BAEL-8640 (#17861)

* BAEL-8640

This commit is related to How to Add a Utf-8 Bom in Java?

* Update BomFileWriterUnitTest.java

* Update BomFileWriterUnitTest.java

Author: MohamedHelmyKassab

core-java-modules/core-java-io-7/src/test/bomfilewriter/BomFileWriterUnitTest.java

@@ -0,0 +1,72 @@
+package com.baeldung.bomfilewriter;
+
+import org.apache.commons.io.FileUtils;
+import org.junit.jupiter.api.Test;
+import org.apache.commons.lang3.ArrayUtils;
+
+import java.io.*;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.Path;
+
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+public class BomFileWriterUnitTest {
+    private static final String TEST_CONTENT = "This is the content of the file.";
+    private static final byte[] UTF8_BOM = {(byte) 0xEF, (byte) 0xBB, (byte) 0xBF};
+    private static final String UTF8_BOM_UNICODE = "\uFEFF";
+    private static final String FILE_PATH_OUTPUT_STREAM = "output_with_bom_output_stream.txt";
+    private static final String FILE_PATH_BUFFERED_WRITER = "output_with_bom_buffered.txt";
+    private static final String FILE_PATH_PRINT_WRITER = "output_with_bom_print_writer.txt";
+    private static final String FILE_PATH_COMMONS_IO = "output_with_bom_commons_io.txt";
+
+    @Test
+    public void givenText_whenAddingBomWithFileOutputStream_thenBOMAdded() throws IOException {
+        try (FileOutputStream fos = new FileOutputStream(FILE_PATH_OUTPUT_STREAM)) {
+            fos.write(UTF8_BOM);
+            fos.write(TEST_CONTENT.getBytes(StandardCharsets.UTF_8));
+        }
+
+        String result = Files.readString(Path.of(FILE_PATH_OUTPUT_STREAM), StandardCharsets.UTF_8);
+        assertTrue(result.startsWith(UTF8_BOM_UNICODE));
+        assertTrue(result.contains(TEST_CONTENT));
+    }
+
+    @Test
+    public void givenText_whenAddingBomWithBufferedWriter_thenBOMAdded() throws IOException {
+        try (OutputStreamWriter osw = new OutputStreamWriter(
+                new FileOutputStream(FILE_PATH_BUFFERED_WRITER), StandardCharsets.UTF_8);
+             BufferedWriter writer = new BufferedWriter(osw)) {
+
+            writer.write(UTF8_BOM_UNICODE);
+            writer.write(TEST_CONTENT);
+        }
+
+        String result = Files.readString(Path.of(FILE_PATH_BUFFERED_WRITER), StandardCharsets.UTF_8);
+        assertTrue(result.startsWith(UTF8_BOM_UNICODE));
+        assertTrue(result.contains(TEST_CONTENT));
+    }
+
+    @Test
+    public void givenText_whenUsingPrintWriter_thenBOMAdded() throws IOException {
+        try (PrintWriter writer = new PrintWriter(
+                new OutputStreamWriter(new FileOutputStream(FILE_PATH_PRINT_WRITER), StandardCharsets.UTF_8))) {
+            writer.write(UTF8_BOM_UNICODE);
+            writer.println(TEST_CONTENT);
+        }
+
+        String result = Files.readString(Path.of(FILE_PATH_PRINT_WRITER), StandardCharsets.UTF_8);
+        assertTrue(result.startsWith(UTF8_BOM_UNICODE));
+        assertTrue(result.contains(TEST_CONTENT));
+    }
+
+    @Test
+    public void givenText_whenUsingCommonsIO_thenBOMAdded() throws IOException {
+        byte[] bomAndContent = ArrayUtils.addAll(UTF8_BOM, TEST_CONTENT.getBytes(StandardCharsets.UTF_8));
+        FileUtils.writeByteArrayToFile(new File(FILE_PATH_COMMONS_IO), bomAndContent);
+
+        String result = FileUtils.readFileToString(new File(FILE_PATH_COMMONS_IO), StandardCharsets.UTF_8);
+        assertTrue(result.startsWith(UTF8_BOM_UNICODE));
+        assertTrue(result.contains(TEST_CONTENT));
+    }
+}