[output-to-file] Write Console Output to Text File in Java (#14082)

* [output-to-file] Write Console Output to Text File in Java

* [output-to-file] rebase on the master

core-java-modules/core-java-io-apis-2/pom.xml

@@ -99,9 +99,6 @@
             <scope>compile</scope>
         </dependency>
     </dependencies>
-    <properties>
-        <junit-jupiter-version>5.2.0</junit-jupiter-version>
-    </properties>
     <build>
         <finalName>core-java-io-apis-2</finalName>
         <resources>
@@ -111,5 +108,7 @@
             </resource>
         </resources>
     </build>
-
+    <properties>
+        <junit-jupiter-version>5.9.3</junit-jupiter-version>
+    </properties>
 </project>

core-java-modules/core-java-io-apis-2/src/test/java/com/baeldung/outputtofile/ConsoleOutputToFileUnitTest.java

@@ -0,0 +1,94 @@
+package com.baeldung.outputtofile;
+
+import static org.junit.jupiter.api.Assertions.assertLinesMatch;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.io.IOException;
+import java.io.OutputStream;
+import java.io.PrintStream;
+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;
+
+import com.google.common.collect.Lists;
+
+class DualPrintStream extends PrintStream {
+    private final PrintStream second;
+
+    public DualPrintStream(OutputStream main, PrintStream second) {
+        super(main);
+        this.second = second;
+    }
+
+    @Override
+    public void close() {
+        super.close();
+        second.close();
+    }
+
+    @Override
+    public void flush() {
+        super.flush();
+        second.flush();
+    }
+
+    @Override
+    public void write(byte[] buf, int off, int len) {
+        super.write(buf, off, len);
+        second.write(buf, off, len);
+    }
+
+    @Override
+    public void write(int b) {
+        super.write(b);
+        second.write(b);
+    }
+
+    @Override
+    public void write(byte[] b) throws IOException {
+        super.write(b);
+        second.write(b);
+    }
+}
+
+public class ConsoleOutputToFileUnitTest {
+
+    // @formatter:off
+    private final static List<String> OUTPUT_LINES = Lists.newArrayList(
+      "I came",
+      "I saw",
+      "I conquered");
+    // @formatter:on
+
+    @Test
+    void whenReplacingSystemOutPrintStreamWithFileOutputStream_thenOutputsGoToFile(@TempDir Path tempDir) throws IOException {
+        PrintStream originalOut = System.out;
+        Path outputFilePath = tempDir.resolve("file-output.txt");
+        PrintStream out = new PrintStream(Files.newOutputStream(outputFilePath), true);
+        System.setOut(out);
+
+        OUTPUT_LINES.forEach(line -> System.out.println(line));
+        assertTrue(outputFilePath.toFile()
+          .exists(), "The file exists");
+        assertLinesMatch(OUTPUT_LINES, Files.readAllLines(outputFilePath));
+        System.setOut(originalOut);
+    }
+
+    @Test
+    void whenUsingDualPrintStream_thenOutputsGoToConsoleAndFile(@TempDir Path tempDir) throws IOException {
+        PrintStream originalOut = System.out;
+        Path outputFilePath = tempDir.resolve("dual-output.txt");
+        DualPrintStream dualOut = new DualPrintStream(Files.newOutputStream(outputFilePath), System.out);
+        System.setOut(dualOut);
+
+        OUTPUT_LINES.forEach(line -> System.out.println(line));
+        assertTrue(outputFilePath.toFile()
+          .exists(), "The file exists");
+        assertLinesMatch(OUTPUT_LINES, Files.readAllLines(outputFilePath));
+        System.setOut(originalOut);
+
+    }
+}