Реализация CSV writer

src/main/java/org/writer/Csv.java

@@ -0,0 +1,20 @@
+package org.writer;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+/**
+ * Аннотация для помечания полей,
+ * которые необходимо сохранять в
+ * CSV файл
+ */
+@Retention(RetentionPolicy.RUNTIME)
+@Target(ElementType.FIELD)
+public @interface Csv {
+    /**
+     * Имя колонки в CSV файле
+     */
+    String name() default "";
+}

src/main/java/org/writer/Main.java

@@ -1,7 +1,27 @@
 package org.writer;
 
+import org.writer.model.Months;
+import org.writer.model.Person;
+import org.writer.model.Student;
+
+import java.util.List;
+
 public class Main {
     public static void main(String[] args) {
-        System.out.println("Hello world!");
+        Writable writable = new WritableImpl();
+
+        List<Student> students = List.of(
+            new Student("Nick", List.of("5,4,3,3,4")),
+            new Student("Paul", List.of("4,4,4,4,4")),
+            new Student("Ann", List.of("5,5,5,5,5"))
+        );
+
+        List<Person> persons = List.of(
+            new Person("John", "Doe", 1, Months.JANUARY, 1991),
+            new Person("Alexandr", "Pushkin", 6, Months.JUNE, 1799)
+        );
+
+        writable.writeToFile(students, "students.csv");
+        writable.writeToFile(persons, "persons.csv");
     }
 }

src/main/java/org/writer/WritableImpl.java

@@ -0,0 +1,67 @@
+package org.writer;
+
+import java.io.BufferedWriter;
+import java.io.FileWriter;
+import java.io.IOException;
+import java.lang.reflect.Field;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+import java.util.stream.Collectors;
+
+/**
+ * Класс, предназанченный для записи коллекции объектов в CSV-файл
+ */
+public class WritableImpl implements Writable {
+
+    /**
+     * Метод, для записи коллекуии в файл формата CSV
+     *
+     * @param data     - коллекция объектов для сохранения
+     * @param fileName - имя файла, в который будут сохранены данные
+     */
+    @Override
+    public void writeToFile(List<?> data, String fileName) {
+        if (data == null || data.isEmpty()) {
+            throw new IllegalArgumentException("data is null or empty");
+        }
+
+        Class<?> clazz = data.get(0).getClass();
+        List<Field> fields = fieldsToCsv(clazz);
+
+        try (
+            BufferedWriter fileWriter = new BufferedWriter(new FileWriter(fileName))
+        ) {
+            fileWriter.write(
+                fields.stream()
+                    .map(f -> f.getAnnotation(Csv.class).name())
+                    .collect(Collectors.joining(",")));
+
+            fileWriter.newLine();
+
+            for (Object o : data) {
+                List<String> values = new ArrayList<>();
+                for (Field f : o.getClass().getDeclaredFields()) {
+                    if (f.isAnnotationPresent(Csv.class)) {
+                        f.setAccessible(true);
+                        Object val = f.get(o);
+                        if (val != null) {
+                            values.add(val.toString());
+                        }
+                    }
+                }
+                fileWriter.write(String.join(",", values));
+                fileWriter.newLine();
+            }
+
+        } catch (IOException | IllegalAccessException e) {
+            throw new RuntimeException(e.getMessage());
+        }
+    }
+
+    private List<Field> fieldsToCsv(Class<?> clazz) {
+        return Arrays.stream(clazz.getDeclaredFields())
+            .filter(field -> field.isAnnotationPresent(Csv.class))
+            .toList();
+    }
+}

src/main/java/org/writer/model/Person.java

@@ -3,20 +3,24 @@
 import lombok.AllArgsConstructor;
 import lombok.Builder;
 import lombok.Data;
+import org.writer.Csv;
 
 @Data
 @Builder
 @AllArgsConstructor
 public class Person {
 
     private String firstName;
-
+    @Csv(name = "lastName")
     private String lastName;
 
+    @Csv(name = "dayOfBirth")
     private int dayOfBirth;
 
+    @Csv(name = "monthOfBirth")
     private Months monthOfBirth;
 
+    @Csv(name = "yearOfBirth")
     private int yearOfBirth;
 
 }

src/main/java/org/writer/model/Student.java

@@ -3,6 +3,7 @@
 import lombok.AllArgsConstructor;
 import lombok.Builder;
 import lombok.Data;
+import org.writer.Csv;
 
 import java.util.List;
 
@@ -11,7 +12,9 @@
 @AllArgsConstructor
 public class Student {
 
+    @Csv(name = "name")
     private String name;
 
+    @Csv(name = "score")
     private List<String> score;
 }

src/test/java/org/writer/WritableImplTest.java

@@ -0,0 +1,65 @@
+package org.writer;
+
+import org.junit.jupiter.api.Test;
+
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.ArrayList;
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+class WritableImplTest {
+    private final Writable writable = new WritableImpl();
+
+    static class TestData{
+        @Csv(name = "name")
+        private final String name;
+        @Csv(name = "age")
+        private final int age;
+
+        private byte ignoredData;
+
+        public TestData(String name, int age){
+            this.name = name;
+            this.age = age;
+        }
+    }
+
+    @Test
+    void testWriteWithNullData() {
+
+        assertThrows(IllegalArgumentException.class,
+            () -> writable.writeToFile(null, "some.csv"));
+
+    }
+
+    @Test
+    void testWriteWithEmptyData() {
+
+        assertThrows(IllegalArgumentException.class,
+            () -> writable.writeToFile(new ArrayList<>(), "some.csv"));
+
+    }
+
+    @Test
+    void testWriteSuccess() throws IOException {
+        String fileName = "some.csv";
+        List<TestData> testData = new ArrayList<>();
+        testData.add(new TestData("name1", 1));
+        testData.add(new TestData("name2", 2));
+        testData.add(new TestData("name3", 3));
+
+        writable.writeToFile(testData, fileName);
+
+        List<String> lines = Files.readAllLines(Path.of(fileName));
+
+        assertEquals(4, lines.size());
+        assertEquals("name,age", lines.get(0));
+        assertEquals("name1,1", lines.get(1));
+        assertEquals("name2,2", lines.get(2));
+        assertEquals("name3,3", lines.get(3));
+    }
+}