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Throwing Unchecked Exceptions

Learning Objectives Covered

By the end of this lesson, you will be able to throw an unchecked exception to alert the user when input is invalid.

Learning Materials

Time Commitment

Approximately 35-45 minutes

Read (Approximately 20-25 minutes)

Throwing Exceptions - Reading

Try (Approximately 15-20 minutes)

Shoddy Practices High School is trying to calculate final grades for students. Unfortunately, all the grades are handwritten and in big piles of paper. We need to create a program that lets people type in the grades that a particular student achieved in a year, exclude the smallest grade, then return the average of the rest of the grades. Since people will be typing in these grades manually, they make mistakes! We need to make sure each subject’s grade is greater than or equal to 0.0 and less than or equal to 4.0. We also assume that students have taken at least 2 subjects during the year.

Please download the GradeCalculator code from here: ThrowingExceptions.zip

How to run the code

You can run the code in BlueJ by…

  1. instantiating the GradeCalculator class,
  2. running the gradeEntryForStudent method,
  3. specifying the number of subjects,
  4. entering one grade per subject in the terminal window.

After instantiating the GradeCalculator class, you run the gradeEntryForStudent method. You will then see a window asking you for the value for int numberOfSubjects for the method:

  • This is where you pass in the number of subjects as a method argument.

After providing the number of subjects you can enter in grades in the Terminal window:

  • First, type in grades, then press enter; the prompt for the next grades appear.

Goal

We need to make sure we protect our assumptions and not let people calling us make mistakes. What would you change in the code?

Test cases

  1. Input: 5 subjects, each with grades [1, 2, 3, 4, 4]
    Output: Final grade for this student: 3.25

  2. Input: 2 subjects, each with grades [1, 2]
    Output: Final grade for this student: 2.00

  3. Input: 1 subject, with a grade of 4
    Output: IllegalArgumentException thrown because there must be at least two subjects worth of grades

  4. Input: 2 subjects, each with grades [4, 5]
    Output: IllegalArgumentException thrown because the grade at index 1 is not a valid grade