Adds lecture note for SO in SOLID.

oop/code/oop/src/main/java/com/scaler/lld/questions/Book.java

@@ -0,0 +1,16 @@
+package com.scaler.lld.questions;
+
+import lombok.AllArgsConstructor;
+import lombok.Getter;
+import lombok.Setter;
+
+@AllArgsConstructor
+@Getter
+@Setter
+public class Book {
+    private String name;
+	private String authorName;
+	private int year;
+	private int price;
+	private String isbn;
+}

oop/code/oop/src/main/java/com/scaler/lld/questions/Invoice.java

@@ -0,0 +1,34 @@
+package com.scaler.lld.questions;
+
+import lombok.AccessLevel;
+import lombok.AllArgsConstructor;
+import lombok.Getter;
+
+@Getter
+@AllArgsConstructor
+public class Invoice {
+    private Book book;
+    private Integer quantity;
+    private Double discountRate;
+    private Double taxRate;
+
+    @Getter(AccessLevel.NONE)
+    private double total; // Will not generate a getter
+
+    public Double getTotal() {
+        double price = ((book.getPrice() - book.getPrice() * discountRate) * this.quantity);
+        return price * (1 + taxRate);
+    }
+
+    public void printInvoice() {
+        System.out.println(quantity + "x " + book.getName() + " " + book.getPrice() + "$");
+        System.out.println("Discount Rate: " + discountRate);
+        System.out.println("Tax Rate: " + taxRate);
+        System.out.println("Total: " + total);
+    }
+
+    public void saveToFile(String filename) {
+        // Creates a file with given name and writes the invoice
+    }
+
+}