How to use toString method in Java and test it?

Question

I'm trying to create a method where it outputs a certain number of stars based on the rating of a movie. I'm using the toString method, but am unsure of how to call this in my tests. When I attempt to write the tests, it says that "Non-static method cannot be referenced from a static context". I'm trying to use assertEquals to see if it's outputting the desired result with "assertEquals("***", Rating.toString());". My other methods had the same problem so I made them static, but this is not an option with the toString method. Is it acceptable to simply make the other methods static, or is the message likely hinting at a different flaw (or flaws) in my methods and/or tests? This may actually result in the same answer if it's a bigger problem within the code, but how would I go about testing toString when it gives the same message (non-static method cannot referenced..) when toString cannot be static?

I have reviewed other questions about the toString method and testing, but am having trouble relating it to my own code. Also, I'm not sure if I'm dealing with one error or multiple which is making it a difficult problem to solve. Help would be very much appreciated.

public class Rating {

    public static int votes;
    public static double score;
    public static String result;


    public Rating() {
        votes = 0;
        score = 0;
    }

    public static void resetVotes(){
        Rating.votes = 0;
        Rating.score = 0;
    }

    public static int getNumVotes(){
        return Rating.votes;
    }

    public static void addVote(double rating){
        Rating.votes = Rating.votes + 1;
        Rating.score = Rating.score + rating;
    }

    public static double computeScore() {
        return (Rating.score / Rating.votes);
    }


    public String toString() {

        if (Rating.votes > 1)
        {
            if (computeScore() > 1 && computeScore() < 2)
            {
                result = "*";
            }
            else if (computeScore() > 2 && computeScore() < 3)
            {
                result = "**";
            }
            else if (computeScore() > 3)
            {
                result = "***";
            }
            else if (computeScore() < 1)
            {
                result =  " ";
            }
        }

        return result;
    }

Answer 1

There is no reason to have any static variables or methods here.

What probably happened is that you called your tests this way:

AssertEquals(Rating.getNumVotes(), 5);
Rating.addVote(5.5d);
AssertEquals(Rating.getNumVotes(), 6);
AssertEquals(Rating.computeScore(), 42);

What this means is that you have only one Rating, the one you're modifying. What you want to do is create an instance of this class, that will represent a specific Rating. So your test should look like this:

Rating r1 = new Rating(); //An instance of Rating
r1.addVote(4.3d);
r1.addVote(4d);
r1.addVote(2.5d);
r1.addVote(5d);

Rating r2 = new Rating(); // Another instance of Rating
r2.addVote(4d);

AssertEquals(r1.getNumVotes(), 4);
AssertEquals(r2.getNumVotes(), 1);

---

And your Rating class should look like this:

• I deleted the statics, and changed the toString() method to make it simpler. Check that I got the good number of stars though, as I'm not sure if a 2.5 movie is 2-star or 3-star.

• Notice the @Override on top of the toString() ? This means this method is inherited, I suggest you look it up. This has no impact on your actual code.

• You see the this keyword? it refers to the current instance of the class: when you add a vote for example, you incerement the number of vote of this specific rating, not of all ratings, as you did with the static.

  public class Rating {
  
  public int votes;
  public double score;
  public String result;
  
  
  public Rating() {
      votes = 0;
      score = 0;
  }
  
  public void resetVotes(){
      this.votes = 0;
      this.score = 0;
  }
  
  public int getNumVotes(){
      return this.votes;
  }
  
  public void addVote(double rating){
      this.votes = this.votes + 1;
      this.score = this.score + rating;
  }
  
  public double computeScore() {
      return (this.score / this.votes);
  }
  
  @Override
  public String toString() {
    String result;
    double score = computeScore();
    if (this.votes > 1)
    {
      if (score <= 0)
          result = "";
      else if (score <= 1)
          result = "*";
      else if (score <= 2)
          result = "**";
      else if (score <= 3)
          result = "***";
      else
          result =  "****";
     }
     else
          result = "No votes.";
  
     return result;
   }
  }
  

Answer 2

You can do this:

public class yourclass {
    public static String toString(bool votes, int computeScore ) {

        if (votes)
        {
            if (computeScore > 1 && computeScore < 2)
            {
                result = "*";
            }
            else if (computeScore > 2 && computeScore < 3)
            {
                result = "**";
            }
            else if (computeScore > 3)
            {
                result = "***";
            }
            else if (computeScore < 1)
            {
                result =  " ";
            }
        }

        return result;
    }
}

then you can call that in your test somethin like that

assertEquals(yourclass.toString(true, 1), "*"))
assertEquals(yourclass.toString(true, 2), "**"))
assertEquals(yourclass.toString(true, 3), "***"))
assertEquals(yourclass.toString(true, 4), "***"))