      return 0;
    }
    int dateCmp = this.creationDate.compareTo(other.this.creationDate);
    if (dateCmp == 0){
      return this.description.compareTo(other.this.description);
    }
    return dateCmp;
  }

  public String getAuthor(){
    return this.author;
  }

  public Date getCreationDate(){
    return this.creationDate;
  }

  public String getDescription(){
    return this.description;
  }

  public String[] getWords(){
   return this.description.toLowerCase().split("[^\\w@#']");
  }
}

import com.teamtreehouse.Treet;
import java.util.Arrays;
import java.util.Date;

public class Example{

  public static void main(String[] args){
    Treet treet = new Treet(
      "Mike", 
      "This is a cool post, man. @treehouse "+
      "is teaching me a lot",
      new Date(1488402048000L)
     );

    Treet secondTreet = new Treet(
      "journeytocode",
      "@treehouse makes learning Java sooooo fun! #treet",
      new Date(1421878767000L)
    );

    System.out.printf("This is a new Treet: %s %n", treet);
    System.out.println("The words are:");

    for (String word: treet.getWords()){
      System.out.println(word);
    }

    Treet[] treets = {treet, secondTreet};
    Arrays.sort(treets);
    for (Treet exampleTreet : treets){
      System.out.println(exampleTreet);
    }
  }
}