From 4dadefd87f644426d53ef8a47581a4e3a197997e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=D0=92=D0=BE=D0=BB=D0=BE=D0=B4=D0=B8=D0=BD=20=D0=93=D0=B5?=
 =?UTF-8?q?=D1=80=D0=BC=D0=B0=D0=BD?= <make_life_better@mail.ru>
Date: Mon, 31 Aug 2026 22:17:51 +0400
Subject: [PATCH] feat(io): add StudentCsvWriter

- add CSV writer for individual students
- support sequential writing of student lists
- preserve existing CSV data and avoid duplicate headers
- reject missing files with IOException
---
 .../java/ru/project/io/StudentCsvWriter.java  | 88 +++++++++++++++++++
 1 file changed, 88 insertions(+)
 create mode 100644 app/src/main/java/ru/project/io/StudentCsvWriter.java

diff --git a/app/src/main/java/ru/project/io/StudentCsvWriter.java b/app/src/main/java/ru/project/io/StudentCsvWriter.java
new file mode 100644
index 0000000..8795fbc
--- /dev/null
+++ b/app/src/main/java/ru/project/io/StudentCsvWriter.java
@@ -0,0 +1,88 @@
+package ru.project.io;
+
+import java.io.BufferedWriter;
+import java.io.File;
+import java.io.FileOutputStream;
+import java.io.IOException;
+import java.io.OutputStreamWriter;
+import java.nio.charset.StandardCharsets;
+import org.apache.commons.csv.CSVFormat;
+import org.apache.commons.csv.CSVPrinter;
+import ru.project.collection.MyList;
+import ru.project.student.Student;
+
+public class StudentCsvWriter {
+
+  private static final String GROUP_NUMBER_HEADER = "groupNumber";
+  private static final String AVERAGE_GRADE_HEADER = "averageGrade";
+  private static final String RECORD_BOOK_NUMBER_HEADER = "recordBookNumber";
+
+  private final String absoluteFilePath;
+
+  public StudentCsvWriter(String absoluteFilePath) {
+    this.absoluteFilePath = absoluteFilePath;
+  }
+
+  public void write(Student student) throws IOException {
+    ensureFileExists();
+
+    boolean fileIsEmpty = isFileEmpty();
+
+    try (CSVPrinter printer = createPrinter()) {
+      if (fileIsEmpty) {
+        writeHeader(printer);
+      }
+
+      writeStudent(printer, student);
+    }
+  }
+
+  public void write(MyList<Student>[] students) throws IOException {
+    ensureFileExists();
+
+    boolean fileIsEmpty = isFileEmpty();
+
+    try (CSVPrinter printer = createPrinter()) {
+      if (fileIsEmpty) {
+        writeHeader(printer);
+      }
+
+      for (MyList<Student> studentList : students) {
+        for (int i = 0; i < studentList.size(); i++) {
+          writeStudent(printer, studentList.get(i));
+        }
+      }
+    }
+  }
+
+  private CSVPrinter createPrinter() throws IOException {
+    BufferedWriter writer =
+        new BufferedWriter(
+            new OutputStreamWriter(
+                new FileOutputStream(absoluteFilePath, true), StandardCharsets.UTF_8));
+
+    return new CSVPrinter(writer, CSVFormat.DEFAULT);
+  }
+
+  private void writeHeader(CSVPrinter printer) throws IOException {
+    printer.printRecord(GROUP_NUMBER_HEADER, AVERAGE_GRADE_HEADER, RECORD_BOOK_NUMBER_HEADER);
+  }
+
+  private void writeStudent(CSVPrinter printer, Student student) throws IOException {
+
+    printer.printRecord(
+        student.getGroupNumber(), student.getAverageGrade(), student.getRecordBookNumber());
+  }
+
+  private void ensureFileExists() throws IOException {
+    File file = new File(absoluteFilePath);
+
+    if (!file.isFile()) {
+      throw new IOException("CSV file does not exist: " + absoluteFilePath);
+    }
+  }
+
+  private boolean isFileEmpty() {
+    return new File(absoluteFilePath).length() == 0;
+  }
+}