Essential Code Snippets Library for Java Developers
About 26 min
30 Seconds of Java - Essential Code Snippets Library for Java Developers
Inspired by 30 seconds of code, this is a collection of reusable, tested, and copy-pasteable Java 21 compatible code snippets that you can understand in 30 seconds or less. If you're interested in contributing to this library, please see the contribution guidelines.
Algorithm
Binary Search In 2d Array
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};
}
}Binary Search
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;
}
}Bubble Sort
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;
}
}
}
}
}Counting Sort
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;
}
}
}Cycle Sort
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;
}
}Insertion Sort
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;
}
}
}Linear Search In 2d Array
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};
}
}Linear Search
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;
}
}Luhn Mod N
public class LuhnModnSnippet {
private static final String CODE_POINTS = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
/**
* Generates a check character using the Luhn mod N algorithm.
*
* @param character the input string consisting of valid alphanumeric characters
* @return the generated check character
* @throws IllegalArgumentException if the input contains invalid characters
*/
public static int codePointFromCharacter(char character) {
if (CODE_POINTS.indexOf(character) == -1) {
throw new IllegalArgumentException("Invalid character: " + character);
}
return CODE_POINTS.indexOf(character);
}
/**
* Converts a code point to its corresponding character.
*
* @param codePoint the code point to be converted
* @return the character representation of the code point
* @throws IllegalArgumentException if the code point is out of range.
*/
public static char characterFromCodePoint(int codePoint) {
if (codePoint < 0 || codePoint >= CODE_POINTS.length()) {
throw new IllegalArgumentException("Invalid code point: " + codePoint);
}
return CODE_POINTS.charAt(codePoint);
}
public static int numberOfValidInputCharacters() {
return CODE_POINTS.length();
}
/**
* Helper method to calculate the sum for both check character generation and validation.
*
* @param input the input string
* @param factorStart the initial factor to start with (1 or 2)
* @return the calculated sum, reminder, and the numberOfValidInputCharacters
*/
private static int[] calculateSum(String input, int factorStart) {
if (input == null || input.isEmpty()) {
throw new IllegalArgumentException("Input cannot be empty");
}
int factor = factorStart;
int sum = 0;
int n = numberOfValidInputCharacters();
for (int i = input.length() - 1; i >= 0; i--) {
int codePoint = codePointFromCharacter(input.charAt(i));
int addend = factor * codePoint;
factor = (factor == 2) ? 1 : 2;
addend = (addend / n) + (addend % n);
sum += addend;
}
return new int[]{sum, sum % n, n};
}
/**
* Generates a check character for the given input string using the Luhn mod N algorithm.
*
* @param input the input string (non-empty)
* @return the generated check character
* @throws IllegalArgumentException if the input is null or empty
*/
public static char generateCheckCharacter(String input) {
int[] result = calculateSum(input, 2);
return characterFromCodePoint((result[2] - result[1]) % result[2]);
}
/**
* Validates a check character by applying the Luhn mod N algorithm.
*
* @param input the input string (including the check character)
* @return true if the input passes validation, false otherwise
* @throws IllegalArgumentException if the input is null or empty
*/
public static boolean validateCheckCharacter(String input) {
int[] result = calculateSum(input, 1);
return (result[1] == 0);
}
}Merge Sort
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];
}
}
}Quick Sort
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);
}
}
}
}Selection Sort
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;
}
}
}Sieve of Eratosthenes
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;
}
}Verhoeff
public class VerhoeffSnippet {
private static final int[][] d = {
{0, 1, 2, 3, 4, 5, 6, 7, 8, 9},
{1, 0, 3, 2, 5, 4, 7, 6, 9, 8},
{2, 3, 0, 1, 6, 7, 4, 5, 8, 9},
{3, 2, 1, 0, 7, 6, 5, 4, 9, 8},
{4, 5, 6, 7, 0, 1, 2, 3, 8, 9},
{5, 4, 7, 6, 1, 0, 3, 2, 9, 8},
{6, 7, 4, 5, 2, 3, 0, 1, 8, 9},
{7, 6, 5, 4, 3, 2, 1, 0, 9, 8},
{8, 9, 8, 9, 8, 9, 8, 9, 0, 1},
{9, 8, 9, 8, 9, 8, 9, 8, 1, 0}
};
private static final int[][] p = {
{0, 1, 2, 3, 4, 5, 6, 7, 8, 9},
{1, 5, 7, 6, 2, 8, 3, 0, 9, 4},
{5, 8, 0, 3, 7, 9, 6, 1, 4, 2},
{8, 9, 1, 6, 0, 4, 3, 5, 2, 7},
{9, 4, 5, 3, 1, 2, 6, 8, 7, 0},
{4, 2, 8, 6, 5, 7, 3, 9, 0, 1},
{2, 7, 9, 3, 8, 0, 6, 4, 1, 5},
{7, 0, 4, 6, 9, 1, 3, 2, 5, 8}
};
private static final int[] inv = {0, 4, 3, 2, 1, 5, 6, 7, 8, 9};
/**
* Validates a number using the Verhoeff checksum algorithm.
*
* @param num the numeric string to validate
* @return true if the number is valid according to Verhoeff algorithm, false otherwise
*/
public static boolean validateVerhoeff(String num) {
int c = 0;
int length = num.length();
// Adjust index for validation of the full number (including check digit)
for (int i = 0; i < length; i++) {
int digit = Character.getNumericValue(num.charAt(length - i - 1));
c = d[c][p[(i + 1) % 8][digit]]; // Correct permutation index
}
return c == 0; // Final checksum must be zero
}
/**
* Generates a Verhoeff check digit for a given numeric string.
*
* @param num the numeric string for which to generate the check digit
* @return the generated Verhoeff check digit as a string
*/
public static String generateVerhoeff(String num) {
int c = 0;
int length = num.length();
for (int i = 0; i < length; i++) {
int digit = Character.getNumericValue(num.charAt(length - i - 1));
c = d[c][p[(i % 8)][digit]];
}
return Integer.toString(inv[c]);
}
}Array
All Equal
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;
}
}Array Concat
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;
}
}Array Mean
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;
}
}Array Median
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;
}
}Array Mode In Place
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;
}
}Array Mode
public class ArrayModeSnippet {
/**
* Private constructor to prevent instantiation.
*/
private ArrayModeSnippet() {
throw new IllegalStateException("Utility class");
}
/**
* Returns the mode(s) of the array.
* If multiple modes exist, it returns them in a list.
*/
public static List<Integer> modeArray(int[] arr) {
int maxCount = 0;
HashMap<Integer, Integer> frequencyMap = new HashMap<>();
for (int num : arr) {
frequencyMap.put(num, frequencyMap.getOrDefault(num, 0) + 1);
maxCount = Math.max(maxCount, frequencyMap.get(num));
}
List<Integer> modes = new ArrayList<>();
for (Map.Entry<Integer, Integer> entry : frequencyMap.entrySet()) {
if (entry.getValue() == maxCount) {
modes.add(entry.getKey());
}
}
return modes;
}
}Array Sum
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();
}
}Find Max
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);
}
}Find Min
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);
}
}Multi Array Concatenation
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;
}
}Reverse Array
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;
}
}Class
Creating Object
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();
}
}Get All Field Names
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;
}
}Get All Methods
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());
}
}Safe Cast
public class SafeCastSnippet {
/**
* Safely casts an object to the specified type.
*
* @param obj object to cast
* @param clazz target type
* @param <T> target type parameter
* @return optional containing the cast object, or empty if the object is not
* assignable to the target type
*/
public static <T> Optional<T> safeCast(final Object obj, final Class<T> clazz) {
return clazz.isInstance(obj) ? Optional.of(clazz.cast(obj)) : Optional.empty();
}
}Get All Public Field Names
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());
}
}Date
Add Days To Date
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;
}
}Date Difference
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);
}
}Encoding
Base64Decode
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()));
}
}Base64Encode
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());
}
}File
List All Files
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;
}
}List Directories
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);
}
}List Files In Directory
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);
}
}Read Lines
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));
}
}Zip Directory
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);
}
}
}
}Zip File
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);
}
}
}
}Zip Files
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
InputStream To String
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()));
}
}Read File Snippet
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());
}
}
}System
Get Environment Variable Or Default
public class GetEnvOrDefaultSnippet {
/**
* Read an environment variable or return a default value when it is missing.
*
* @param key environment variable name
* @param defaultValue value returned when the variable is missing
* @return environment variable value or default value
*/
public static String getEnvOrDefault(String key, String defaultValue) {
return System.getenv().getOrDefault(key, defaultValue);
}
}Math
Dice Throw
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 Rating
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;
}
}Even Odd
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";
}
}
}Factorial
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);
}
}Fibonacci
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);
}
}
/**
* Fibonacci series using dynamic programming. Works for larger ns as well.
*
* @param n given number
* @return fibonacci number for given n
*/
public static int fibonacciBig(int n) {
int previous = 0;
int current = 1;
for (int i = 0; i < n - 1; i++) {
int t = previous + current;
previous = current;
current = t;
}
return current;
}
/**
* 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);
}
}Greatest Common Divisor
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);
}
}Haversine Formula
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;
}
}Least Common Multiple
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
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;
}
}Natural Number Binary Conversion
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();
}
}Perform Lottery
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]);
}
}Prime Number
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;
}
}Random Number
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 Short && end instanceof Short) {
return (short) (start.shortValue()
+ random.nextInt(end.shortValue() - start.shortValue() + 1));
} else if (start instanceof Integer && end instanceof Integer) {
return start.intValue()
+ random.nextInt(end.intValue() - start.intValue() + 1);
} else if (start instanceof Long && end instanceof Long) {
return start.longValue()
+ (long) (random.nextDouble() * end.longValue() - start.longValue() + 1);
} else if (start instanceof Float && end instanceof Float) {
return start.floatValue()
+ random.nextFloat() * (end.floatValue() - start.floatValue());
} else if (start instanceof Double && end instanceof Double) {
return start.doubleValue()
+ random.nextDouble() * (end.doubleValue() - start.doubleValue());
} else {
throw new IllegalArgumentException("Invalid Numbers As Arguments "
+ start.getClass() + " and " + end.getClass());
}
}
}Square Root
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;
}
}Media
Capture Screen
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));
}
}Network
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());
}
}String
Anagram
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);
}
}Common Letters
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);
}
}Compare Version
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("\\.");
}
}Duplicate Character
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);
}
}Levenshtein Distance
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()];
}
}Lindenmayer System
public class LindenmayerSystemSnippet {
/**
* Generates an L-system string based on axiom, production rules, and a number of iterations.
*
* @param axiom initial string to begin the L-system
* @param productionRules map of character rules where each symbol can be replaced with a string
* @param iterations number of iterations to apply the production rules
* @return the generated string after all iterations
*/
public static String generateLindenmayerSystem(
String axiom,
Map<Character, String> productionRules,
int iterations
) {
String current = axiom;
for (int i = 0; i < iterations; i++) {
StringBuilder nextIteration = new StringBuilder(current.length() * 2);
// Replace each symbol with the corresponding production rule or the symbol itself
current.chars()
.mapToObj(c -> (char) c)
.forEach(symbol ->
nextIteration.append(
productionRules.getOrDefault(symbol, String.valueOf(symbol))
)
);
current = nextIteration.toString();
}
return current;
}
}Max Character Count
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;
}
}Palindrome Check
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;
}
}Reverse String
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();
}
}String To Date
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);
}
}KMP Substring Search Algorithm
public class KMPSubstringSearchSnippet {
/**
* Implements the Knuth-Morris-Pratt (KMP) algorithm to find the index of the first occurrence of a substring in a given text.
*
* @param text The text in which the substring is to be searched.
* @param pattern The substring pattern to search for.
* @return The index of the first occurrence of the pattern in the text, or -1 if the pattern is not found.
*/
public static int kmpSearch(String text, String pattern) {
if (pattern == null || pattern.length() == 0) {
return 0; // Trivial case: empty pattern
}
int[] lps = computeLPSArray(pattern);
int i = 0; // index for text
int j = 0; // index for pattern
while (i < text.length()) {
if (pattern.charAt(j) == text.charAt(i)) {
i++;
j++;
}
if (j == pattern.length()) {
return i - j; // Found pattern at index (i - j)
} else if (i < text.length() && pattern.charAt(j) != text.charAt(i)) {
if (j != 0) {
j = lps[j - 1]; // Use the LPS array to skip characters
} else {
i++; // If no match and j is 0, move to the next character in text
}
}
}
return -1; // Pattern not found
}
/**
* Computes the LPS (Longest Prefix Suffix) array for the pattern, which indicates the longest proper prefix which is also a suffix.
*
* @param pattern The pattern for which the LPS array is to be computed.
* @return The LPS array.
*/
private static int[] computeLPSArray(String pattern) {
int length = 0;
int i = 1;
int[] lps = new int[pattern.length()];
lps[0] = 0; // LPS for the first character is always 0
while (i < pattern.length()) {
if (pattern.charAt(i) == pattern.charAt(length)) {
length++;
lps[i] = length;
i++;
} else {
if (length != 0) {
length = lps[length - 1]; // Fall back to the previous LPS value
} else {
lps[i] = 0;
i++;
}
}
}
return lps;
}
}Format Bytes
public class FormatBytesSnippet {
/**
* Convert bytes into Human readable form.
*
* @param bytes The value in bytes needed to be converted
* @return String the converted human-readable form
*/
public static String formatBytes(long bytes) {
double kb = 1024;
double mb = kb * 1024;
double gb = mb * 1024;
double tb = gb * 1024;
if ((bytes >= 0) && (bytes < kb)) {
return bytes + " B";
} else if ((bytes >= kb) && (bytes < mb)) {
return String.format("%.2f KB", bytes / kb);
} else if ((bytes >= mb) && (bytes < gb)) {
return String.format("%.2f MB", bytes / mb);
} else if ((bytes >= gb) && (bytes < tb)) {
return String.format("%.2f GB", bytes / gb);
} else if (bytes >= tb) {
return String.format("%.2f TB", bytes / tb);
} else {
return "Invalid Input";
}
}
}Thread
Thread Pool
Thread
public class ThreadSnippet {
/**
* Creates and returns a new thread with the task assigned to it (task will be performed parallel to the main thread).
*
* @param task the task to be executed by this thread
* @return new thread with task assigned to it.
*/
public static Thread createThread(Runnable task) {
return new Thread(task);
}
}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());
}
}Damm Algorithm
public class DammSnippet {
/**
* Private constructor to prevent instantiation of utility class.
*/
private DammSnippet() {
throw new UnsupportedOperationException("Utility class - instantiation is not allowed.");
}
private static final int[][] matrix = new int[][] {
{ 0, 3, 1, 7, 5, 9, 8, 6, 4, 2 },
{ 7, 0, 9, 2, 1, 5, 4, 8, 6, 3 },
{ 4, 2, 0, 6, 8, 7, 1, 3, 5, 9 },
{ 1, 7, 5, 0, 9, 8, 3, 4, 2, 6 },
{ 6, 1, 2, 3, 0, 4, 5, 9, 7, 8 },
{ 3, 6, 7, 4, 2, 0, 9, 5, 8, 1 },
{ 5, 8, 6, 9, 7, 2, 0, 1, 3, 4 },
{ 8, 9, 4, 5, 3, 6, 2, 0, 1, 7 },
{ 9, 4, 3, 8, 6, 1, 7, 2, 0, 5 },
{ 2, 5, 8, 1, 4, 3, 6, 7, 9, 0 }
};
public static int calculateCheckSumDigit(String number) {
if (number == null || number.isEmpty()) {
throw new IllegalArgumentException("Input number cannot be null or empty.");
}
int interim = 0;
for (int index = 0; index < number.length(); index++) {
char currCh = number.charAt(index);
if (!Character.isDigit(currCh)) {
throw new IllegalArgumentException("Input number contains invalid characters: " + number);
}
int currentIndex = currCh - '0';
interim = matrix[interim][currentIndex];
}
return interim;
}
public static int calculateCheckSumDigit(int number) {
return calculateCheckSumDigit(String.valueOf(number));
}
public static int calculateCheckSumDigit(long number) {
return calculateCheckSumDigit(String.valueOf(number));
}
public static String generateCheckSum(String number) {
int checkSumDigit = calculateCheckSumDigit(number);
return number + checkSumDigit;
}
public static int generateCheckSum(int number) {
int checkSumDigit = calculateCheckSumDigit(number);
return (number * 10) + checkSumDigit;
}
public static long generateCheckSum(long number) {
int checkSumNumber = calculateCheckSumDigit(number);
return (number * 10) + checkSumNumber;
}
public static boolean validate(String number) {
return calculateCheckSumDigit(number) == 0;
}
public static boolean validate(int number) {
return calculateCheckSumDigit(number) == 0;
}
public static boolean validate(long number) {
return calculateCheckSumDigit(number) == 0;
}
}