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core-java-modules/core-java-io-2/src/main/java/com/baeldung/filereader/FileReaderExample.java

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+package com.baeldung.filereader;
+
+import java.io.*;
+
+public class FileReaderExample {
+
+    public static String readAllCharactersOneByOne(Reader reader) throws IOException {
+        StringBuilder content = new StringBuilder();
+        int nextChar;
+        while ((nextChar = reader.read()) != -1) {
+            content.append((char) nextChar);
+        }
+        return String.valueOf(content);
+    }
+
+    public static String readMultipleCharacters(Reader reader, int length) throws IOException {
+        char[] buffer = new char[length];
+        int charactersRead = reader.read(buffer, 0, length);
+
+
+        if (charactersRead != -1) {
+            return new String(buffer, 0, charactersRead);
+        } else {
+            return "";
+        }
+    }
+
+    public static String readFile(String path) {
+        FileReader fileReader = null;
+        try {
+            fileReader = new FileReader(path);
+            return readAllCharactersOneByOne(fileReader);
+        } catch (IOException e) {
+            e.printStackTrace();
+        } finally {
+            if (fileReader != null) {
+                try {
+                    fileReader.close();
+                } catch (IOException e) {
+                    e.printStackTrace();
+                }
+            }
+        }
+        return null;
+    }
+
+    public static String readFileUsingTryWithResources(String path) {
+        try (FileReader fileReader = new FileReader(path)) {
+            return readAllCharactersOneByOne(fileReader);
+        } catch (IOException e) {
+            e.printStackTrace();
+        }
+        return null;
+    }
+
+
+}

core-java-modules/core-java-io-2/src/test/java/com/baeldung/createfiles/CreateFilesUnitTest.java

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+package com.baeldung.createfiles;
+
+import com.google.common.io.Files;
+import org.junit.Test;
+
+import java.io.File;
+import java.io.IOException;
+
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+
+public class CreateFilesUnitTest {
+    @Test
+    public void givenAnExistingDirectory_whenCreatingAFileWithAbsolutePath_thenFileIsCreated() throws IOException {
+        File tempDirectory = new File(System.getProperty("java.io.tmpdir"));
+        File fileWithAbsolutePath = new File(tempDirectory.getAbsolutePath() + "/myDirectory/testFile.txt");
+
+        assertFalse(fileWithAbsolutePath.exists());
+
+        Files.touch(fileWithAbsolutePath);
+
+        assertTrue(fileWithAbsolutePath.exists());
+    }
+
+    @Test
+    public void givenAnExistingDirectory_whenCreatingANewDirectoryAndFileWithRelativePath_thenFileIsCreated() throws IOException {
+        File tempDirectory = new File(System.getProperty("java.io.tmpdir"));
+        File fileWithRelativePath = new File(tempDirectory, "myDirectory/newFile.txt");
+
+        assertFalse(fileWithRelativePath.exists());
+
+        Files.touch(fileWithRelativePath);
+
+        assertTrue(fileWithRelativePath.exists());
+    }
+
+    @Test
+    public void whenCreatingAFileWithFileSeparator_thenFileIsCreated() throws IOException {
+        File tempDirectory = new File(System.getProperty("java.io.tmpdir"));
+        File newFile = new File(tempDirectory.getAbsolutePath() + File.separator + "newFile.txt");
+
+        assertFalse(newFile.exists());
+
+        Files.touch(newFile);
+
+        assertTrue(newFile.exists());
+    }
+}

core-java-modules/core-java-io-2/src/test/java/com/baeldung/file/FileClassDemoUnitTest.java

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+package com.baeldung.file;
+
+import org.junit.Test;
+
+import java.io.*;
+import java.util.Arrays;
+import java.util.List;
+
+import static org.junit.Assert.*;
+
+public class FileClassDemoUnitTest {
+
+    @Test
+    public void givenDirectoryCreated_whenMkdirIsInvoked_thenDirectoryIsDeleted() {
+        File directory = new File("testDirectory");
+        if (!directory.isDirectory() || !directory.exists()) {
+            directory.mkdir();
+        }
+
+        assertTrue(directory.delete());
+    }
+
+    @Test
+    public void givenFileCreated_whenCreateNewFileIsInvoked_thenFileIsDeleted() throws IOException {
+        File file = new File("testFile.txt");
+        if (!file.isFile() || !file.exists()) {
+            file.createNewFile();
+        }
+
+        assertTrue(file.delete());
+    }
+
+
+    @Test
+    public void givenFileCreated_whenCreateNewFileInvoked_thenMetadataIsAsExpected() throws IOException {
+
+        // different Operating systems have different separator characters
+        String separatorCharacter = System.getProperty("file.separator");
+
+        File parentDirectory = makeDirectory("filesDirectory");
+
+        File childFile = new File(parentDirectory, "file1.txt");
+        childFile.createNewFile();
+
+        assertTrue(childFile.getName().equals("file1.txt"));
+        assertTrue(childFile.getParentFile().getName().equals(parentDirectory.getName()));
+        assertTrue(childFile.getPath().equals(parentDirectory.getPath() + separatorCharacter + "file1.txt"));
+
+        removeDirectory(parentDirectory);
+    }
+
+
+    @Test(expected = FileNotFoundException.class)
+    public void givenReadOnlyFileCreated_whenCreateNewFileInvoked_thenFileCannotBeWrittenTo() throws IOException {
+        File parentDirectory = makeDirectory("filesDirectory");
+
+        File childFile = new File(parentDirectory, "file1.txt");
+        childFile.createNewFile();
+
+        childFile.setWritable(false);
+
+        FileOutputStream fos = new FileOutputStream(childFile);
+        fos.write("Hello World".getBytes()); // write operation
+        fos.flush();
+        fos.close();
+
+        removeDirectory(parentDirectory);
+    }
+
+    @Test(expected = FileNotFoundException.class)
+    public void givenWriteOnlyFileCreated_whenCreateNewFileInvoked_thenFileCannotBeReadFrom() throws IOException {
+        File parentDirectory = makeDirectory("filesDirectory");
+
+        File childFile = new File(parentDirectory, "file1.txt");
+        childFile.createNewFile();
+
+        childFile.setReadable(false);
+
+        FileInputStream fis = new FileInputStream(childFile);
+        fis.read(); // read operation
+        fis.close();
+
+        removeDirectory(parentDirectory);
+    }
+
+    @Test
+    public void givenFilesCreatedInDirectory_whenCreateNewFileInvoked_thenTheyCanBeListedAsExpected() throws IOException {
+        File directory = makeDirectory("filtersDirectory");
+
+        File csvFile = new File(directory, "csvFile.csv");
+        csvFile.createNewFile();
+
+        File txtFile = new File(directory, "txtFile.txt");
+        txtFile.createNewFile();
+
+        //normal listing
+        assertEquals(2, directory.list().length);
+
+        //filtered listing
+        FilenameFilter csvFilter = (dir, ext) -> ext.endsWith(".csv");
+        assertEquals(1, directory.list(csvFilter).length);
+
+        removeDirectory(directory);
+    }
+
+    @Test
+    public void givenDirectoryIsCreated_whenMkdirInvoked_thenDirectoryCanBeRenamed() {
+
+        File source = makeDirectory("source");
+        File destination = makeDirectory("destination");
+        source.renameTo(destination);
+
+        assertFalse(source.isDirectory());
+        assertTrue(destination.isDirectory());
+
+        removeDirectory(destination);
+    }
+
+    @Test
+    public void givenDataIsWrittenToFile_whenWriteIsInvoked_thenFreeSpaceOnSystemDecreases() throws IOException {
+
+        String name = System.getProperty("user.home") + System.getProperty("file.separator") + "test";
+        File testDir = makeDirectory(name);
+        File sample = new File(testDir, "sample.txt");
+
+        long freeSpaceBeforeWrite = testDir.getFreeSpace();
+        writeSampleDataToFile(sample);
+
+        long freeSpaceAfterWrite = testDir.getFreeSpace();
+        assertTrue(freeSpaceAfterWrite < freeSpaceBeforeWrite);
+
+        removeDirectory(testDir);
+    }
+
+    private static File makeDirectory(String directoryName) {
+        File directory = new File(directoryName);
+        directory.mkdir();
+        if (directory.isDirectory()) {
+            return directory;
+        }
+        throw new RuntimeException("Directory not created for " + directoryName);
+    }
+
+    private static void removeDirectory(File directory) {
+        // make sure you don't delete your home directory here
+        if (directory.getPath().equals(System.getProperty("user.home"))) {
+            return;
+        }
+
+        // remove directory and its files from system
+        if (directory != null && directory.exists()) {
+            // delete all files inside the directory
+            File[] filesInDirectory = directory.listFiles();
+            if (filesInDirectory != null) {
+                List<File> files = Arrays.asList(filesInDirectory);
+                files.forEach(f -> deleteFile(f));
+            }
+
+            // finally delete the directory itself
+            deleteFile(directory);
+        }
+    }
+
+    private static void deleteFile(File fileToDelete) {
+        if (fileToDelete != null && fileToDelete.exists()) {
+            fileToDelete.delete();
+        }
+    }
+
+    private static void writeSampleDataToFile(File sample) throws IOException {
+        //write sample text to file
+        try (FileOutputStream out = new FileOutputStream(sample)) {
+            for (int i = 1; i <= 100000; i++) {
+                String sampleText = "Sample line number " + i + "\n";
+                out.write(sampleText.getBytes());
+            }
+        }
+    }
+}

core-java-modules/core-java-io-2/src/test/java/com/baeldung/filereader/FileReaderExampleUnitTest.java

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+package com.baeldung.filereader;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+import java.io.File;
+import java.io.FileReader;
+import java.io.IOException;
+
+public class FileReaderExampleUnitTest {
+
+
+    private static final String FILE_PATH = "src/test/resources/HelloWorld.txt";
+
+
+    @Test
+    public void givenFileReader_whenReadAllCharacters_thenReturnsContent() throws IOException {
+        String expectedText = "Hello, World!";
+        File file = new File(FILE_PATH);
+        try (FileReader fileReader = new FileReader(file)) {
+            String content = FileReaderExample.readAllCharactersOneByOne(fileReader);
+            Assert.assertEquals(expectedText, content);
+        }
+    }
+
+    @Test
+    public void givenFileReader_whenReadMultipleCharacters_thenReturnsContent() throws IOException {
+        String expectedText = "Hello";
+        File file = new File(FILE_PATH);
+        try (FileReader fileReader = new FileReader(file)) {
+            String content = FileReaderExample.readMultipleCharacters(fileReader, 5);
+            Assert.assertEquals(expectedText, content);
+        }
+    }
+
+    @Test
+    public void whenReadFile_thenReturnsContent() {
+        String expectedText = "Hello, World!";
+        String content = FileReaderExample.readFile(FILE_PATH);
+        Assert.assertEquals(expectedText, content);
+    }
+
+    @Test
+    public void whenReadFileUsingTryWithResources_thenReturnsContent() {
+        String expectedText = "Hello, World!";
+        String content = FileReaderExample.readFileUsingTryWithResources(FILE_PATH);
+        Assert.assertEquals(expectedText, content);
+    }
+
+}

core-java-modules/core-java-io-2/src/test/java/com/baeldung/filewriter/FileWriterExampleUnitTest.java

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+package com.baeldung.filewriter;
+
+import java.io.FileWriter;
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+
+import org.junit.After;
+import org.junit.Assert;
+import org.junit.Test;
+
+public class FileWriterExampleUnitTest {
+
+    @After
+    public void tearDown() throws IOException {
+        Files.delete(Path.of("src/test/resources/FileWriterTest.txt"));
+    }
+
+    @Test
+    public void testWriteString() throws IOException {
+        try (FileWriter fileWriter = new FileWriter("src/test/resources/FileWriterTest.txt")) {
+            fileWriter.write("Hello Folks!");
+        }
+        Assert.assertEquals("Hello Folks!", new String(Files.readAllBytes(Path.of("src/test/resources/FileWriterTest.txt"))));
+    }
+
+    @Test
+    public void testAppendString() throws IOException {
+        try (FileWriter fileWriter = new FileWriter("src/test/resources/FileWriterTest.txt")) {
+            fileWriter.write("Hello Folks!");
+        }
+        // using another try with resources to reopen the file in append mode
+        try (FileWriter fileWriter = new FileWriter("src/test/resources/FileWriterTest.txt", true)) {
+            fileWriter.write("Hello Folks Again!");
+        }
+
+        Assert.assertEquals("Hello Folks!" + "Hello Folks Again!", new String(Files.readAllBytes(Path.of("src/test/resources/FileWriterTest.txt"))));
+    }
+
+    @Test
+    public void testWriteCharArray() throws IOException {
+        try (FileWriter fileWriter = new FileWriter("src/test/resources/FileWriterTest.txt")) {
+            fileWriter.write("Hello Folks!".toCharArray());
+        }
+        Assert.assertEquals("Hello Folks!", new String(Files.readAllBytes(Path.of("src/test/resources/FileWriterTest.txt"))));
+    }
+}

core-java-modules/core-java-io-2/src/test/resources/HelloWorld.txt

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+Hello, World!