Inspired by 30 seconds of code, this is a collection of reusable tested Java code snippets.
Update the sample application with the snippet and add a test for it. After proving that it works update this README.md.
- List directories
- List files in directory
- List files in directory recursively
- Read lines from file to string list
- Zip file
- Zip multiple files
public static void quickSort(int[] arr, int left, int right) {
int pivotIndex = left + (right - left) / 2;
int pivotValue = arr[pivotIndex];
int i = left, j = right;
while (i <= j) {
while (arr[i] < pivotValue) {
i++;
}
while (arr[j] > pivotValue) {
j--;
}
if (i <= j) {
int tmp = arr[i];
arr[i] = arr[j];
arr[j] = tmp;
i++;
j--;
}
if (left < i) {
quickSort(arr, left, j);
}
if (right > i) {
quickSort(arr, i, right);
}
}
} public static <T> T[] arrayConcat(T[] first, T[] second) {
T[] result = Arrays.copyOf(first, first.length + second.length);
System.arraycopy(second, 0, result, first.length, second.length);
return result;
} public static <T> T[] nArrayConcat(T[] first, T[]... rest) {
int totalLength = first.length;
for (T[] array : rest) {
totalLength += array.length;
}
T[] result = Arrays.copyOf(first, totalLength);
int offset = first.length;
for (T[] array : rest) {
System.arraycopy(array, 0, result, offset, array.length);
offset += array.length;
}
return result;
} public static File[] listDirectories(String path) {
return new File(path).listFiles(File::isDirectory);
} public static File[] listFilesInDirectory(final File folder) {
return folder.listFiles(File::isFile);
} public static List<File> listAllFiles(String path) {
List<File> all = new ArrayList<>();
File[] list = new File(path).listFiles();
if (list != null) { // In case of access error, list is null
for (File f : list) {
if (f.isDirectory()) {
all.addAll(listAllFiles(f.getAbsolutePath()));
} else {
all.add(f.getAbsoluteFile());
}
}
}
return all;
} public static List<String> readLines(String filename) throws IOException {
return Files.readAllLines(new File(filename).toPath());
} public static void zipFile(String srcFilename, String zipFilename) throws IOException {
File srcFile = new File(srcFilename);
try (
FileOutputStream fileOut = new FileOutputStream(zipFilename);
ZipOutputStream zipOut = new ZipOutputStream(fileOut);
FileInputStream fileIn = new FileInputStream(srcFile);
) {
ZipEntry zipEntry = new ZipEntry(srcFile.getName());
zipOut.putNextEntry(zipEntry);
final byte[] bytes = new byte[1024];
int length;
while ((length = fileIn.read(bytes)) >= 0) {
zipOut.write(bytes, 0, length);
}
}
} public static void zipFiles(String[] srcFilenames, String zipFilename) throws IOException {
try (
FileOutputStream fileOut = new FileOutputStream(zipFilename);
ZipOutputStream zipOut = new ZipOutputStream(fileOut);
) {
for (int i=0; i<srcFilenames.length; i++) {
File srcFile = new File(srcFilenames[i]);
try (FileInputStream fileIn = new FileInputStream(srcFile)) {
ZipEntry zipEntry = new ZipEntry(srcFile.getName());
zipOut.putNextEntry(zipEntry);
final byte[] bytes = new byte[1024];
int length;
while ((length = fileIn.read(bytes)) >= 0) {
zipOut.write(bytes, 0, length);
}
}
}
}
} public static int fibonacci(int n) {
if (n <= 1) return n;
else return fibonacci(n-1) + fibonacci(n-2);
} public static int factorial(int number) {
int result = 1;
for (int factor = 2; factor <= number; factor++) {
result *= factor;
}
return result;
} public static Integer[] performLottery(int numNumbers, int numbersToPick) {
List<Integer> numbers = new ArrayList<>();
for(int i = 0; i < numNumbers; i++) {
numbers.add(i+1);
}
Collections.shuffle(numbers);
return numbers.subList(0, numbersToPick).toArray(new Integer[numbersToPick]);
} public static void captureScreen(String filename) throws AWTException, IOException {
Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
Rectangle screenRectangle = new Rectangle(screenSize);
Robot robot = new Robot();
BufferedImage image = robot.createScreenCapture(screenRectangle);
ImageIO.write(image, "png", new File(filename));
} public static int httpGet(URL address) throws IOException {
HttpURLConnection con = (HttpURLConnection) address.openConnection();
return con.getResponseCode();
} public static boolean isPalindrome(String s) {
StringBuilder sb = new StringBuilder();
for (char c : s.toCharArray()) {
if (Character.isLetter(c)) {
sb.append(c);
}
}
String forward = sb.toString().toLowerCase();
String backward = sb.reverse().toString().toLowerCase();
return forward.equals(backward);
} public static String reverseString(String s) {
return new StringBuilder(s).reverse().toString();
} public static Date stringToDate(String date, String format) throws ParseException {
SimpleDateFormat simpleDateFormat = new SimpleDateFormat(format);
return simpleDateFormat.parse(date);
}