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Java

Posted by Ben Cavolick
Ben Cavolick
371 Points

Constructor - why not just initialize the variables?

Hi all, I'm having some trouble getting my head around the necessity of a constructor in Java. If I take the following code:

package exams;

public class StudentResults {

Private String Full_Name; Private String Exam_Name; Private String Exam_Score; Private String Exam_Grade;

StudentResults() { Full_Name = "No Name Given"; Exam_Name = "Unknown"; Exam_Score = "No Score"; Exam_Grade = "Unknown"; } }

I'm unsure what would be the difference between just doing this:

package exams;

public class StudentResults {

Private String Full_Name = "No Name Given"; Private String Exam_Name = "Unknown"; Private String Exam_Score = "No Score"; Private String Exam_Grade = "Unknown";

}

Thanks

Ben Cavolick
Ben Cavolick
371 Points

I'm sorry about the formatting, it looked good when I posted it!

1 Answer

Samuel Ferree
Samuel Ferree
31,723 Points
Samuel Ferree
Samuel Ferree
31,723 Points

In this case, they would work the same. But as a practice, the second method can have some problems going forward. Imagine that StudentResults was inherited from, like so.

public class StudentResults {

    protected String Full_Name = "No Name Given";
    protected String Exam_Name = "Unknown";
    protected String Exam_Score = "No Score";
    protected String Exam_Grade = "Unknown";

}

public class PerfectStudentResults extends StudentResults {

  public PerfectStudentResults()
  {
    Exam_Score = "100";
    Exam_Grade = "A+";
   }
  public String getExamName() throws Exception {
    if(Exam_Name == null) {
      throw new Exception("Exam not set!"); //This code will never trigger, causing confusion.
    }
  }
}

Somebody without access to the code for StudentResults might assumed they've subclassed it correctly, and since they didn't set the Exam_Name variable, it should be null, but their error handling code won't trigger, because it won't be null.

While the functionality for the most part is the same, for these special cases, it is considered best practice to initialize/assign instance variables in the constructor.

Ben Cavolick
Ben Cavolick
371 Points

Thank you Samuel, I'm afraid I don't understand when you say "they didn't set the Exam_Name variable, it should be null" as even if it was set to the same Exam_Name = "Unknown" in the constructor wouldn't it also fail?

Samuel Ferree
Samuel Ferree
31,723 Points

No, unless explicity called in the subclass constructor, the superclass constructor isn't called.