LibraryServiceTest

src/test/java/com/example/library/LibraryServiceTest.java

+package com.example.library;
+
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.*;
+import org.mockito.junit.jupiter.MockitoExtension;
+
+import java.util.Optional;
+
+import static org.junit.jupiter.api.Assertions.*;
+import static org.mockito.Mockito.*;
+
+@ExtendWith(MockitoExtension.class)
+public class LibraryServiceTest {
+
+  @Mock
+  private BookRepository repository;
+
+  @InjectMocks
+  private LibraryService service;
+
+  @Captor
+  private ArgumentCaptor<Book> bookCaptor;
+
+  @Test
+  void borrowBook_shouldUpdateAvailabilityAndSave() {
+    // Arrange
+    Book book = new Book("Effective Java", "Joshua Bloch", true);
+    book.setAvailable(true);
+
+    when(repository.findById(1L)).thenReturn(Optional.of(book));
+    when(repository.save(any(Book.class))).thenAnswer(inv -> inv.getArgument(0));
+
+    // Act
+    boolean result = service.borrowBook(1L);
+
+    // Assert
+    assertTrue(result);
+
+    verify(repository).save(bookCaptor.capture());
+    Book savedBook = bookCaptor.getValue();
+    assertFalse(savedBook.isAvailable(), "Book should no longer be available after borrowing");
+  }
+
+  @Test
+  void returnBook_shouldUpdateAvailabilityAndSave() {
+    Book book = new Book("Domain-Driven Design", "Eric Evans", false);
+
+    when(repository.findById(2L)).thenReturn(Optional.of(book));
+    when(repository.save(any(Book.class))).thenAnswer(inv -> inv.getArgument(0));
+
+    boolean result = service.returnBook(2L);
+
+    assertTrue(result);
+
+    verify(repository).save(bookCaptor.capture());
+    Book savedBook = bookCaptor.getValue();
+    assertTrue(savedBook.isAvailable(), "Book should be available after return");
+  }
+
+  @Test
+  void borrowBook_shouldFail_whenBookNotFound() {
+    when(repository.findById(99L)).thenReturn(Optional.empty());
+
+    boolean result = service.borrowBook(99L);
+
+    assertFalse(result);
+    verify(repository, never()).save(any());
+  }
+
+  @Test
+  void returnBook_shouldFail_whenBookAlreadyAvailable() {
+    Book book = new Book("Clean Architecture", "Robert C. Martin", true);
+
+    when(repository.findById(3L)).thenReturn(Optional.of(book));
+
+    boolean result = service.returnBook(3L);
+
+    assertFalse(result);
+    verify(repository, never()).save(any());
+  }
+}