diff --git a/refactoring/src/main/java/com/example/refactoring/_24_comments/_43_introduce_assertion/Customer.java b/refactoring/src/main/java/com/example/refactoring/_24_comments/_43_introduce_assertion/Customer.java
new file mode 100644
index 00000000..bf0134b6
--- /dev/null
+++ b/refactoring/src/main/java/com/example/refactoring/_24_comments/_43_introduce_assertion/Customer.java
@@ -0,0 +1,19 @@
+package com.example.refactoring._24_comments._43_introduce_assertion;
+
+public class Customer {
+
+    private Double discountRate;
+
+    public double applyDiscount(double amount) {
+        return (this.discountRate != null) ? amount - (this.discountRate * amount) : amount;
+    }
+
+    public Double getDiscountRate() {
+        return discountRate;
+    }
+
+    public void setDiscountRate(Double discountRate) {
+        assert discountRate != null && discountRate > 0;
+        this.discountRate = discountRate;
+    }
+}
diff --git a/refactoring/src/main/java/com/example/refactoring/_24_comments/_43_introduce_assertion/_before/Customer.java b/refactoring/src/main/java/com/example/refactoring/_24_comments/_43_introduce_assertion/_before/Customer.java
new file mode 100644
index 00000000..4d716a49
--- /dev/null
+++ b/refactoring/src/main/java/com/example/refactoring/_24_comments/_43_introduce_assertion/_before/Customer.java
@@ -0,0 +1,18 @@
+package com.example.refactoring._24_comments._43_introduce_assertion._before;
+
+public class Customer {
+
+    private Double discountRate;
+
+    public double applyDiscount(double amount) {
+        return (this.discountRate != null) ? amount - (this.discountRate * amount) : amount;
+    }
+
+    public Double getDiscountRate() {
+        return discountRate;
+    }
+
+    public void setDiscountRate(Double discountRate) {
+        this.discountRate = discountRate;
+    }
+}
diff --git a/refactoring/src/test/java/com/example/refactoring/_24_comments/_43_introduce_assertion/CustomerTest.java b/refactoring/src/test/java/com/example/refactoring/_24_comments/_43_introduce_assertion/CustomerTest.java
new file mode 100644
index 00000000..95fa2ffc
--- /dev/null
+++ b/refactoring/src/test/java/com/example/refactoring/_24_comments/_43_introduce_assertion/CustomerTest.java
@@ -0,0 +1,15 @@
+package com.example.refactoring._24_comments._43_introduce_assertion;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class CustomerTest {
+
+    @Test
+    void setDiscountRate() {
+        Customer customer = new Customer();
+        customer.setDiscountRate(-10d);
+    }
+
+}
\ No newline at end of file