diff --git a/pom.xml b/pom.xml
index f301482..9b69af4 100644
--- a/pom.xml
+++ b/pom.xml
@@ -14,6 +14,7 @@
         <maven.compiler.target>${maven.compiler.source}</maven.compiler.target>
         <junit.jupiter.version>5.5.1</junit.jupiter.version>
         <assertj.version>3.13.2</assertj.version>
+        <mockito.version>1.10.19</mockito.version>
     </properties>
 
     <dependencies>
@@ -29,7 +30,12 @@
             <version>${assertj.version}</version>
             <scope>test</scope>
         </dependency>
-
+        <dependency>
+            <groupId>org.mockito</groupId>
+            <artifactId>mockito-all</artifactId>
+            <version>${mockito.version}</version>
+            <scope>test</scope>
+        </dependency>
     </dependencies>
 
     <build>
diff --git a/src/test/java/com/github/isenng/libraryunittesting/library/LibraryBookImplTests.java b/src/test/java/com/github/isenng/libraryunittesting/library/LibraryBookImplTests.java
new file mode 100644
index 0000000..21bbc1f
--- /dev/null
+++ b/src/test/java/com/github/isenng/libraryunittesting/library/LibraryBookImplTests.java
@@ -0,0 +1,66 @@
+package com.github.isenng.libraryunittesting.library;
+
+import com.github.isenng.libraryunittesting.library.duedates.DueDateCalculator;
+import com.github.isenng.libraryunittesting.library.exceptions.AlreadyCheckedOutException;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Nested;
+import org.junit.jupiter.api.Test;
+
+import java.time.LocalDate;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.catchThrowable;
+import static org.mockito.Mockito.*;
+
+class LibraryBookImplTests {
+    private LibraryBook libraryBook;
+    private DueDateCalculator mockDueDateCalculator;
+
+    @BeforeEach
+    void setup() {
+        mockDueDateCalculator = mock(DueDateCalculator.class);
+        libraryBook = new LibraryBookImpl(mockDueDateCalculator);
+    }
+
+    @Nested
+    class Constructor {
+        @Test
+        void ShouldThrowIllegalArgumentExceptionIfDueDateCalculatorIsNull() {
+            // act
+            Throwable t = catchThrowable(() -> new LibraryBookImpl(null));
+
+            // assert
+            assertThat(t).isInstanceOf(IllegalArgumentException.class);
+        }
+    }
+
+    @Nested
+    class CheckOut {
+        @Test
+        void ShouldThrowAlreadyCheckedOutExceptionIfBookIsCheckedOut() throws Exception {
+            // arrange
+            when(mockDueDateCalculator.calculateDueDate()).thenReturn(LocalDate.now());
+            libraryBook.checkOut();
+
+            // act
+            Throwable t = catchThrowable(() -> libraryBook.checkOut());
+
+            // assert
+            assertThat(t).isInstanceOf(AlreadyCheckedOutException.class);
+        }
+
+        @Test
+        void ShouldReturnDueDateIfBookIsNotCheckedOut() throws Exception {
+            // arrange
+            LocalDate expected = LocalDate.now();
+            when(mockDueDateCalculator.calculateDueDate()).thenReturn(expected);
+
+            // act
+            LocalDate result = libraryBook.checkOut();
+
+            // assert
+            assertThat(result).isEqualTo(expected);
+            verify(mockDueDateCalculator).calculateDueDate();
+        }
+    }
+}