عرض عملي: بناء طابعة الإيصالات مع التحقق من المدخلات

Written Explanation

The plan

We want this output (the numbers come from the module lab):

Item        Qty    Price     Total
Pen           2     1.50      3.00
Notebook      1     4.25      4.25
Subtotal                      7.25
Tax (15%)                     1.09
Total                         8.34

In steps:

1. Ask for an item name. An empty name means "no more items".
2. Ask for the quantity. Re-ask until it is a whole number of 1 or more.
3. Ask for the price. Re-ask until it is a number above zero.
4. Calculate the line total (quantity times price) and add it to the subtotal.
5. Keep the formatted line for later.
6. After the last item, calculate tax (15%) and print the receipt.

The tax rate is a constant. A named final constant (BEG-009) means the 0.15 is written once and its meaning is clear.

Why the receipt is built in a String

The receipt has a header, then the item lines, then the totals. But we only learn the items while the user answers questions, and the prompts would mix into the receipt if we printed each line immediately. So we collect each formatted line as text with String.format, and print them all at the end. lines += ... appends a new row to the text collected so far. (A StringBuilder class does this job more efficiently, but you meet it later; for a handful of rows this is fine.)

Step 1: the skeleton and the planned mistake

Start with the simplest version that reads an item. First, the version with the mistake, where the quantity comes before the name and uses nextInt():

import java.util.Scanner;

public class Bug {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        System.out.print("Quantity: ");
        int qty = in.nextInt();
        System.out.print("Item name: ");
        String name = in.nextLine();
        System.out.printf("[%-10s] x %d%n", name, qty);
    }
}

Run it with 2, then Pen as the answers:

Quantity: Item name: [          ] x 2

The name prompt never waited: name is empty (the brackets show ten spaces of padding around nothing). This is exactly the trap from Lesson 2. The nextInt() left the line break, and nextLine() read it. Why it matters here: in a receipt, an empty item name is a silent data error: the program runs, the receipt prints, and a row has no name.

Step 2: the fix, and a better design

Lesson 2 gave the fixes. Here we choose the order: ask for the name first, since a line followed by a number is safe, and keep one throw-away in.nextLine(); after the last number of each item, because the loop then returns to the name question. That is the design in the final program below.

Step 3: the quantity loop

The validation loop from Lesson 5, with the minimum of 1 for the quantity:

int qty;
while (true) {
    System.out.print("Quantity: ");
    if (in.hasNextInt()) {
        qty = in.nextInt();
        if (qty >= 1) {
            break;
        }
    } else {
        in.next();
    }
    System.out.println("Enter a whole number, 1 or more.");
}

This rejects text, decimals such as 2.5, zero, and negatives. The lab demands exactly this: not a whole number, or less than 1, means ask again.

Step 4: the price loop

The price loop is the same shape with hasNextDouble() and price > 0. Note that it is the same shape as the quantity loop: when the checks follow a pattern, you can copy the structure and change the type and the condition. (Later you will learn to avoid this repetition with methods.)

double price;
while (true) {
    System.out.print("Price: ");
    if (in.hasNextDouble()) {
        price = in.nextDouble();
        if (price > 0) {
            break;
        }
    } else {
        in.next();
    }
    System.out.println("Enter a price above zero, like 1.50.");
}
in.nextLine();              // finish the price line

price > 0 rejects NaN too, as Lesson 5 explained, because every comparison with NaN is false. The last line is the throw-away call: the price was the last token on its line, and the next thing the program reads is the next item's name with nextLine().

Step 5: calculate, collect, print

double lineTotal = qty * price;
subtotal += lineTotal;
lines += String.format("%-12s %3d %8.2f %9.2f%n", name, qty, price, lineTotal);

The format string is what makes the columns: name 12 wide, left-aligned; quantity 3 wide; price 8 wide with two decimals; total 9 wide with two decimals; each separated by one space. The header and the totals rows use the same widths, with %s in place of the numbers for the header, and a wider left column for the labels of the totals.

At the end:

double tax = subtotal * TAX_RATE;
System.out.printf("%-12s %3s %8s %9s%n", "Item", "Qty", "Price", "Total");
System.out.print(lines);
System.out.printf("%-25s %9.2f%n", "Subtotal", subtotal);
System.out.printf("%-25s %9.2f%n", "Tax (15%)", tax);
System.out.printf("%-25s %9.2f%n", "Total", subtotal + tax);

%-25s is 12 + 1 + 3 + 1 + 8 = 25 characters, which is exactly the width of the three left columns plus their spaces, so the totals land under the Total column. Counting widths like this is how you design a table on paper before coding it.

One detail: the label Tax (15%) is passed as a value for %s, not as part of the format string, so the % in it is safe. If you wrote it inside a format string, you would need %%.