From 9b6a2a0236d8d5b1dad74496269230b189f871ba Mon Sep 17 00:00:00 2001
From: Kai Yuan <kent.yuan@Gmail.com>
Date: Fri, 12 Jan 2024 03:12:52 +0100
Subject: [PATCH] [fileWriter-flush-close] FileWriter flush() and close()
 (#15559)

---
 .../FileWriterFlushVsCloseUnitTest.java       | 101 ++++++++++++++++++
 1 file changed, 101 insertions(+)
 create mode 100644 core-java-modules/core-java-io-apis-2/src/test/java/com/baeldung/filewriterflushandclose/FileWriterFlushVsCloseUnitTest.java

diff --git a/core-java-modules/core-java-io-apis-2/src/test/java/com/baeldung/filewriterflushandclose/FileWriterFlushVsCloseUnitTest.java b/core-java-modules/core-java-io-apis-2/src/test/java/com/baeldung/filewriterflushandclose/FileWriterFlushVsCloseUnitTest.java
new file mode 100644
index 0000000000..8d59605d7d
--- /dev/null
+++ b/core-java-modules/core-java-io-apis-2/src/test/java/com/baeldung/filewriterflushandclose/FileWriterFlushVsCloseUnitTest.java
@@ -0,0 +1,101 @@
+package com.baeldung.filewriterflushandclose;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.io.File;
+import java.io.FileWriter;
+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;
+
+public class FileWriterFlushVsCloseUnitTest {
+
+    @Test
+    void whenUsingFileWriterWithTryWithResources_thenAutoClosed(@TempDir Path tmpDir) throws IOException {
+        Path filePath = tmpDir.resolve("auto-close.txt");
+        File file = filePath.toFile();
+
+        try (FileWriter fw = new FileWriter(file)) {
+            fw.write("Catch Me If You Can");
+        }
+
+        List<String> lines = Files.readAllLines(filePath);
+        assertEquals(List.of("Catch Me If You Can"), lines);
+    }
+
+    @Test
+    void whenUsingFileWriterWithoutFlush_thenDataWontBeWritten(@TempDir Path tmpDir) throws IOException {
+        Path filePath = tmpDir.resolve("noFlush.txt");
+        File file = filePath.toFile();
+
+        FileWriter fw = new FileWriter(file);
+        fw.write("Catch Me If You Can");
+
+        List<String> lines = Files.readAllLines(filePath);
+        assertEquals(0, lines.size());
+        fw.close(); //close the resource
+    }
+
+    @Test
+    void whenUsingFileWriterWithFlush_thenGetExpectedResult(@TempDir Path tmpDir) throws IOException {
+        Path filePath = tmpDir.resolve("flush1.txt");
+        File file = filePath.toFile();
+
+        FileWriter fw = new FileWriter(file);
+        fw.write("Catch Me If You Can");
+        fw.flush();
+
+        List<String> lines = Files.readAllLines(filePath);
+        assertEquals(List.of("Catch Me If You Can"), lines);
+        fw.close(); //close the resource
+    }
+
+    @Test
+    void whenUsingFileWriterWithClose_thenGetExpectedResult(@TempDir Path tmpDir) throws IOException {
+        Path filePath = tmpDir.resolve("close1.txt");
+        File file = filePath.toFile();
+        FileWriter fw = new FileWriter(file);
+        fw.write("Catch Me If You Can");
+        fw.close();
+
+        List<String> lines = Files.readAllLines(filePath);
+        assertEquals(List.of("Catch Me If You Can"), lines);
+
+    }
+
+    @Test
+    void whenUsingFileWriterWithFlushMultiTimes_thenGetExpectedResult(@TempDir Path tmpDir) throws IOException {
+        List<String> lines = List.of("Catch Me If You Can", "A Man Called Otto", "Saving Private Ryan");
+        Path filePath = tmpDir.resolve("flush2.txt");
+        File file = filePath.toFile();
+        FileWriter fw = new FileWriter(file);
+        for (String line : lines) {
+            fw.write(line + System.lineSeparator());
+            fw.flush();
+        }
+
+        List<String> linesInFile = Files.readAllLines(filePath);
+        assertEquals(lines, linesInFile);
+        fw.close(); //close the resource
+    }
+
+    @Test
+    void whenUsingFileWriterWithCloseMultiTimes_thenGetIOExpectedException(@TempDir Path tmpDir) throws IOException {
+        List<String> lines = List.of("Catch Me If You Can", "A Man Called Otto", "Saving Private Ryan");
+        Path filePath = tmpDir.resolve("close2.txt");
+        File file = filePath.toFile();
+        FileWriter fw = new FileWriter(file);
+        //write and close
+        fw.write(lines.get(0) + System.lineSeparator());
+        fw.close();
+
+        //writing again throws IOException
+        Throwable throwable = assertThrows(IOException.class, () -> fw.append(lines.get(1)));
+        assertEquals("Stream closed", throwable.getMessage());
+    }
+}
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