eugenp · eric-martin · Oct 15, 2025 · Oct 12, 2025 · Oct 12, 2025
diff --git a/...java-io-8/src/test/java/com/baeldung/modifyFileByPattern/ModifyFileByPatternUnitTest.java b/...java-io-8/src/test/java/com/baeldung/modifyFileByPattern/ModifyFileByPatternUnitTest.java
@@ -0,0 +1,108 @@
+package com.baeldung.replacewordinfile;
+
+import static org.junit.jupiter.api.Assertions.assertLinesMatch;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.io.BufferedReader;
+import java.io.BufferedWriter;
+import java.io.File;
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.StandardCopyOption;
+import java.util.List;
+import java.util.regex.Pattern;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
+
+public class ModifyFileByPatternUnitTest {
+
+    @TempDir
+    private File fileDir;
+
+    private File myFile;
+
+    private static final List<String> ORIGINAL_LINES = List.of(
+        //@formatter:off
+      "Both JAVA and KOTLIN applications can run on the JVM.",
+      "But python is a simpler language",
+      "PYTHON application is also platform independent.",
+      "java and kotlin are statically typed languages.",
+      "On the other hand, python is a dynamically typed language.");
+    //@formatter:on
+
+    private static final List<String> EXPECTED_LINES = List.of(
+        //@formatter:off
+      "Both Java and Kotlin applications can run on the JVM (Java Virtual Machine).",
+      "Python application is also platform independent.",
+      "Java and Kotlin are statically typed languages.",
+      "On the other hand, Python is a dynamically typed language.");
+    //@formatter:on
+
+    @BeforeEach
+    void initFile() throws IOException {
+        myFile = new File(fileDir, "myFile.txt");
+        Files.write(myFile.toPath(), ORIGINAL_LINES);
+    }
+
+    @Test
+    void whenLoadTheFileContentToMemModifyThenWriteBack_thenCorrect() throws IOException {
+        assertTrue(Files.exists(myFile.toPath()));
+        List<String> lines = Files.readAllLines(myFile.toPath());
+        lines.remove(1); // remove the 2nd line
+        List<String> newLines = lines.stream()
+            .map(line -> line.replaceAll("(?i)java", "Java")
+                .replaceAll("(?i)kotlin", "Kotlin")
+                .replaceAll("(?i)python", "Python")
+                .replaceAll("JVM", "$0 (Java Virtual Machine)"))
+            .toList();
+
+        Files.write(myFile.toPath(), newLines);
+        assertLinesMatch(EXPECTED_LINES, Files.readAllLines(myFile.toPath()));
+    }
+
+    @Test
+    void whenUsingBufferedReaderAndModifyViaTempFile_thenCorrect(@TempDir Path tempDir) throws IOException {
+
+        Pattern javaPat = Pattern.compile("(?i)java");
+        Pattern kotlinPat = Pattern.compile("(?i)kotlin");
+        Pattern pythonPat = Pattern.compile("(?i)python");
+        Pattern jvmPat = Pattern.compile("JVM");
+
+        Path modifiedFile = tempDir.resolve("modified.txt");
+        //@formatter:off
+        try ( BufferedReader reader = Files.newBufferedReader(myFile.toPath());
+              BufferedWriter writer = Files.newBufferedWriter(modifiedFile)) {
+            //@formatter:on
+
+            int lineNumber = 0;
+            String line;
+            while ((line = reader.readLine()) != null) {
+                lineNumber++;
+                if (lineNumber == 2) {
+                    continue; // skip the 2nd line
+                }
+
+                String replaced = line;
+                replaced = javaPat.matcher(replaced)
+                    .replaceAll("Java");
+                replaced = kotlinPat.matcher(replaced)
+                    .replaceAll("Kotlin");
+                replaced = pythonPat.matcher(replaced)
+                    .replaceAll("Python");
+                replaced = jvmPat.matcher(replaced)
+                    .replaceAll("JVM (Java Virtual Machine)");
+
+                writer.write(replaced);
+                writer.newLine();
+            }
+        }
+
+        Files.move(modifiedFile, myFile.toPath(), StandardCopyOption.REPLACE_EXISTING);
+        assertTrue(myFile.exists());
+        assertLinesMatch(EXPECTED_LINES, Files.readAllLines(myFile.toPath()));
+
+    }
+}