spring-boot-demo/pdf-invoice/src/main/java/com/ankurm/pdfinvoice/Invoice.java

package com.ankurm.pdfinvoice;

import java.math.BigDecimal;
import java.math.RoundingMode;
import java.time.LocalDate;
import java.util.List;

/**
 * The one invoice all three libraries render. Plain records, no framework types, so each writer
 * is a function {@code Invoice -> bytes} and the comparison is about the PDF libraries, not about Spring.
 */
public record Invoice(String number, LocalDate date, Party seller, Party buyer, List<Line> lines, String note) {

    public record Party(String name, String address, String gstin) {}

    /** {@code description} may be Devanagari; that is the point of the exercise. */
    public record Line(String description, int quantity, BigDecimal unitPrice) {
        public BigDecimal amount() {
            return unitPrice.multiply(BigDecimal.valueOf(quantity));
        }
    }

    /** GST at 18 percent, split evenly into CGST and SGST as an intra-state sale would be. */
    public static final BigDecimal GST_RATE = new BigDecimal("0.18");

    public BigDecimal subtotal() {
        return lines.stream().map(Line::amount).reduce(BigDecimal.ZERO, BigDecimal::add);
    }

    public BigDecimal gst() {
        return subtotal().multiply(GST_RATE).setScale(2, RoundingMode.HALF_UP);
    }

    public BigDecimal total() {
        return subtotal().add(gst());
    }
}