ContractorCalculator.java

package payroll;

public class ContractorCalculator extends PayCalculator {
  public double calculate(double hours, double rate) {
    validateHours(hours);
    return hours * rate;
  }
}

HourlyCalculator.java

package payroll;

public class HourlyCalculator extends PayCalculator {
  public double calculate(double hours, double rate) {
    validateHours(hours);
    double overtime = Math.max(0, hours - 40);
    return hours * rate + overtime * rate * 0.5;
  }
}

PayCalculator.java

package payroll;

public abstract class PayCalculator {
  public abstract double calculate(double hours, double rate);

  protected void validateHours(double hours) {
    if (hours < 0 || hours > 80) {
      throw new RuntimeException("Hours out of range: " + hours);
    }
  }
}