[fileWriter-flush-close] FileWriter flush() and close() (#15559)

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());
+    }
+}