public abstract class TaxableItem implements Item {
    private double taxRate;

    public abstract double getListPrice();

    public TaxableItem(double rate) {
        taxRate = rate;
    }

    /*************  Part (a) *************/
    // returns the price of the item including the tax
    public double purchasePrice() {
        return this.getListPrice() * (1 + this.taxRate);
    }
}

Create TaxableItem.java