30 Seconds of Java
약 7 분
30 Seconds of Java
30 Seconds of Java는 30초 안에 이해 가능한 Java 11 코드를 모아놓은 프로젝트 입니다. 이 프로젝트는 30 seconds of code에서 영감을 받아 만들어졌습니다. 코드 기여를 하실 때는 instructions 문서를 참고하여 주시길 바랍니다.
알고리즘
퀵 정렬(Quicksort)
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); } } }버블 정렬(Bubblesort)
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; } } } }선택 정렬(Selectionsort)
public static void selectionSort(int[] arr) { var len = arr.length; for (var i = 0; i < len - 1; i++) { var minIndex = i; for (var j = i + 1; j < len; j++) { if(arr[j] < arr[minIndex]) minIndex = j; } var tmp = arr[minIndex]; arr[minIndex] = arr[i]; arr[i] = tmp; } }삽입 정렬(InsertionSort)
public static void insertionSort(int[] arr) { for (var i = 1; i < arr.length; i++) { var tmp = arr[i]; var j = i - 1; while (j >= 0 && arr[j] > tmp) { arr[j + 1] = arr[j]; j--; } arr[j + 1] = tmp; } }계수 정렬(CountingSort)
public static void countingSort(int[] arr) { var max = Arrays.stream(arr).max().getAsInt(); var count = new int[max + 1]; for (var num : arr) { count[num]++; } for (var i = 1; i <= max; i++) { count[i] += count[i - 1]; } var sorted = new int[arr.length]; for (var i = arr.length - 1; i >= 0; i--) { var cur = arr[i]; sorted[count[cur] - 1] = cur; count[cur]--; } var index = 0; for (var num : sorted) { arr[index++] = num; } }배열
두 개의 배열 연결
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; }N 개의 배열 연결
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; }어레이의 모든 요소가 동일한지 확인
public static <T> boolean allEqual(T[] arr) { return Arrays.stream(arr).distinct().count() == 1; }배열의 평균 값
public static double arrayMean(int[] arr) { return (double) Arrays.stream(arr).sum() / arr.length; }배열의 중앙값
public static double arrayMedian(int[] arr) { Arrays.sort(arr); var mid = arr.length / 2; return arr.length % 2 != 0 ? (double) arr[mid] : (double) (arr[mid] + arr[mid - 1]) / 2; }Mode in Array
public static int modeArray(int arr[]) { int mode=0,maxcount=0; for(int i=0;i<arr.length;i++) { int count=0; for(int j=0;j<arr.length;j++) { if(arr[i]==arr[j]) count++; } if(count>maxcount) { maxcount=count; mode=arr[i]; } } return mode; }배열의 합
public static int arraySum(int[] arr) { return Arrays.stream(arr).sum(); }배열의 최댓값
public static int findMax(int[] arr) { return Arrays.stream(arr).reduce(Integer.MIN_VALUE, Integer::max); }인코딩
Base64 인코딩
public static String encodeBase64(String input) { return Base64.getEncoder().encodeToString(input.getBytes()); }Base64 디코딩
public static String decodeBase64(String input) { return new String(Base64.getDecoder().decode(input.getBytes())); }파일
디렉토리 목록 가져오기
public static File[] listDirectories(String path) { return new File(path).listFiles(File::isDirectory); }디렉토리의 파일 리스트 가져오기
public static File[] listFilesInDirectory(final File folder) { return folder.listFiles(File::isFile); }재귀적으로 디렉토리의 파일 리스트 가져오기
public static List<File> listAllFiles(String path) { var all = new ArrayList<File>(); var list = new File(path).listFiles(); if (list != null) { // list가 null일 시 오류 발생 for (var f : list) { if (f.isDirectory()) { all.addAll(listAllFiles(f.getAbsolutePath())); } else { all.add(f.getAbsoluteFile()); } } } return all; }파일을 읽고 String 리스트에 저장
public static List<String> readLines(String filename) throws IOException { return Files.readAllLines(new File(filename).toPath()); }파일 압축하기
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); } } }파일 여러개 압축하기
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); } } } } }디렉토리 압축하기
public static void zipDirectory (String srcDirectoryName, String zipFileName) throws IOException { var srcDirectory = new File(srcDirectoryName); try ( var fileOut = new FileOutputStream(zipFileName); var zipOut = new ZipOutputStream(fileOut) ) { zipFile(srcDirectory, srcDirectory.getName(), zipOut); } } public static void zipFile(File fileToZip, String fileName, ZipOutputStream zipOut) throws IOException { if (fileToZip.isHidden()) { // 숨겨진 파일들은 무시하도록 설정되어있음 return; } if (fileToZip.isDirectory()) { if (fileName.endsWith("/")) { zipOut.putNextEntry(new ZipEntry(fileName)); // 나중에 압축되어야함 zipOut.closeEntry(); } else { // 압축 해제 작업이 수행되는 동안 구조를 유지하기 위해 명시적으로 역슬래시 "/" 부호 추가 zipOut.putNextEntry(new ZipEntry(fileName + "/")); zipOut.closeEntry(); } var children = fileToZip.listFiles(); for (var childFile : children) { // Recursively apply function to all children zipFile(childFile, fileName + "/" + childFile.getName(), zipOut); } return; } try ( var fis = new FileInputStream(fileToZip) // 파일일 시 압축 시작 ) { var zipEntry = new ZipEntry(fileName); zipOut.putNextEntry(zipEntry); var bytes = new byte[1024]; var length = 0; while ((length = fis.read(bytes)) >= 0) { zipOut.write(bytes, 0, length); } } }수학
재귀함수를 활용한 피보나치
Even or Odd
public static String evenodd(int num) { if(num%2==0) return "even"; else return "odd"; } public static int fibonacci(int n) { if (n <= 1) { return n; } else { return fibonacci(n - 1) + fibonacci(n - 2); } }피보나치 반복문으로 처리
public static int iterativeFibonacci(int number) { List<Integer> list = new ArrayList<>(); list.add(0); list.add(1); for (int i = 2; i < number + 1; i++) { list.add(list.get(i - 2) + list.get(i - 1)); } return list.get(number); }팩토리얼
public static int factorial(int number) { var result = 1; for (var factor = 2; factor <= number; factor++) { result *= factor; } return result; }재귀 함수를 활용한 팩토리얼
public static int recursiveFactorial(int number) { var initial = 0; if (number == initial) { return initial + 1; } return number * recursiveFactorial(number - 1); }하버사인 공식
// 지구 반지름 KM private static final double SPHERE_RADIUS_IN_KM = 6372.8; public static double findHaversineDistance(double latA, double longA, double latB, double longB) { if (!isValidLatitude(latA) || !isValidLatitude(latB) || !isValidLongitude(longA) || !isValidLongitude(longB)) { throw new IllegalArgumentException(); } // 위도와 경도 차이 계산 var latitudeDiff = Math.toRadians(latB - latA); var longitudeDiff = Math.toRadians(longB - longA); var latitudeA = Math.toRadians(latA); var latitudeB = Math.toRadians(latB); // 하버 사인 공식을 통해 두 개의 좌표 사이의 거리 구하기 var a = Math.pow(Math.sin(latitudeDiff / 2), 2) + Math.pow(Math.sin(longitudeDiff / 2), 2) * Math.cos(latitudeA) * Math.cos(latitudeB); var c = 2 * Math.asin(Math.sqrt(a)); return SPHERE_RADIUS_IN_KM * c; } // 올바른 위도값인지 확인 private static boolean isValidLatitude(double latitude) { return latitude >= -90 && latitude <= 90; } // 올바른 경도값인지 확인 private static boolean isValidLongitude(double longitude) { return longitude >= -180 && longitude <= 180; }로또
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]); }룬(Luhn) 알고리즘
public static int calculateLuhnChecksum(long num) { if (num < 0) { throw new IllegalArgumentException("Non-negative numbers only."); } final var numStr = String.valueOf(num); var sum = 0; var isOddPosition = true; // numStr의 숫자를 오른쪽에서 왼쪽(끝에서 처음) 방향으로 반복 for (var i = numStr.length() - 1; i >= 0; i--) { final var digit = Integer.parseInt(Character.toString(numStr.charAt(i))); final var substituteDigit = (isOddPosition ? 2 : 1) * digit; final var tensPlaceDigit = substituteDigit / 10; final var onesPlaceDigit = substituteDigit % 10; sum += tensPlaceDigit + onesPlaceDigit; isOddPosition = !isOddPosition; } final var checksumDigit = (10 - (sum % 10)) % 10; // Outermost modulus handles edge case `num = 0`. return checksumDigit; }SquareRoot of a Number
static double sqrt(int num,int p) //p-precision(till how many decimal numbers we want) { int start=0,end=num; double root=0.0; while(start<=end) { int mid=start+(end-start)/2; if((mid*mid)>num) end=mid-1; else if((mid*mid)<num) start=mid+1; else return mid; } //root=end; double incr=0.1; for(int i=0;i<p;i++) { while(root*root < num) root=root+incr; root=root-incr; incr=incr/10; } return root; }최대공약수(GCD)
public static int gcd(int a, int b) { if (b == 0) return a; return gcd(b, a % b); }소수
public static boolean isPrime(int number) { if (number < 3) { return true; } // n이 짝수인지 확인 if (number % 2 == 0) { return false; } // 짝수가 아니라면 홀수인지 확인 for (var i = 3; i * i <= number; i += 2) { if (number % i == 0) { return false; } } return true; }이진법
public static String toBinary(long naturalNumber) { if (naturalNumber < 0) { throw new NumberFormatException("Negative Integer, this snippet only accepts " + "positive integers"); } if (naturalNumber == 0) { return "0"; } final Stack<Long> binaryBits = Stream.iterate(naturalNumber, n -> n > 0, n -> n / 2).map(n -> n % 2) .collect(Stack::new, Stack::push, Stack::addAll); return Stream.generate(binaryBits::pop) .limit(binaryBits.size()).map(String::valueOf).collect(Collectors.joining()); } public static Long fromBinary(String binary) { binary.chars().filter(c -> c != '0' && c != '1').findFirst().ifPresent(in -> { throw new NumberFormatException( "Binary string contains values other than '0' and '1'"); }); return IntStream.range(0, binary.length()) .filter(in -> binary.charAt(binary.length() - 1 - in) == '1') .mapToLong(in -> ((long) 0b1) << in).sum(); }미디어
화면 캡쳐
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)); }Networking
HTTP GET
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()); }HTTP POST
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()); }문자열
팰린드롬 알고리즘
public static boolean isPalindrome(String s) { for (int i = 0, j = s.length() - 1; i < j; i++, j--) { while (i < j && !Character.isLetter(s.charAt(i))) { i++; } while (i < j && !Character.isLetter(s.charAt(j))) { j--; } if (Character.toLowerCase(s.charAt(i)) != Character.toLowerCase(s.charAt(j))) { return false; } } return true; }문자열 뒤집기
public static String reverseString(String s) { return new StringBuilder(s).reverse().toString(); }문자열을 date(날짜) 타입으로 변환
public static Date stringToDate(String date, String format) throws ParseException { var simpleDateFormat = new SimpleDateFormat(format); return simpleDateFormat.parse(date); }아나그램 체크
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); }레벤슈타인 거리 찾기
public static int findLevenshteinDistance(String word1, String word2) { // word2 문자열이 비어있을 시, 삭제 int[][] ans = new int[word1.length() + 1][word2.length() + 1]; for (int i = 0; i <= word1.length(); i++) { ans[i][0] = i; } // word1 문자열이 비어있을 시, 추가 for (int i = 0; i <= word2.length(); i++) { ans[0][i] = i; } // 빈 문자열이 없을 때 for (int i = 1; i <= word1.length(); i++) { for (int j = 1; j <= word2.length(); j++) { int min = Math.min(Math.min(ans[i][j - 1], ans[i - 1][j]), ans[i - 1][j - 1]); ans[i][j] = word1.charAt(i - 1) == word2.charAt(j - 1) ? ans[i - 1][j - 1] : min + 1; } } return ans[word1.length()][word2.length()]; }버전 비교
public static int compareVersion(String v1, String v2) { Function<String, String[]> getVersionComponents = version -> version.replaceAll(".*?((?<!\\w)\\d+([.-]\\d+)*).*", "$1", "$1").split("\\."); var components1 = getVersionComponents.apply(v1); var components2 = getVersionComponents.apply(v2); int length = Math.max(components1.length, components2.length); for (int i = 0; i < length; i++) { Integer c1 = i < components1.length ? Integer.parseInt(components1[i]) : 0; Integer c2 = i < components2.length ? Integer.parseInt(components2[i]) : 0; int result = c1.compareTo(c2); if (result != 0) { return result; } } return 0; }클래스
메소드 이름
public static List<String> getAllMethods(final Class<?> cls) { return Arrays.stream(cls.getDeclaredMethods()) .map(Method::getName) .collect(Collectors.toList()); }퍼블릭 필드 이름들 가져오기
public static List<String> getAllFieldNames(final Class<?> cls) { return Arrays.stream(cls.getFields()) .map(Field::getName) .collect(Collectors.toList()); }모든 필드 이름들 가져오기
public static List<String> getAllFieldNames(final Class<?> cls) { var fields = new ArrayList<String>(); var currentCls = cls; while (currentCls != null) { fields.addAll( Arrays.stream(currentCls.getDeclaredFields()) .filter(field -> !field.isSynthetic()) .map(Field::getName) .collect(Collectors.toList())); currentCls = currentCls.getSuperclass(); } return fields; }Object 생성
public static Object createObject(String cls) throws NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException, ClassNotFoundException { var objectClass = Class.forName(cls); var objectConstructor = objectClass.getConstructor(); return objectConstructor.newInstance(); }I/O
Stream을 통해 파일 읽기
public static List<String> readFile(String fileName) throws FileNotFoundException { try (Stream<String> stream = new BufferedReader(new FileReader(fileName)).lines()) { return stream.collect(Collectors.toList()); } }InputStream을 문자열로
public static String inputStreamToString(InputStream inputStream) throws IOException { try (var reader = new BufferedReader(new InputStreamReader(inputStream))) { var stringBuilder = new StringBuilder(); var data = reader.read(); while (data != -1) { stringBuilder.append((char) data); data = reader.read(); } return stringBuilder.toString(); } }쓰레드
쓰레드 풀 생성
public static ExecutorService createFixedThreadPool() { return Executors.newFixedThreadPool(Runtime.getRuntime().availableProcessors()); }날짜
현재 날짜
public static Date addDaysToDate(Date date, int noOfDays){ if(date!=null){ Calendar cal = Calendar.getInstance(); cal.setTime(date); cal.add(Calendar.DAY_OF_MONTH, noOfDays); return cal.getTime(); } return null; }로컬 날짜
public static LocalDate addDaysToLocalDate(LocalDate date, long noOfDays){ return date!=null ? date.plusDays(noOfDays) : null; }