Challenges

Remember the rules for this are

• Try to use only the information given up to this point in this book.
• Try not to give up until you've given it a solid attempt

Challenge 1

The following Ratio class should have an invariant where denominator cannot be 0.

class Ratio {
    int numerator;
    int denominator;

    Ratio(int numerator, int denominator) {
        if (denominator == 0) {
            throw new RuntimeException("Denominator cannot be zero");
        }
    }

    double value() {
        return ((double) numerator) / denominator;
    }
}

Unfortunately other classes can directly access the denominator field.

Make the denominator field private and add an accessor method for other classes to use. You can name this accessor .denominator() or .getDenominator().

Challenge 2

Alter the code from the last challenge such that numerator is also private and only usable via an accessor method.

Challenge 3

Alter the code from the last challenge such that you can not only access but also mutate the numerator and denominator fields via exposed methods. The method for mutating the denominator should throw an exception if someone tries to set it to zero.

Challenge 4

Given this Pineapple class make every method except digest private.

class Pineapple {
    void digestProtein(String name) {
        IO.println("Pineapple juice is digesting the proteins in " + name);
    }
    
    void eatPork() {
        digestProtein("pork");
    }

    void eatChicken() {
        digestProtein("chicken");
    }

    void eatBeef() {
        digestProtein("beef");
    }

    void eatYou() {
        IO.println("Pineapple juice is eating you from the inside.");
    }

    void digest(String thing) {
        switch (thing) {
            case "pork" -> eatPork();
            case "chicken" -> eatChicken();
            case "beef" -> eatBeef();
            default -> eatYou();
        }
    }
}

Challenge 5

Rewrite the code from the previous challenge such that the digest method on Pineapple does the same thing but no other private methods exist.

Note when doing this that you wouldn't need to consider other code. The behavior of the only exposed method does not change so those private methods would only be "implementation details."