test(io): add StudentCsvWriter tests

- test writing to empty and existing CSV files
- verify append behavior and header handling
- test writing arrays of student lists
- verify student order and empty lists
- test IOException for missing files

app/src/test/java/ru/project/io/StudentCsvWriterTest.java

@@ -0,0 +1,194 @@
+package ru.project.io;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.List;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
+import ru.project.collection.MyLinkedList;
+import ru.project.collection.MyList;
+import ru.project.student.Student;
+
+class StudentCsvWriterTest {
+
+  @TempDir Path tempDir;
+
+  @Test
+  void writeShouldWriteHeaderAndStudentToEmptyFile() throws Exception {
+    Path file = tempDir.resolve("students.csv");
+    Files.createFile(file);
+
+    Student student = new Student("A12", 4.5, 10001);
+
+    StudentCsvWriter writer = new StudentCsvWriter(file.toAbsolutePath().toString());
+
+    writer.write(student);
+
+    List<String> lines = Files.readAllLines(file);
+
+    assertEquals(List.of("groupNumber,averageGrade,recordBookNumber", "A12,4.5,10001"), lines);
+  }
+
+  @Test
+  void writeShouldAppendStudentToExistingFile() throws Exception {
+    Path file = tempDir.resolve("students.csv");
+
+    Files.write(file, List.of("groupNumber,averageGrade,recordBookNumber", "A12,4.5,10001"));
+
+    Student student = new Student("B07", 3.8, 10002);
+
+    StudentCsvWriter writer = new StudentCsvWriter(file.toAbsolutePath().toString());
+
+    writer.write(student);
+
+    List<String> lines = Files.readAllLines(file);
+
+    assertEquals(
+        List.of("groupNumber,averageGrade,recordBookNumber", "A12,4.5,10001", "B07,3.8,10002"),
+        lines);
+  }
+
+  @Test
+  void writeShouldNotCreateMissingFile() {
+    Path file = tempDir.resolve("students.csv");
+
+    Student student = new Student("A12", 4.5, 10001);
+
+    StudentCsvWriter writer = new StudentCsvWriter(file.toAbsolutePath().toString());
+
+    assertThrows(IOException.class, () -> writer.write(student));
+
+    assertFalse(Files.exists(file));
+  }
+
+  @Test
+  void writeShouldAppendSeveralStudentsInOrder() throws Exception {
+    Path file = tempDir.resolve("students.csv");
+    Files.createFile(file);
+
+    Student student1 = new Student("A12", 4.5, 10001);
+    Student student2 = new Student("B07", 3.8, 10002);
+    Student student3 = new Student("C25", 4.9, 10003);
+
+    StudentCsvWriter writer = new StudentCsvWriter(file.toAbsolutePath().toString());
+
+    writer.write(student1);
+    writer.write(student2);
+    writer.write(student3);
+
+    List<String> lines = Files.readAllLines(file);
+
+    assertEquals(
+        List.of(
+            "groupNumber,averageGrade,recordBookNumber",
+            "A12,4.5,10001",
+            "B07,3.8,10002",
+            "C25,4.9,10003"),
+        lines);
+  }
+
+  @Test
+  void writeArrayShouldAppendStudentsFromAllListsInOrder() throws Exception {
+
+    Path file = tempDir.resolve("students.csv");
+    Files.createFile(file);
+
+    Student student1 = new Student("A12", 4.5, 10001);
+    Student student2 = new Student("B07", 3.8, 10002);
+    Student student3 = new Student("C25", 4.9, 10003);
+    Student student4 = new Student("D15", 4.2, 10004);
+    Student student5 = new Student("E20", 3.6, 10005);
+
+    MyList<Student> firstList = new MyLinkedList<>();
+    firstList.add(student1);
+    firstList.add(student2);
+
+    MyList<Student> secondList = new MyLinkedList<>();
+    secondList.add(student3);
+
+    MyList<Student> thirdList = new MyLinkedList<>();
+    thirdList.add(student4);
+    thirdList.add(student5);
+
+    @SuppressWarnings("unchecked")
+    MyList<Student>[] students = new MyList[] {firstList, secondList, thirdList};
+
+    StudentCsvWriter writer = new StudentCsvWriter(file.toAbsolutePath().toString());
+
+    writer.write(students);
+
+    List<String> lines = Files.readAllLines(file);
+
+    assertEquals(
+        List.of(
+            "groupNumber,averageGrade,recordBookNumber",
+            "A12,4.5,10001",
+            "B07,3.8,10002",
+            "C25,4.9,10003",
+            "D15,4.2,10004",
+            "E20,3.6,10005"),
+        lines);
+  }
+
+  @Test
+  void writeArrayShouldSkipEmptyLists() throws Exception {
+    Path file = tempDir.resolve("students.csv");
+    Files.createFile(file);
+
+    Student student = new Student("A12", 4.5, 10001);
+
+    MyList<Student> emptyList = new MyLinkedList<>();
+
+    MyList<Student> nonEmptyList = new MyLinkedList<>();
+    nonEmptyList.add(student);
+
+    @SuppressWarnings("unchecked")
+    MyList<Student>[] students = new MyList[] {emptyList, nonEmptyList, new MyLinkedList<>()};
+
+    StudentCsvWriter writer = new StudentCsvWriter(file.toAbsolutePath().toString());
+
+    writer.write(students);
+
+    List<String> lines = Files.readAllLines(file);
+
+    assertEquals(List.of("groupNumber,averageGrade,recordBookNumber", "A12,4.5,10001"), lines);
+  }
+
+  @Test
+  void writeEmptyArrayShouldNotModifyFile() throws Exception {
+    Path file = tempDir.resolve("students.csv");
+
+    List<String> initialContent =
+        List.of("groupNumber,averageGrade,recordBookNumber", "A12,4.5,10001");
+
+    Files.write(file, initialContent);
+
+    @SuppressWarnings("unchecked")
+    MyList<Student>[] students = new MyList[0];
+
+    StudentCsvWriter writer = new StudentCsvWriter(file.toAbsolutePath().toString());
+
+    writer.write(students);
+
+    assertEquals(initialContent, Files.readAllLines(file));
+  }
+
+  @Test
+  void writeEmptyArrayShouldNotCreateMissingFile() {
+    Path file = tempDir.resolve("students.csv");
+
+    @SuppressWarnings("unchecked")
+    MyList<Student>[] students = new MyList[0];
+
+    StudentCsvWriter writer = new StudentCsvWriter(file.toAbsolutePath().toString());
+
+    assertThrows(IOException.class, () -> writer.write(students));
+
+    assertFalse(Files.exists(file));
+  }
+}