Inspired by 30 seconds of code, this is a collection of reusable tested Java 11 compatible code snippets that you can understand in 30 seconds or less.
Find an open issue that needs help at https://github.com/iluwatar/30-seconds-of-java/issues. Or if there's nothing that interests you, you can just raise a pull request. If you have an interesting snippet you would like to see in this library but can't implement yourself, please raise an issue.
For new snippets the general steps are
Library.java with the new code snippet LibraryTest.javaREADME.mdmaster branch public static void quickSort(int[] arr, int left, int right) {
var pivotIndex = left + (right - left) / 2;
var pivotValue = arr[pivotIndex];
var i = left;
var j = right;
while (i <= j) {
while (arr[i] < pivotValue) {
i++;
}
while (arr[j] > pivotValue) {
j--;
}
if (i <= j) {
var 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 void bubbleSort(int[] arr) {
var lastIndex = arr.length - 1;
for(var j = 0; j < lastIndex; j++) {
for(var i = 0; i < lastIndex - j; i++) {
if(arr[i] > arr[i + 1]) {
var tmp = arr[i];
arr[i] = arr[i + 1];
arr[i + 1] = tmp;
}
}
}
} public static <T> T[] arrayConcat(T[] first, T[] second) {
var 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) {
var totalLength = first.length;
for (var array : rest) {
totalLength += array.length;
}
var result = Arrays.copyOf(first, totalLength);
var offset = first.length;
for (var array : rest) {
System.arraycopy(array, 0, result, offset, array.length);
offset += array.length;
}
return result;
} public static <T> boolean allEqual(T[] arr) {
return Arrays.stream(arr).distinct().count() == 1;
} 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) {
var all = new ArrayList<File>();
var list = new File(path).listFiles();
if (list != null) { // In case of access error, list is null
for (var 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 {
var srcFile = new File(srcFilename);
try (
var fileOut = new FileOutputStream(zipFilename);
var zipOut = new ZipOutputStream(fileOut);
var fileIn = new FileInputStream(srcFile);
) {
var zipEntry = new ZipEntry(srcFile.getName());
zipOut.putNextEntry(zipEntry);
final var 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 (
var fileOut = new FileOutputStream(zipFilename);
var zipOut = new ZipOutputStream(fileOut);
) {
for (var i=0; i<srcFilenames.length; i++) {
var srcFile = new File(srcFilenames[i]);
try (var fileIn = new FileInputStream(srcFile)) {
var zipEntry = new ZipEntry(srcFile.getName());
zipOut.putNextEntry(zipEntry);
final var 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) {
var result = 1;
for (var factor = 2; factor <= number; factor++) {
result *= factor;
}
return result;
} public static Integer[] performLottery(int numNumbers, int numbersToPick) {
var numbers = new ArrayList<Integer>();
for(var i = 0; i < numNumbers; i++) {
numbers.add(i+1);
}
Collections.shuffle(numbers);
return numbers.subList(0, numbersToPick).toArray(new Integer[numbersToPick]);
} public static int gcd(int a, int b) {
if (b == 0)
return a;
return gcd(b, a % b);
} public static boolean isPrime(int number) {
if (number < 3) {
return true;
}
// check if n is a multiple of 2
if (number % 2 == 0) {
return false;
}
// if not, then just check the odds
for (var i = 3; i * i <= number; i += 2) {
if (number % i == 0) {
return false;
}
}
return true;
} public static void captureScreen(String filename) throws AWTException, IOException {
var screenSize = Toolkit.getDefaultToolkit().getScreenSize();
var screenRectangle = new Rectangle(screenSize);
var robot = new Robot();
var image = robot.createScreenCapture(screenRectangle);
ImageIO.write(image, "png", new File(filename));
} public static HttpResponse<String> httpGet(String uri) throws Exception {
var client = HttpClient.newHttpClient();
var request = HttpRequest.newBuilder()
.uri(URI.create(uri))
.build();
return client.send(request, BodyHandlers.ofString());
} public static HttpResponse<String> httpPost(String address, HashMap<String,String> arguments) throws IOException, InterruptedException{
var sj = new StringJoiner("&");
for(var entry : arguments.entrySet()) {
sj.add(URLEncoder.encode(entry.getKey(), "UTF-8") + "="
+ URLEncoder.encode(entry.getValue(), "UTF-8"));
}
var out = sj.toString().getBytes(StandardCharsets.UTF_8);
var request = HttpRequest.newBuilder()
.uri(URI.create(address))
.headers("Content-Type", "application/x-www-form-urlencoded; charset=UTF-8")
.POST(BodyPublishers.ofByteArray(out))
.build();
return HttpClient.newHttpClient().send(request, BodyHandlers.ofString());
} public static boolean isPalindrome(String s) {
var sb = new StringBuilder();
for (var c : s.toCharArray()) {
if (Character.isLetter(c)) {
sb.append(c);
}
}
var forward = sb.toString().toLowerCase();
var 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 {
var simpleDateFormat = new SimpleDateFormat(format);
return simpleDateFormat.parse(date);
}public boolean isAnagram(String s1, String s2) {
var l1 = s1.length();
var l2 = s2.length();
var arr1 = new int[256];
var arr2 = new int[256];
if (l1 != l2) {
return false;
}
for (var i = 0; i < l1; i++) {
arr1[s1.charAt(i)]++;
arr2[s2.charAt(i)]++;
}
return Arrays.equals(arr1, arr2);
} public static List<String> getAllMethods(final Class<?> cls) {
var list = new ArrayList<String>();
for (var method : cls.getDeclaredMethods()) {
list.add(method.getName());
}
return list;
} public static List<String> getAllFieldNames(final Class<?> cls) {
return Arrays.stream(cls.getFields())
.map(Field::getName)
.collect(Collectors.toList());
}