Java 开发人员必备代码片段库
约 20 分钟
30 秒 Java - Java 开发人员必备代码片段库
受 30 秒代码 的启发，这是一个可重用、经过测试且可复制粘贴的 Java 17 兼容代码片段集合，您可以在 30 秒内或更短时间内理解。如果您有兴趣为这个库做出贡献，请查看 贡献指南。
算法
二维数组中的二分查找
public class BinarySearchIn2dArraySnippet {
/**
* Search an item with binarySearch algorithm.
*
* @param matrix should be sorted
* @param target an item to search
* @return if location of item is found, otherwise return {-1,-1}
*/
public static int[] binarySearchIn2darr(int[][] matrix, int target) {
int rows = matrix.length - 1;
int cols = matrix[0].length - 1;
if (rows == 1) {
return binarySearch(matrix, target, 0, 0, cols);
}
int rstart = 0;
int rend = rows;
int cmid = cols / 2;
while (rstart < rend - 1) {
int rmid = rstart + (rend - rstart) / 2;
if (matrix[rmid][cmid] > target) {
rend = rmid;
} else if (matrix[rmid][cmid] < target) {
rstart = rmid;
} else {
return new int[]{rmid, cmid};
}
}
if (matrix[rstart][cmid] == target) {
return new int[]{rstart, cmid};
}
if (matrix[rend][cmid] == target) {
return new int[]{rend, cmid};
}
if (target <= matrix[rstart][cmid - 1]) {
return binarySearch(matrix, target, rstart, 0, cmid - 1);
}
if (target >= matrix[rstart][cmid + 1]) {
return binarySearch(matrix, target, rstart, cmid + 1, cols);
}
if (target <= matrix[rend][cmid - 1]) {
return binarySearch(matrix, target, rend, 0, cmid - 1);
}
if (target <= matrix[rend][cmid + 1]) {
return binarySearch(matrix, target, rend, cmid + 1, cols);
}
return new int[]{-1, -1};
}
static int[] binarySearch(int[][] matrix, int target, int row, int cstart, int cend) {
while (cstart <= cend) {
int cmid = cstart + (cend - cstart) / 2;
if (matrix[row][cmid] > target) {
cend = cmid - 1;
} else if (matrix[row][cmid] < target) {
cstart = cend + 1;
} else {
return new int[]{row, cmid};
}
}
return new int[]{-1, -1};
}
}二分查找
public class BinarySearchSnippet {
/**
* Search an item with binarySearch algorithm.
*
* @param arr sorted array to search
* @param item an item to search
* @return if item is found, return the index position of the array item otherwise return -1
*/
public static int binarySearch(int[] arr, int left, int right, int item) {
if (right >= left) {
int mid = left + (right - left) / 2;
if (arr[mid] == item) {
return mid;
}
if (arr[mid] > item) {
return binarySearch(arr, left, mid - 1, item);
}
return binarySearch(arr, mid + 1, right, item);
}
return -1;
}
}冒泡排序
public class BubbleSortSnippet {
/**
* Sort an array with bubbleSort algorithm.
*
* @param arr array to sort
*/
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;
}
}
}
}
}计数排序
public class CountingSortSnippet {
/**
* Sort an array having zero or positive numbers with countingSort algorithm.
*
* @param arr array to sort
*/
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;
}
}
}循环排序
public class CycleSortSnippet {
/**
* Sort an array with cycleSort algorithm.
*
* @param arr array to sort
*/
public static int[] cycleSort(int[] arr) {
int n = arr.length;
int i = 0;
while (i < n) {
int correctpos = arr[i] - 1;
if (arr[i] != arr[correctpos]) {
int temp = arr[i];
arr[i] = arr[correctpos];
arr[correctpos] = temp;
} else {
i++;
}
}
return arr;
}
}插入排序
public class InsertionSortSnippet {
/**
* Sort an array with insertionSort algorithm.
*
* @param arr array to sort
*/
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;
}
}
}二维数组中的线性查找
public class LinearSearchIn2dArraySnippet {
/**
* Search an item with linearSearch algorithm.
*
* @param arr array to search
* @param target an item to search
* @return if location of target is found,otherwise return {-1,-1}
*/
public static int[] linearSearch2dArray(int[][] arr, int target) {
for (int i = 0; i < arr.length; i++) {
for (int j = 0; j < arr[i].length; j++) {
if (arr[i][j] == target) {
return new int[]{i, j};
}
}
}
return new int[]{-1, -1};
}
}线性查找
public class LinearSearchSnippet {
/**
* Search an item with linearSearch algorithm.
*
* @param arr array to search
* @param item an item to search
* @return if item is found, return the index position of the array item otherwise return -1
*/
public static int linearSearch(int[] arr, int item) {
for (int i = 0; i < arr.length; i++) {
if (item == arr[i]) {
return i;
}
}
return -1;
}
}归并排序
public class MergeSortSnippet {
/**
* Sort an array with qmergesort algorithm.
*
* @param arr array to sort
* @low low index where to begin sort (e.g. 0)
* @high high index where to end sort (e.g. array length - 1)
*/
public static void mergeSort(int[] arr, int low, int high) {
if (low >= high) {
return;
}
var mid = (low + high) / 2;
mergeSort(arr, low, mid);
mergeSort(arr, mid + 1, high);
merge(arr, low, high, mid);
}
private static void merge(int[] arr, int low, int high, int mid) {
int[] temp = new int[(high - low + 1)];
var i = low;
var j = mid + 1;
var k = 0;
while (i <= mid && j <= high) {
if (arr[i] < arr[j]) {
temp[k++] = arr[i];
i++;
} else {
temp[k++] = arr[j];
j++;
}
}
while (i <= mid) {
temp[k++] = arr[i];
i++;
}
while (j <= high) {
temp[k++] = arr[j];
j++;
}
for (int m = 0, n = low; m < temp.length; m++, n++) {
arr[n] = temp[m];
}
}
}快速排序
public class QuickSortSnippet {
/**
* Sort an array with quicksort algorithm.
*
* @param arr array to sort
* @param left left index where to begin sort (e.g. 0)
* @param right right index where to end sort (e.g. array length - 1)
*/
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);
}
}
}
}选择排序
public class SelectionSortSnippet {
/**
* Sort an array with selectionSort algorithm.
*
* @param arr array to sort
*/
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;
}
}
}埃拉托斯特尼筛法
public class SieveOfEratosthenesSnippet {
/**
* Search an item with binarySearch algorithm.
*
* @param n range of number.
* @return isPrime boolean array where prime number 0 to n are mark true.
*/
public static boolean[] sieveOfEratosthenes(int n) {
boolean[] isPrime = new boolean[n + 1];
for (int i = 0; i < isPrime.length; i++) {
isPrime[i] = true;
}
for (int i = 2; i * i <= n; i++) {
if (isPrime[i] == true) {
for (int j = i * i; j <= n; j += i) {
isPrime[j] = false;
}
}
}
return isPrime;
}
}数组
全部相等
public class AllEqualSnippet {
/**
* Returns true if all elements in array are equal.
*
* @param arr the array to check (not null)
* @param <T> the element type
* @return true if all elements in the array are equal
*/
public static <T> boolean allEqual(T[] arr) {
return Arrays.stream(arr).distinct().count() == 1;
}
}数组连接
public class ArrayConcatSnippet {
/**
* Generic 2 array concatenation Credits: Joachim Sauer https://stackoverflow.com/questions/80476/how-can-i-concatenate-two-arrays-in-java
*
* @param first is the first array (not null)
* @param second is the second array (not null)
* @param <T> the element type
* @return concatenated array
*/
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;
}
}数组平均值
public class ArrayMeanSnippet {
/**
* Returns the mean of the integers in the array.
*
* @param arr the array of integers (not null)
* @return a double representing the mean of the array
*/
public static double arrayMean(int[] arr) {
return (double) Arrays.stream(arr).sum() / arr.length;
}
}数组中位数
public class ArrayMedianSnippet {
/**
* Returns the median of the array.
*
* @param arr the array of integers (not null)
* @return a double representing the median of the array
*/
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;
}
}数组众数（就地运算）
public class ArrayModeInPlaceSnippet {
/**
* Returns the mode of the array.
*
* @param arr array to find mode in it
* @return mode of array
*/
public static int modeArrayInPlace(int[] arr) {
if (arr.length == 0) {
return 0;
}
Arrays.sort(arr);
int mode = arr[0];
int maxcount = 1;
int count = 1;
for (int i = 1; i < arr.length; i++) {
if (arr[i] == arr[i - 1]) {
count++;
} else {
if (count > maxcount) {
maxcount = count;
mode = arr[i - 1];
}
count = 1;
}
}
if (count > maxcount) {
mode = arr[arr.length - 1];
}
return mode;
}
}数组众数
public class ArrayModeSnippet {
/**
* Returns the mode of the array.
*
* @param arr array to find mode in it
* @return mode of array
*/
public static int modeArray(int[] arr) {
int mode = 0;
int 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;
}
}数组求和
public class ArraySumSnippet {
/**
* Returns sum of the integers in the array.
*
* @param arr the array of integers (not null)
* @return the sum of the elements from the array
*/
public static int arraySum(int[] arr) {
return Arrays.stream(arr).sum();
}
}查找最大值
public class FindMaxSnippet {
/**
* Returns the maximum integer from the array using reduction.
*
* @param arr the array of integers (not null)
* @return the maximum element from the array
*/
public static int findMax(int[] arr) {
return Arrays.stream(arr).reduce(Integer.MIN_VALUE, Integer::max);
}
}查找最小值
public class FindMinSnippet {
/**
* Returns the minimum integer from the array using reduction.
*
* @param arr the array of integers (not null)
* @return the minimum element from the array
*/
public static int findMin(int[] arr) {
return Arrays.stream(arr).reduce(Integer.MAX_VALUE, Integer::min);
}
}多维数组连接
public class MultiArrayConcatenationSnippet {
/**
* Generic N array concatenation Credits: Joachim Sauer https://stackoverflow.com/questions/80476/how-can-i-concatenate-two-arrays-in-java
*
* @param first is the first array (not null)
* @param rest the rest of the arrays (optional)
* @param <T> the element type
* @return concatenated array
*/
public static <T> T[] multiArrayConcat(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;
}
}反转数组
public class ReverseArraySnippet {
/**
* The function then reverses the elements of the array between the starting and ending
* indices using a while loop and a temporary variable `temp`. Finally, the function returns
* the reversed array.
*
* @param array a array
* @param start start index array
* @param end end index array
* @return reverses elements in the array
* @throws IllegalArgumentException if the [start] index is greater
* than the [end] index or if the array is null
**/
public static <T> T[] reverseArray(T[] array, int start, int end) {
if (start > end || array == null) {
throw new
IllegalArgumentException("Invalid argument!");
}
int minimumSizeArrayForReversal = 2;
if (start == end || array.length < minimumSizeArrayForReversal) {
return array;
}
while (start < end) {
T temp = array[start];
array[start] = array[end];
array[end] = temp;
start++;
end--;
}
return array;
}
}类
创建对象
public class CreatingObjectSnippet {
/**
* Create object using reflection.
*
* @param cls fully qualified name of class includes the package name as String
* @return object
* @throws NoSuchMethodException if a method that does not exist at runtime.
* @throws IllegalAccessException <p>if an currently executing method does not have access to
* the definition of the specified class, field, method or constructor</p>
* @throws InvocationTargetException <p>InvocationTargetException is a checked exception
* that wraps an exception thrown by an invoked method or constructor.</p>
* @throws InstantiationException <p>when an method tries to create an instance of a class
* using the newInstance method in class Class.</p>
* @throws ClassNotFoundException <p>when an application tries to load in a class
* through its string name.</p>
*/
public static Object createObject(String cls)
throws NoSuchMethodException,
IllegalAccessException,
InvocationTargetException,
InstantiationException,
ClassNotFoundException {
var objectClass = Class.forName(cls);
var objectConstructor = objectClass.getConstructor();
return objectConstructor.newInstance();
}
}获取所有字段名称
public class GetAllFieldNamesSnippet {
/**
* Print all declared field names of the class or the interface the class extends.
*
* @param clazz Tested class
* @return list of names of all fields
*/
public static List<String> getAllFieldNames(final Class<?> clazz) {
var fields = new ArrayList<String>();
var currentClazz = clazz;
while (currentClazz != null) {
fields.addAll(
Arrays.stream(currentClazz.getDeclaredFields())
.filter(field -> !field.isSynthetic())
.map(Field::getName)
.collect(Collectors.toList()));
currentClazz = currentClazz.getSuperclass();
}
return fields;
}
}获取所有方法
public class GetAllMethodsSnippet {
/**
* Print all declared methods of the class.
*
* @param cls Tested class
* @return list of methods name
*/
public static List<String> getAllMethods(final Class<?> cls) {
return Arrays.stream(cls.getDeclaredMethods())
.map(Method::getName)
.collect(Collectors.toList());
}
}获取所有公共字段名称
public class GetAllPublicFieldNamesSnippet {
/**
* Print all declared public field names of the class or the interface the class extends.
*
* @param clazz Tested class
* @return list of name of public fields
*/
public static List<String> getAllPublicFieldNames(final Class<?> clazz) {
return Arrays.stream(clazz.getFields())
.map(Field::getName)
.collect(Collectors.toList());
}
}日期
向日期添加天数
public class AddDaysToDateSnippet {
/**
* Add days to given date.
*
* @param date given date
* @param noOfDays number of days to add
* @return modified date
*/
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;
}
/**
* Add days to local date.
*
* @param date given local date
* @param noOfDays number of days to add
* @return modified date
*/
public static LocalDate addDaysToLocalDate(LocalDate date, long noOfDays) {
return date != null ? date.plusDays(noOfDays) : null;
}
}日期差
public class DateDifferenceSnippet {
/**
* This function calculates the number of years between two LocalDate objects.
* If the result is negative, it returns the absolute value of the difference.
*
* @param firstTime The first LocalDate object representing the starting date
* @param secondTime The second LocalDate object representing the ending date
* @return The number of years between the two LocalDate objects as a long data type
*/
public static long getYearsDifference(LocalDate firstTime, LocalDate secondTime) {
var yearsDifference = ChronoUnit.YEARS.between(firstTime, secondTime);
return Math.abs(yearsDifference);
}
/**
* This function calculates the number of months between two LocalDate objects.
* If the result is negative, it returns the absolute value of the difference.
*
* @param firstTime The first LocalDate object representing the starting date
* @param secondTime The second LocalDate object representing the ending date
* @return The number of months between the two LocalDate objects as a long data type
*/
public static long getMonthsDifference(LocalDate firstTime, LocalDate secondTime) {
var monthsDifference = ChronoUnit.MONTHS.between(firstTime, secondTime);
return Math.abs(monthsDifference);
}
/**
* This function calculates the number of days between two LocalDate objects.
* If the result is negative, it returns the absolute value of the difference.
*
* @param firstTime The first LocalDate object representing the starting date
* @param secondTime The second LocalDate object representing the ending date
* @return The number of days between the two LocalDate objects as a long data type
*/
public static long getDaysDifference(LocalDate firstTime, LocalDate secondTime) {
var daysDifference = ChronoUnit.DAYS.between(firstTime, secondTime);
return Math.abs(daysDifference);
}
}编码
Base64 解码
public class Base64DecodeSnippet {
/**
* Decodes a Base64 encoded string to the actual representation.
*
* @param input base64 encoded string
* @return decoded string
*/
public static String decodeBase64(String input) {
return new String(Base64.getDecoder().decode(input.getBytes()));
}
}Base64 编码
public class Base64EncodeSnippet {
/**
* Encodes the input string to a Base64 encoded string.
*
* @param input string to be encoded
* @return base64 encoded string
*/
public static String encodeBase64(String input) {
return Base64.getEncoder().encodeToString(input.getBytes());
}
}文件
列出所有文件
public class ListAllFilesSnippet {
/**
* Recursively list all the files in directory.
*
* @param path the path to start the search from
* @return list of all files
*/
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;
}
}列出目录
public class ListDirectoriesSnippet {
/**
* List directories.
*
* @param path the path where to look
* @return array of File
*/
public static File[] listDirectories(String path) {
return new File(path).listFiles(File::isDirectory);
}
}列出目录中的文件
public class ListFilesInDirectorySnippet {
/**
* List files in directory.
*
* @param folder the path where to look
* @return array of File
*/
public static File[] listFilesInDirectory(String folder) {
return new File(folder).listFiles(File::isFile);
}
}读取行
public class ReadLinesSnippet {
/**
* Read file as list of strings.
*
* @param filename the filename to read from
* @return list of strings
* @throws IOException if an I/O error occurs
*/
public static List<String> readLines(String filename) throws IOException {
return Files.readAllLines(Paths.get(filename));
}
}压缩目录
public class ZipDirectorySnippet {
/**
* Zip a complete directory.
*
* @param srcDirectoryName The path to the directory to be zipped
* @param zipFileName The location and name of the zipped file.
* @throws IOException if an I/O error occurs
* */
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);
}
}
/**
* Utility function which either zips a single file, or recursively calls itself for
* a directory to traverse down to the files contained within it.
*
* @param fileToZip The file as a resource
* @param fileName The actual name of the file
* @param zipOut The output stream to which all data is being written
* */
public static void zipFile(File fileToZip, String fileName, ZipOutputStream zipOut)
throws IOException {
if (fileToZip.isHidden()) { // Ignore hidden files as standard
return;
}
if (fileToZip.isDirectory()) {
if (fileName.endsWith("/")) {
zipOut.putNextEntry(new ZipEntry(fileName)); // To be zipped next
zipOut.closeEntry();
} else {
// Add the "/" mark explicitly to preserve structure while unzipping action is performed
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) // Start zipping once we know it is a file
) {
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);
}
}
}
}压缩文件
public class ZipFileSnippet {
/**
* Zip single file.
*
* @param srcFilename the filename of the source file
* @param zipFilename the filename of the destination zip file
* @throws IOException if an I/O error occurs
*/
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);
}
}
}
}压缩多个文件
public class ZipFilesSnippet {
/**
* Zip multiples files.
*
* @param srcFilenames array of source file names
* @param zipFilename the filename of the destination zip file
* @throws IOException if an I/O error occurs
*/
public static void zipFiles(String[] srcFilenames, String zipFilename) throws IOException {
try (
var fileOut = new FileOutputStream(zipFilename);
var zipOut = new ZipOutputStream(fileOut)
) {
for (String srcFilename : srcFilenames) {
var srcFile = new File(srcFilename);
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);
}
}
}
}
}
}I/O
输入流转字符串
public class InputStreamToStringSnippet {
/**
* Convert InputStream to String.
*
* @param inputStream InputStream to convert
* @return String
*/
public static String inputStreamToString(InputStream inputStream) {
return new BufferedReader(new InputStreamReader(inputStream, StandardCharsets.UTF_8))
.lines().collect(Collectors.joining(System.lineSeparator()));
}
}读取文件代码片段
public class ReadFileSnippet {
/**
* Read file using stream and return list of string lines.
*
* @param fileName file to read
* @throws FileNotFoundException if an I/O error occurs
*/
public static List<String> readFile(String fileName) throws FileNotFoundException {
try (Stream<String> stream = new BufferedReader(new FileReader(fileName)).lines()) {
return stream.collect(Collectors.toList());
}
}
}数学
掷骰子
public class DiceThrow {
private static Random random = new Random();
/**
* Enum for standardized sided dice (4,6,8,10,12 and 20).
*/
public enum DiceSides {
FOUR(4), SIX(6), EIGHT(8), TEN(10), TWELVE(12), TWENTY(20);
private final int diSides;
DiceSides(int diceSides) {
this.diSides = diceSides;
}
/**
* Returns the number of sides of a dice.
*
* @return int denoting number of sides of a dice
*/
public int getDiceSides() {
return this.diSides;
}
}
/**
* Returns the sum of sides for the given number of sides of each dice.
*
* @param noOfDice number of dice
* @param sides sides of a dice
* @return int sum of sides for number of dice
*/
public static int throwDice(int noOfDice, DiceSides sides) {
int sum = 0;
for (int i = 0; i < noOfDice; i++) {
sum = sum + (1 + random.nextInt(sides.getDiceSides()));
}
return sum;
}
}Elo 评分
public class EloRatingSnippet {
static final int BASE = 400; //Two types are popular - 400 and 480. We will choose 400 here
static final int RATING_ADJUSTMENT_FACTOR = 32; //32 is the standard for Beginner Games
/**
* Elo Rating Snippet to calculate result after a single match.
*
* @param firstPlayerRating Rating of the first player.
* @param secondPlayerRating Rating of the second player.
* @param result Result of the match, always considered with respect to the first player.
* 1 indicates a win, 0.5 indicates a draw and 0 indicates a loss.
* @return Returns the new rating of the first player.
*/
public static double calculateMatchRating(double firstPlayerRating, double secondPlayerRating,
double result) {
double ratingDiff = ((secondPlayerRating - firstPlayerRating) * 1.0) / BASE;
double logisticDiff = Math.pow(10, ratingDiff);
double firstPlayerExpectedScore = 1.0 / (1 + logisticDiff);
double firstPlayerActualScore = result;
double newRating = firstPlayerRating + RATING_ADJUSTMENT_FACTOR * (firstPlayerActualScore
- firstPlayerExpectedScore);
return newRating;
}
}奇偶性
public class EvenOdd {
/**
* Returns string denoting number is odd or even.
*
* @param num To check whether its even or odd
* @return string denoting its even or odd
*/
public static String evenodd(int num) {
if (num % 2 == 0) {
return "even";
} else {
return "odd";
}
}
}阶乘
public class FactorialSnippet {
/**
* Factorial. Works only for small numbers
*
* @param number for which factorial is to be calculated for
* @return factorial
*/
public static int factorial(int number) {
var result = 1;
for (var factor = 2; factor <= number; factor++) {
result *= factor;
}
return result;
}
/**
* Factorial. Example of what the recursive implementation looks like.
*
* @param number for which factorial is to be calculated for
* @return factorial
*/
public static int recursiveFactorial(int number) {
var initial = 0;
if (number == initial) {
return initial + 1;
}
return number * recursiveFactorial(number - 1);
}
}斐波那契数列
public class FibonacciSnippet {
/**
* Recursive Fibonacci series. Works only for small n and is spectacularly inefficient
*
* @param n given number
* @return fibonacci number for given n
*/
public static int fibonacci(int n) {
if (n <= 1) {
return n;
} else {
return fibonacci(n - 1) + fibonacci(n - 2);
}
}
/**
* Example of what an iterative implementation of Fibonacci looks like.
*
* @param number given number
* @return fibonacci number for given n
*/
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);
}
}最大公约数
public class GreatestCommonDivisorSnippet {
/**
* Greatest common divisor calculation.
*
* @param a one of the numbers whose gcd is to be computed
* @param b other number whose gcd is to be computed
* @return gcd of the two numbers
*/
public static int gcd(int a, int b) {
if (b == 0) {
return a;
}
return gcd(b, a % b);
}
}海弗森公式
public class HaversineFormulaSnippet {
// Radius of sphere on which the points are, in this case Earth.
private static final double SPHERE_RADIUS_IN_KM = 6372.8;
/**
* Haversine formula for calculating distance between two latitude, longitude points.
*
* @param latA Latitude of point A
* @param longA Longitude of point A
* @param latB Latitude of point B
* @param longB Longitude of point B
* @return the distance between the two points.
*/
public static double findHaversineDistance(double latA, double longA, double latB, double longB) {
if (!isValidLatitude(latA)
|| !isValidLatitude(latB)
|| !isValidLongitude(longA)
|| !isValidLongitude(longB)) {
throw new IllegalArgumentException();
}
// Calculate the latitude and longitude differences
var latitudeDiff = Math.toRadians(latB - latA);
var longitudeDiff = Math.toRadians(longB - longA);
var latitudeA = Math.toRadians(latA);
var latitudeB = Math.toRadians(latB);
// Calculating the distance as per haversine formula
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;
}
// Check for valid latitude value
private static boolean isValidLatitude(double latitude) {
return latitude >= -90 && latitude <= 90;
}
// Check for valid longitude value
private static boolean isValidLongitude(double longitude) {
return longitude >= -180 && longitude <= 180;
}
}最小公倍数
public class LeastCommonMultipleSnippet {
/**
* Least common multiple calculation.
*
* @param a one of the numbers whose lcm is to be computed
* @param b other number whose lcm is to be computed
* @return lcm of the two numbers
*/
public static int lcm(int a, int b) {
int max = a > b ? a : b;
int min = a < b ? a : b;
for (int i = 1; i <= min; i += 1) {
int prod = max * i;
if (prod % min == 0) {
return prod;
}
}
return max * min;
}
}Luhn 算法
public class LuhnSnippet {
/**
* Calculates checksum for a given number with Luhn's algorithm. Works only on non-negative
* integers not greater than {@link Long#MAX_VALUE} i.e., all numbers with a maximum of 18
* digits, plus 19-digit-long numbers start with 1..8 (also some with 9, too). For
* demonstration purposes, algorithm is not optimized for efficiency.
*
* @param num number whose checksum is to be calculated
* @return checksum value for num
* @see <a href="https://patents.google.com/patent/US2950048A">Hans P. LUHN's patent US2950048A</a>
* @see <a href="https://en.wikipedia.org/wiki/Luhn_algorithm">Luhn algorithm on Wikipedia</a>
*/
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;
// Loop on digits of numStr from right to left.
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;
}
}自然数二进制转换
public class NaturalNumberBinaryConversionSnippet {
/**
* Convert natural number to binary string. Only supports positive integers.Throws exception
* for negative integers
*
* @param naturalNumber given number
* @return Binary string representation of naturalNumber
*/
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());
}
/**
* Convert binary string representation to Long valued Integer. Throws exception if input
* string contains characters other than '0' and '1'
*
* @param binary given number
* @return Unsigned Long value for the binary number
*/
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();
}
}执行抽奖
public class PerformLotterySnippet {
/**
* Generate random lottery numbers.
*
* @param numNumbers how many performLottery numbers are available (e.g. 49)
* @param numbersToPick how many numbers the player needs to pick (e.g. 6)
* @return array with the random numbers
*/
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]);
}
}素数
public class PrimeNumberSnippet {
/**
* Checks if given number is a prime number. Prime number is a number that is greater than 1 and
* divided by 1 or itself only Credits: https://en.wikipedia.org/wiki/Prime_number
*
* @param number number to check prime
* @return true if prime
*/
public static boolean isPrime(int number) {
//if number < 2 its not a prime number
if (number < 2) {
return false;
}
// 2 and 3 are prime numbers
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;
}
}随机数
public class RandomNumber {
private RandomNumber() {}
private static Random random = new Random();
/**
* Return a random number between two given numbers.
*
* @param start Starting point to find the random number
* @param end Ending point to find the random number
* @return Number denoting the random number generated
*/
public static <T extends Number> Number getRandomNumber(T start, T end) {
if ((start instanceof Byte && end instanceof Byte)) {
return (byte) (start.byteValue() + random.nextInt(end.byteValue() - start.byteValue() + 1));
} else if ((start instanceof Byte && end instanceof Byte)
|| (start instanceof Short && end instanceof Short)) {
return (short) (start.shortValue()
+ random.nextInt(end.shortValue() - start.shortValue() + 1));
} else if ((start instanceof Integer && end instanceof Integer)) {
return (int) (start.intValue()
+ random.nextInt(end.intValue() - start.intValue() + 1));
} else if (start instanceof Long && end instanceof Long) {
return (long) (start.longValue()
+ random.nextLong(end.longValue() - start.longValue() + 1));
} else if (start instanceof Float && end instanceof Float) {
return (float) (start.floatValue()
+ random.nextFloat(end.floatValue() - start.floatValue() + 1));
} else if (start instanceof Double && end instanceof Double) {
return (double) (start.doubleValue()
+ random.nextDouble(end.doubleValue() - start.doubleValue() + 1));
} else {
throw new IllegalArgumentException("Invalid Numbers As Arguments "
+ start.getClass() + " and " + end.getClass());
}
}
}平方根
public class SquareRoot {
/**
* Returns square root of a number.
*
* @param num To find SquareRoot
* @param p precision till how many decimal numbers we want accurate ans
*/
public static double sqrt(int num, int p) {
int start = 0;
int 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;
}
}
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;
}
}媒体
截取屏幕
public class CaptureScreenSnippet {
/**
* Capture screenshot and save it to PNG file. Credits: https://viralpatel.net/blogs/how-to-take-screen-shots-in-java-taking-screenshots-java/
*
* @param filename the name of the file
* @throws AWTException if the platform configuration does not allow low-level input control
* @throws IOException if an I/O error occurs
*/
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));
}
}网络
HTTP GET
public class HttpGetSnippet {
/**
* Performs HTTP GET request.
*
* @param uri the URI of the connection
* @return response object
* @throws Exception i/o error, interruption error, etc
*/
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, HttpResponse.BodyHandlers.ofString());
}
}HTTP POST
public class HttpPostSnippet {
/**
* Performs HTTP POST request. Credits https://stackoverflow.com/questions/3324717/sending-http-post-request-in-java
*
* @param address the URL of the connection in String format, like "http://www.google.com"
* @param arguments the body of the POST request, as a HashMap
* @return response object
* @throws IOException if an I/O error occurs
* @throws InterruptedException if the operation is interrupted
*/
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(), StandardCharsets.UTF_8) + "="
+ URLEncoder.encode(entry.getValue(), StandardCharsets.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(HttpRequest.BodyPublishers.ofByteArray(out))
.build();
return HttpClient.newHttpClient().send(request, HttpResponse.BodyHandlers.ofString());
}
}字符串
字母异位词
public class AnagramSnippet {
/**
* Checks if two words are anagrams (contains same characters with same frequency in any order).
*
* @param s1 The first string to be checked
* @param s2 The second string to be checked
* @return true if they are anagrams of each other
*/
public static boolean isAnagram(String s1, String s2) {
var l1 = s1.length();
var l2 = s2.length();
if (l1 != l2) {
return false;
}
var arr1 = new int[256];
var arr2 = new int[256];
for (var i = 0; i < l1; i++) {
arr1[s1.charAt(i)]++;
arr2[s2.charAt(i)]++;
}
return Arrays.equals(arr1, arr2);
}
}公共字母
public class CommonLettersSnippet {
/**
* Find Common Characters inside given two strings.
*
* @param firstStr first string
* @param secondStr second string
* @return Common Characters.
*/
public static String getCommonLetters(String firstStr, String secondStr) {
Set<String> commonLetters = new HashSet<>();
for (Character currentCharacter : firstStr.toCharArray()) {
if (isCommonLetter(secondStr, currentCharacter)) {
commonLetters.add(currentCharacter.toString());
}
}
return String.join(" ", commonLetters);
}
private static boolean isCommonLetter(String str, Character character) {
return str.contains(character.toString()) && Character.isLetter(character);
}
}比较版本
public class CompareVersionSnippet {
private static final String EXTRACT_VERSION_REGEX = ".*?((?<!\\w)\\d+([.-]\\d+)*).*";
/**
* Compares two version strings.
* Credits: https://stackoverflow.com/a/6702000/6645088 and https://stackoverflow.com/a/44592696/6645088
*
* @param v1 the first version string to compare
* @param v2 the second version string to compare
* @return the value {@code 0} if the two strings represent same versions;
* a value less than {@code 0} if {@code v1} is greater than {@code v2}; and
* a value greater than {@code 0} if {@code v2} is greater than {@code v1}
*/
public static int compareVersion(String v1, String v2) {
var components1 = getVersionComponents(v1);
var components2 = getVersionComponents(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;
}
private static String[] getVersionComponents(String version) {
return version.replaceAll(EXTRACT_VERSION_REGEX, "$1").split("\\.");
}
}重复字符
public class DuplicateCharacterSnippet {
/**
* Remove Duplicate Characters from a string.
*
* @param str The string to be processed
* @return A string with no duplicate characters
*/
public static String removeDuplicateCharacters(String str) {
char[] charsOfStr = str.toCharArray();
Set<String> uniqueCharacters = new HashSet<>();
for (char character : charsOfStr) {
uniqueCharacters.add(String.valueOf(character));
}
return String.join("", uniqueCharacters);
}
}莱文斯坦距离
public class LevenshteinDistanceSnippet {
/**
* Find the Levenshtein distance between two words. https://en.wikipedia.org/wiki/Levenshtein_distance
*
* @param word1 first word
* @param word2 second word
* @return distance
*/
public static int findLevenshteinDistance(String word1, String word2) {
// If word2 is empty, removing
int[][] ans = new int[word1.length() + 1][word2.length() + 1];
for (int i = 0; i <= word1.length(); i++) {
ans[i][0] = i;
}
// if word1 is empty, adding
for (int i = 0; i <= word2.length(); i++) {
ans[0][i] = i;
}
// None is empty
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()];
}
}最大字符计数
public class MaxCharacterCountSnippet {
/**
* The maximum count of times a specific character appears in a string.
*
* @param str َA specific string
* @param character A specific character
* @return the maximum count of one character
*/
public static int getMaxCharacterCount(String str, char character) {
int characterCount = 0;
int maxCharacterCount = 0;
for (int i = 0; i < str.length(); i++) {
if ((str.charAt(i)) == character) {
characterCount++;
maxCharacterCount = Math.max(maxCharacterCount, characterCount);
} else {
characterCount = 0;
}
}
return maxCharacterCount;
}
}回文检查
public class PalindromCheckSnippet {
/**
* Checks if given string is palindrome (same forward and backward). Skips non-letter characters
* Credits: https://github.com/kousen/java_8_recipes
*
* @param s string to check
* @return true if palindrome
*/
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;
}
}反转字符串
public class ReverseStringSnippet {
/**
* Reverse string.
*
* @param s the string to reverse
* @return reversed string
*/
public static String reverseString(String s) {
return new StringBuilder(s).reverse().toString();
}
}字符串转日期
public class StringToDateSnippet {
/**
* Convert string to date.
*
* @param date the date string
* @param format expected date format
* @return Date
* @throws ParseException in case of an unparseable date string
*/
public static Date stringToDate(String date, String format) throws ParseException {
var simpleDateFormat = new SimpleDateFormat(format);
return simpleDateFormat.parse(date);
}
}线程
线程池
public class ThreadPool {
/**
* <p>Creates pool of threads. Where the pool is the size of the number of processors
* available to the Java virtual machine.</p>
*
* @return the newly created thread pool
*/
public static ExecutorService createFixedThreadPool() {
return Executors.newFixedThreadPool(Runtime.getRuntime().availableProcessors());
}
}