Build a simple cash register Java application.
Add a JFrame to it , then add to the frame, through code, the necessary components (labels, textfields, etc...) to allow a user to enter the number of donuts purchased and the price of each donuts.
Add a sales button to compute the subtotal, the tax and the total cost.
Add a clear button to clear all the input boxes.
Add a display type component to allow you to display: the amount of donuts purchased, the price per donuts, the subtotal, the tax and the total cost when the sales button is clicked.
Layout your frame however you want. make it appealing and easy to use. (everything through code).