AP CSA 3.3 Exercise 1: Private Data, Public Doors

Unit 3: Class Creation · Lesson 3.3 · Exercise 1

Private Data, Public Doors

Anatomy of a Class. Write real Java, run it, and submit it to be graded against hidden test cases.

Back to the Lesson 3.3 page All Unit 3 lessons

Why this one is worth doing

Instance variables private, methods public. The accessor lets the outside read and the mutator lets it write, and every rule the class enforces lives in the mutator because that is the only way in.

What to write

Write a class named Student with two PRIVATE instance variables: a name and a grade.
The constructor sets both.
getName and getGrade return them.
setGrade changes the grade, but it must refuse anything below 0 or above 100 and leave the grade unchanged in that case.
isPassing returns true when the grade is 60 or more.

Your program reads

The grader supplies the input. Your class does not read anything.

Your program prints

Your class prints nothing. The grader prints what your methods return.

The class the grader will call

Your answer must declare exactly these, spelled exactly like this. The grader creates the object and calls the methods itself, so a method under a different name is a method it cannot find.

class Student
Student(String name, int grade)
String getName()
int getGrade()
void setGrade(int grade)
boolean isPassing()

Do not write a class named Main and do not write a main method. The grader supplies its own.

Worked examples

These are the cases you can see. There are more you cannot, and they use different values, so an answer that prints these numbers as constants will fail.

Example 1 input

Rae 72
3
150
55
-4

Example 1 output

Rae
72
true
72
55
55
false

Example 2 input

Kim 100
1
0

Example 2 output

Kim
100
true
0
false

Your answer

Main.java

class Student {
  // Two private instance variables.

  public Student(String name, int grade) {

  }

  public String getName() {
    return "";
  }

  public int getGrade() {
    return 0;
  }

  public void setGrade(int grade) {
    // Reject anything outside 0 to 100.
  }

  public boolean isPassing() {
    return false;
  }
}

Input for the Run button

Run uses the sample main shown below, which is not the one that grades you. Submitting runs your class against the real cases, including hidden ones.

The sample main the Run button uses

public class Main {
  public static void main(String[] args) {
    Student s = new Student("Rae", 72);
    System.out.println(s.getGrade());
    s.setGrade(150);
    System.out.println(s.getGrade());
    s.setGrade(55);
    System.out.println(s.isPassing());
  }
}

Stuck?

Hint 1

The parameter in setGrade has the same name as the instance variable, so this.grade is the field and grade on its own is the parameter.

Hint 2

A rejected grade changes nothing at all. Do not set it to 0 or to 100, just leave it alone.

Hint 3

The constructor is not asked to validate, so a Student can be built with an out of range grade. Only setGrade guards.

Where to go next

Rework the Lesson 3.3 page Pick another Unit 3 lesson

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