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CsvWriter.java

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+import java.io.File;
+import java.io.FileWriter;
+import java.io.IOException;
+import java.time.LocalDateTime;
+import java.time.format.DateTimeFormatter;
+
+public class CsvWriter {
+    private static final String FILE_NAME = "currency_log.csv";
+    private static final DateTimeFormatter FORMATTER = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
+
+    private static boolean is_header_written = false;
+
+    public static void write(double value) {
+        File file = new File(FILE_NAME);
+        boolean file_exists = file.exists();
+
+        try (FileWriter writer = new FileWriter(FILE_NAME, true)) {
+            if (!is_header_written && !file_exists) {
+                writer.write("Дата и время;Курс USD (руб.)\n");
+                is_header_written = true;
+            }
+
+            String timestamp = LocalDateTime.now().format(FORMATTER);
+            String line = timestamp + ";" + value + "\n";
+            writer.write(line);
+
+        } catch (IOException e) {
+            e.printStackTrace();
+        }
+    }
+}