Java tutorial

File operations

read/write string

// Java 11
Files.writeString(fileName, content);
String content = Files.readString(fileName);

// Java 7
String content = new String (Files.readAllBytes(Paths.get(fileName)));
Files.write(Paths.get(filePath), content.getBytes());

// Java 6 
BufferedWriter bw = new BufferedWriter(new FileWriter(fileName));
bw.write(str);
bw.close();

StringBuilder contentBuilder = new StringBuilder();
BufferedReader br = new BufferedReader(new FileReader(fileName))
String line;
while ((line = br.readLine()) != null){
    contentBuilder.append(line).append("\n");
}
String content = contentBuilder.toString();

// Java 6 with IO streams
FileOutputStream fos = new FileOutputStream(fileName);
fos.write(content.getBytes());
fos.close();

// on Java 7 you can use Writer/Reader with try-with-resource
try(BufferedWriter writer = new BufferedWriter(new FileWriter(fileName))){
   writer.write(str);
}

deep directory traverse

try (Stream<Path> paths = Files.walk(Paths.get("mydir"))) {
    paths.forEach((path)->{
      // ... do something
    });
} catch (IOException e) {
    e.printStackTrace();
}

delete

Following method allows delete file or folder. Folder can be non-empty.

public static void deleteFile(File f) throws IOException {
    try(Stream<Path> stream = Files.walk(f.toPath())) {
        stream.sorted(Comparator.reverseOrder())
            .map(Path::toFile)
            .forEach(File::delete);
        }
}

You can replace File.delete() with Files.delete(), which may throw more exceptions.

clean folder

Following method allows to clean folder, without deleting itself.

public static void cleanDir(File dir) throws IOException {
    if (dir.isDirectory()) {
        try (Stream<Path> stream = Files.walk(dir.toPath())) {
            stream.sorted(Comparator.reverseOrder())
                .map(Path::toFile)
                .forEach(f -> {
                    if (!dir.equals(f)) f.delete();
                });
        }
    }
}

copy files

If you don't need callbacks:

public static void copyFiles(File src, File dst) 
        throws IOException {
    dst.getParentFile().mkdirs();
    Files.copy(src.toPath(), dst.toPath(), StandardCopyOption.REPLACE_EXISTING);
}

Following method allows copy content of one file to another. Code without util classes like Files.

public static boolean copyFiles(File src, File dst,
                                    ProgressListener progressListener,
                                    ErrorHandler errorHandler) {

    dst.getParentFile().mkdirs();
    long total = src.length();
    long current = 0L;

    try (InputStream in = new BufferedInputStream(new FileInputStream(src));
             OutputStream out = new BufferedOutputStream(new FileOutputStream(dst))) {
        byte[] buf = new byte[1024];
        int length;
        while ((length = in.read(buf)) > 0) {
            out.write(buf, 0, length);
            current += length;
            
            if (progressListener != null) {
               progressListener.onProgress(current, total);
            }
        }
    } catch (Exception e) {
        e.printStackTrace();
        if (errorHandler != null) {
            errorHandler.onError(e);
        }
        return false;
    }

    return true;
}

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