Curated collection of useful little Java 8 functions that you can understand quickly.
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chunkcountOccurrencesdeepFlattendifferencedifferenceWithdistinctValuesOfArraydropElementsdropRighteveryNthfilterNonUniqueflattenflattenDepthgroupByheadinitialinitializeArrayWithRangeinitializeArrayWithValuesintersectionisSortedjoinnthElementpickreducedFilterremovesamplesampleSizeshufflesimilaritysortedIndexsymmetricDifferencetailtaketakeRightunionwithoutzipzipObject
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Chunks an array into smaller arrays of specified size.
public static int[][] chunk(int[] numbers, int size) {
return IntStream.iterate(0, i -> i + size)
.limit((long) Math.ceil((double) numbers.length / size))
.mapToObj(cur -> Arrays.copyOfRange(numbers, cur, cur + size > numbers.length ? numbers.length : cur + size))
.toArray(int[][]::new);
}public static <T> T[] concat(T[] first, T[] second) {
return Stream.concat(
Stream.of(first),
Stream.of(second)
).toArray(i -> (T[]) Arrays.copyOf(new Object[0], i, first.getClass()));
}Counts the occurrences of a value in an array.
Use Array.stream().filter().count() to count total number of values that equals the specified value.
public static long countOccurrences(int[] numbers, int value) {
return Arrays.stream(numbers)
.filter(number -> number == value)
.count();
}Deep flattens an array.
Use recursion. Use Array.stream().flatMapToInt()
public static int[] deepFlatten(Object[] input) {
return Arrays.stream(input)
.flatMapToInt(o -> {
if (o instanceof Object[]) {
return Arrays.stream(deepFlatten((Object[]) o));
}
return IntStream.of((Integer) o);
}).toArray();
}Returns the difference between two arrays.
Create a Set from b, then use Array.stream().filter() on a to only keep values not contained in b.
public static int[] difference(int[] first, int[] second) {
Set<Integer> set = Arrays.stream(second).boxed().collect(Collectors.toSet());
return Arrays.stream(first)
.filter(v -> !set.contains(v))
.toArray();
}Filters out all values from an array for which the comparator function does not return true.
The comparator for int is implemented using IntBinaryOperator function.
Uses Array.stream().filter and Array.stream().noneMatch() to find the appropriate values.
public static int[] differenceWith(int[] first, int[] second, IntBinaryOperator comparator) {
return Arrays.stream(first)
.filter(a ->
Arrays.stream(second)
.noneMatch(b -> comparator.applyAsInt(a, b) == 0)
).toArray();
}Returns all the distinct values of an array.
Uses Array.stream().distinct() to discard all duplicated values.
public static int[] distinctValuesOfArray(int[] elements) {
return Arrays.stream(elements).distinct().toArray();
}Removes elements in an array until the passed function returns true. Returns the remaining elements in the array.
Loop through the array, using Arrays.copyOfRange() to drop the first element of the array until the returned value from the function is true. Returns the remaining elements.
public static int[] dropElements(int[] elements, IntPredicate condition) {
while (elements.length > 0 && !condition.test(elements[0])) {
elements = Arrays.copyOfRange(elements, 1, elements.length);
}
return elements;
}Returns a new array with n elements removed from the right.
Check if n is shorter than the given array and use Array.copyOfRange() to slice it accordingly or return an empty array.
public static int[] dropRight(int[] elements, int n) {
if (n < 0) {
throw new IllegalArgumentException("n is less than 0");
}
return n < elements.length
? Arrays.copyOfRange(elements, 0, elements.length - n)
: new int[0];
}Returns every nth element in an array.
Use IntStream.range().filter() to create a new array that contains every nth element of a given array.
public static int[] everyNth(int[] elements, int nth) {
return IntStream.range(0, elements.length)
.filter(i -> i % nth == nth - 1)
.map(i -> elements[i])
.toArray();
}Find index of element in the array. Return -1 in case element does not exist.
Uses IntStream.range().filter() to find index of the element in the array.
public static int indexOf(int[] elements, int el) {
return IntStream.range(0, elements.length)
.filter(idx -> elements[idx] == el)
.findFirst()
.orElse(-1);
}Find last index of element in the array. Return -1 in case element does not exist.
Uses IntStream.iterate().limit().filter() to find index of the element in the array.
public static int lastIndexOf(int[] elements, int el) {
return IntStream.iterate(elements.length - 1, i -> i - 1)
.limit(elements.length)
.filter(idx -> elements[idx] == el)
.findFirst()
.orElse(-1);
}Filters out the non-unique values in an array.
Use Array.stream().filter() for an array containing only the unique values.
public static int[] filterNonUnique(int[] elements) {
return Arrays.stream(elements)
.filter(el -> indexOf(elements, el) == lastIndexOf(elements, el))
.toArray();
}Flattens an array.
Use Array.stream().flatMapToInt().toArray() to create a new array.
public static int[] flatten(Object[] elements) {
return Arrays.stream(elements)
.flatMapToInt(el -> el instanceof int[]
? Arrays.stream((int[]) el)
: IntStream.of((int) el)
).toArray();
}Flattens an array up to the specified depth.
public static Object[] flattenDepth(Object[] elements, int depth) {
if (depth == 0) {
return elements;
}
return Arrays.stream(elements)
.flatMap(el -> el instanceof Object[]
? Arrays.stream(flattenDepth((Object[]) el, depth - 1))
: Arrays.stream(new Object[]{el})
).toArray();
}Groups the elements of an array based on the given function.
Uses Arrays.stream().collect(Collectors.groupingBy()) to group based on the grouping function.
public static <T, R> Map<R, List<T>> groupBy(T[] elements, Function<T, R> func) {
return Arrays.stream(elements).collect(Collectors.groupingBy(func));
}Returns all the elements of an array except the last one. Use Arrays.copyOfRange() to return all except the last one
public static <T> T[] initial(T[] elements) {
return Arrays.copyOfRange(elements, 0, elements.length - 1);
}Initializes an array containing the numbers in the specified range where start and end are inclusive.
public static int[] initializeArrayWithRange(int end, int start) {
return IntStream.rangeClosed(start, end).toArray();
}Initializes and fills an array with the specified values.
public static int[] initializeArrayWithValues(int n, int value) {
return IntStream.generate(() -> value).limit(n).toArray();
}Returns a list of elements that exist in both arrays.
Create a Set from second, then use Array.stream().filter() on a to only keep values contained in b.
public static int[] intersection(int[] first, int[] second) {
Set<Integer> set = Arrays.stream(second).boxed().collect(Collectors.toSet());
return Arrays.stream(first)
.filter(set::contains)
.toArray();
}Returns 1 if the array is sorted in ascending order, -1 if it is sorted in descending order or 0 if it is not sorted.
Calculate the ordering direction for the first two elements.Use for loop to iterate over array items and compare them in pairs. Return 0 if the direction changes or the direction if the last element is reached.
public static <T extends Comparable<? super T>> int isSorted(T[] arr) {
final int direction = arr[0].compareTo(arr[1]) < 0 ? 1 : -1;
for (int i = 0; i < arr.length; i++) {
T val = arr[i];
if (i == arr.length - 1) return direction;
else if ((val.compareTo(arr[i + 1]) * direction > 0)) return 0;
}
return direction;
}Joins all elements of an array into a string and returns this string. Uses a separator and an end separator.
Use IntStream.range to zip index with the array item. Then, use Stream.reduce to combine elements into a string.
public static <T> String join(T[] arr, String separator, String end) {
return IntStream.range(0, arr.length)
.mapToObj(i -> new SimpleEntry<>(i, arr[i]))
.reduce("", (acc, val) -> val.getKey() == arr.length - 2
? acc + val.getValue() + end
: val.getKey() == arr.length - 1 ? acc + val.getValue() : acc + val.getValue() + separator, (fst, snd) -> fst);
}Returns the nth element of an array.
Use Arrays.copyOfRange() to get an array containing the nth element at the first place.
public static <T> T nthElement(T[] arr, int n) {
if (n > 0) {
return Arrays.copyOfRange(arr, n, arr.length)[0];
}
return Arrays.copyOfRange(arr, arr.length + n, arr.length)[0];
}Picks the key-value pairs corresponding to the given keys from an object.
Use Array.stream to filter all the keys that are present in the arr. Then, convert all the keys present into a Map using Collectors.toMap.
public static <T, R> Map<T, R> pick(Map<T, R> obj, T[] arr) {
return Arrays.stream(arr)
.filter(obj::containsKey)
.collect(Collectors.toMap(k -> k, obj::get));
}Filter an array of objects based on a condition while also filtering out unspecified keys.
Use Arrays.stream().filter() to filter the array based on the predicate fn so that it returns the objects for which the condition is true. For each filtered Map object, create a new Map with keys present in the keys. Finally, collect all the Map object into an array.
public static Map<String, Object>[] reducedFilter(Map<String, Object>[] data, String[] keys, Predicate<Map<String, Object>> fn) {
return Arrays.stream(data)
.filter(fn)
.map(el -> Arrays.stream(keys).filter(el::containsKey)
.collect(Collectors.toMap(Function.identity(), el::get)))
.toArray((IntFunction<Map<String, Object>[]>) Map[]::new);
}Returns a random element from an array.
Use Math.random() to generate a random number, multiply it by length and round it of to the nearest whole number using Math.floor(). This method also works with strings.
public static <T> T sample(T[] arr) {
return arr[(int) Math.floor(Math.random() * arr.length)];
}Gets n random elements at unique keys from array up to the size of array.
Shuffle the array using the Fisher-Yates algorithm. Use Array.copyOfRange() to get the first n elements.
public static <T> T[] sampleSize(T[] input, int n) {
T[] arr = Arrays.copyOf(input, input.length);
int length = arr.length;
int m = length;
while (m > 0) {
int i = (int) Math.floor(Math.random() * m--);
T tmp = arr[i];
arr[i] = arr[m];
arr[m] = tmp;
}
return Arrays.copyOfRange(arr, 0, n > length ? length : n);
}Randomizes the order of the values of an array, returning a new array.
Uses the Fisher-Yates algorithm to reorder the elements of the array.
public static <T> T[] shuffle(T[] input) {
T[] arr = Arrays.copyOf(input, input.length);
int length = arr.length;
int m = length;
while (m > 0) {
int i = (int) Math.floor(Math.random() * m--);
T tmp = arr[i];
arr[i] = arr[m];
arr[m] = tmp;
}
return arr;
}Returns an array of elements that appear in both arrays.
Use Arrays.stream().filter() to remove values that are not part of second, determined using Arrays.stream().anyMatch().
public static <T> T[] similarity(T[] first, T[] second) {
return Arrays.stream(first)
.filter(a -> Arrays.stream(second).anyMatch(b -> Objects.equals(a, b)))
// Make a new array of first's runtime type, but empty content:
.toArray(i -> (T[]) Arrays.copyOf(new Object[0], i, first.getClass()));
}Returns the lowest index at which value should be inserted into array in order to maintain its sort order.
Check if the array is sorted in descending order (loosely). Use IntStream.range().filter() to find the appropriate index where the element should be inserted.
public static <T extends Comparable<? super T>> int sortedIndex(T[] arr, T el) {
boolean isDescending = arr[0].compareTo(arr[arr.length - 1]) > 0;
return IntStream.range(0, arr.length)
.filter(i -> isDescending ? el.compareTo(arr[i]) >= 0 : el.compareTo(arr[i]) <= 0)
.findFirst()
.orElse(arr.length);
}Returns the symmetric difference between two arrays.
Create a Set from each array, then use Array.stream().filter() on each of them to only keep values not contained in the other. Finally, concatenate both arrays and create a new array and return it.
public static <T> T[] symmetricDifference(T[] first, T[] second) {
Set<T> sA = new HashSet<>(Arrays.asList(first));
Set<T> sB = new HashSet<>(Arrays.asList(second));
return Stream.concat(
Arrays.stream(first).filter(a -> !sB.contains(a)),
Arrays.stream(second).filter(b -> !sA.contains(b))
).toArray(i -> (T[]) Arrays.copyOf(new Object[0], i, first.getClass()));
}Returns all elements in an array except for the first one.
Return Arrays.copyOfRange(1) if the array's length is more than 1, otherwise, return the whole array.
public static <T> T[] tail(T[] arr) {
return arr.length > 1
? Arrays.copyOfRange(arr, 1, arr.length)
: arr;
}Returns an array with n elements removed from the beginning.
public static <T> T[] take(T[] arr, int n) {
return Arrays.copyOfRange(arr, 0, n);
}Returns an array with n elements removed from the end.
Use Arrays.copyOfRange() to create a slice of the array with n elements taken from the end.
public static <T> T[] takeRight(T[] arr, int n) {
return Arrays.copyOfRange(arr, arr.length - n, arr.length);
}Returns every element that exists in any of the two arrays once.
Create a Set with all values of a and b and convert to an array.
public static <T> T[] union(T[] first, T[] second) {
Set<T> set = new HashSet<>(Arrays.asList(first));
set.addAll(Arrays.asList(second));
return set.toArray((T[]) Arrays.copyOf(new Object[0], 0, first.getClass()));
}Filters out the elements of an array, that have one of the specified values.
Use Arrays.strean().filter() to create an array excluding(using !Arrays.asList(elements).contains()) all given values.
public static <T> T[] without(T[] arr, T... elements) {
List<T> excludeElements = Arrays.asList(elements);
return Arrays.stream(arr)
.filter(el -> !excludeElements.contains(el))
.toArray(i -> (T[]) Arrays.copyOf(new Object[0], i, arr.getClass()));
}Creates an array of elements, grouped based on the position in the original arrays.
public static List<Object[]> zip(Object[]... arrays) {
OptionalInt max = Arrays.stream(arrays).mapToInt(arr -> arr.length).max();
return IntStream.range(0, max.getAsInt())
.mapToObj(i -> Arrays.stream(arrays)
.map(arr -> i < arr.length ? arr[i] : null)
.toArray())
.collect(Collectors.toList());
}Given an array of valid property identifiers and an array of values, return an object associating the properties to the values.
public static Map<String, Object> zipObject(String[] props, Object[] values) {
return IntStream.range(0, props.length)
.mapToObj(i -> new SimpleEntry<>(props[i], i < values.length ? values[i] : null))
.collect(
HashMap::new, (m, v) -> m.put(v.getKey(), v.getValue()), HashMap::putAll);
}Returns the average of an of two or more numbers.
public static double average(int[] arr) {
return IntStream.of(arr)
.average()
.orElseThrow(() -> new IllegalArgumentException("Array is empty"));
}Calculates the greatest common denominator (gcd) of an array of numbers.
Use Array.stream().reduce() and the gcd formula (uses recursion) to calculate the greatest common denominator of an array of numbers.
public static OptionalInt gcd(int[] numbers) {
return Arrays.stream(numbers)
.reduce((a, b) -> gcd(a, b));
}
private static int gcd(int a, int b) {
if (b == 0) {
return a;
}
return gcd(b, a % b);
}Calculates the lowest common multiple (lcm) of an array of numbers.
Use Array.stream().reduce() and the lcm formula (uses recursion) to calculate the lowest common multiple of an array of numbers.
public static OptionalInt lcm(int[] numbers) {
IntBinaryOperator lcm = (x, y) -> (x * y) / gcd(x, y);
return Arrays.stream(numbers)
.reduce((a, b) -> lcm.applyAsInt(a, b));
}
private static int gcd(int a, int b) {
if (b == 0) {
return a;
}
return gcd(b, a % b);
}Finds the next power of two greater than or equal to the value.
This method uses left ship operator to shift 1 by the value on the right side. The right side is calculated using the Integer.numberOfLeadingZeros method.
The Integer.numberOfLeadingZeros give the number of zeros leading the value. For example, calling Integer.numberOfLeadingZeros(3) would give value as 30. This is because 3 is represented in binary as 11. As integer has 32 bits, so there are 30 bits with 0. The right side of the left shift operator becomes 32-30 = 2. Left shifting 1 by 2 i.e. 001 << 2 would be 100. 100 in decimal is equal to 4.
public static int findNextPositivePowerOfTwo(int value) {
return 1 << (32 - Integer.numberOfLeadingZeros(value - 1));
}Check if the number is even.
This method uses bitwise & operator. The 0b1 is the binary representation of 1. Since, Java 7 you can write binary literals by prefixing them with either 0b or 0B. The & operator will return 0 when number is even. For example, IsEven(4) would result in 100 & 001. The result of & will be 000.
public static boolean isEven(final int value) {
return (value & 0b1) == 0;
}Checks if a value is positive power of two.
To understand how it works let's assume we made a call IsPowerOfTwo(4).
As value is greater than 0, so right side of the && operator will be evaluated.
The result of (~value + 1) is equal to value itself. ~100 + 001 => 011 + 001 => 100. This is equal to value.
The result of (value & value) is value. 100 & 100 => 100.
This will value the expression to true as value is equal to value.
public static boolean isPowerOfTwo(final int value) {
return value > 0 && ((value & (~value + 1)) == value);
}Generate a random integer between Integer.MIN_VALUE and Integer.MAX_VALUE.
public static int generateRandomInt() {
return ThreadLocalRandom.current().nextInt();
}Generates all anagrams of a string (contains duplicates).
public static List<String> anagrams(String input) {
if (input.length() <= 2) {
return input.length() == 2
? Arrays.asList(input, input.substring(1) + input.substring(0, 1))
: Collections.singletonList(input);
}
return IntStream.range(0, input.length())
.mapToObj(i -> new SimpleEntry<>(i, input.substring(i, i + 1)))
.flatMap(entry ->
anagrams(input.substring(0, entry.getKey()) + input.substring(entry.getKey() + 1))
.stream()
.map(s -> entry.getValue() + s))
.collect(Collectors.toList());
}Returns the length of a string in bytes.
public static int byteSize(String input) {
return input.getBytes().length;
}Capitalizes the first letter of a string.
public static String capitalize(String input, boolean lowerRest) {
return input.substring(0, 1).toUpperCase() +
(lowerRest
? input.substring(1, input.length()).toLowerCase()
: input.substring(1, input.length()));
}Capitalizes the first letter of every word in a string.
public static String capitalizeEveryWord(final String input) {
return Pattern.compile("\\b(?=\\w)").splitAsStream(input)
.map(w -> capitalize(w, false))
.collect(Collectors.joining());
}Retuns number of vowels in provided string.
public static int countVowels(String input) {
return input.replaceAll("[^aeiouAEIOU]", "").length();
}Escapes a string to use in a regular expression.
public static String escapeRegExp(String input) {
return Pattern.quote(input);
}Converts a string from camelcase.
public static String fromCamelCase(String input, String separator) {
return input
.replaceAll("([a-z\\d])([A-Z])", "$1" + separator + "$2")
.toLowerCase();
}Returns true if the given string is an absolute URL, false otherwise.
public static boolean isAbsoluteUrl(String url) {
return Pattern.compile("^[a-z][a-z0-9+.-]*:").matcher(url).find();
}Checks if a string is lower case.
public static boolean isLowerCase(String input) {
return Objects.equals(input, input.toLowerCase());
}Checks if a string is upper case.
public static boolean isUpperCase(String input) {
return Objects.equals(input, input.toUpperCase());
}Checks if a string is palindrome.
public static boolean isPalindrome(String input) {
String s = input.toLowerCase().replaceAll("[\\W_]", "");
return Objects.equals(
s,
new StringBuilder(s).reverse().toString()
);
}Checks if a string is numeric.
public static boolean isNumeric(final String input) {
return IntStream.range(0, input.length())
.allMatch(i -> Character.isDigit(input.charAt(i)));
}Replaces all but the last num of characters with the specified mask character.
public static String mask(String input, int num, String mask) {
int length = input.length();
return num > 0
?
input.substring(0, length - num).replaceAll(".", mask)
+ input.substring(length - num)
:
input.substring(0, Math.negateExact(num))
+ input.substring(Math.negateExact(num), length).replaceAll(".", mask);
}Reverses a string.
public static String reverseString(String input) {
return new StringBuilder(input).reverse().toString();
}Alphabetically sorts the characters in a string.
public static String sortCharactersInString(String input) {
return Arrays.stream(input.split("")).sorted().collect(Collectors.joining());
}Splits a multiline string into an array of lines.
public static String[] splitLines(String input) {
return input.split("\\r?\\n");
}Converts a string to camelcase.
public static String toCamelCase(String input) {
Matcher matcher = Pattern.compile("[A-Z]{2,}(?=[A-Z][a-z]+[0-9]*|\\b)|[A-Z]?[a-z]+[0-9]*|[A-Z]|[0-9]+").matcher(input);
List<String> matchedParts = new ArrayList<>();
while (matcher.find()) {
matchedParts.add(matcher.group(0));
}
String s = matchedParts.stream()
.map(x -> x.substring(0, 1).toUpperCase() + x.substring(1).toLowerCase())
.collect(Collectors.joining());
return s.substring(0, 1).toLowerCase() + s.substring(1);
}Converts a string to kebab case.
public static String toKebabCase(String input) {
Matcher matcher = Pattern.compile("[A-Z]{2,}(?=[A-Z][a-z]+[0-9]*|\\b)|[A-Z]?[a-z]+[0-9]*|[A-Z]|[0-9]+").matcher(input);
List<String> matchedParts = new ArrayList<>();
while (matcher.find()) {
matchedParts.add(matcher.group(0));
}
return matchedParts.stream()
.map(String::toLowerCase)
.collect(Collectors.joining("-"));
}public static List<String> match(String input, String regex) {
Matcher matcher = Pattern.compile(regex).matcher(input);
List<String> matchedParts = new ArrayList<>();
while (matcher.find()) {
matchedParts.add(matcher.group(0));
}
return matchedParts;
}Converts a string to snake case.
public static String toSnakeCase(String input) {
Matcher matcher = Pattern.compile("[A-Z]{2,}(?=[A-Z][a-z]+[0-9]*|\\b)|[A-Z]?[a-z]+[0-9]*|[A-Z]|[0-9]+").matcher(input);
List<String> matchedParts = new ArrayList<>();
while (matcher.find()) {
matchedParts.add(matcher.group(0));
}
return matchedParts.stream()
.map(String::toLowerCase)
.collect(Collectors.joining("_"));
}Truncates a string up to a specified length.
public static String truncateString(String input, int num) {
return input.length() > num
? input.substring(0, num > 3 ? num - 3 : num) + "..."
: input;
}Converts a given string into an array of words.
public static String[] words(String input) {
return Arrays.stream(input.split("[^a-zA-Z-]+"))
.filter(s -> !s.isEmpty())
.toArray(String[]::new);
}Converts InputStream to a String.
public static String convertInputStreamToString(final InputStream in) throws IOException {
ByteArrayOutputStream result = new ByteArrayOutputStream();
byte[] buffer = new byte[1024];
int length;
while ((length = in.read(buffer)) != -1) {
result.write(buffer, 0, length);
}
return result.toString(StandardCharsets.UTF_8.name());
}Reads content of a file to a String
public String readFileAsString(Path path) throws IOException {
return new String(Files.readAllBytes(path));
}public static String getCurrentWorkingDirectoryPath() {
return FileSystems.getDefault().getPath("").toAbsolutePath().toString();
}Returns the value of java.io.tmpdir system property. It appends separator if not present at the end.
public static String tmpDirName() {
String tmpDirName = System.getProperty("java.io.tmpdir");
if (!tmpDirName.endsWith(File.separator)) {
tmpDirName += File.separator;
}
return tmpDirName;
}Converts exception stack trace to a String.
public static String stackTraceAsString(final Throwable throwable) {
final StringWriter sw = new StringWriter();
throwable.printStackTrace(new PrintWriter(sw));
return sw.toString();
}Gets the name of the operating system as a lower case String.
public static String osName() {
return System.getProperty("os.name").toLowerCase();
}Checks if debugger is attached to the JVM.
public static boolean isDebuggerAttached() {
final RuntimeMXBean runtimeMXBean = ManagementFactory.getRuntimeMXBean();
return runtimeMXBean.getInputArguments()
.stream()
.anyMatch(arg -> arg.contains("-agentlib:jdwp"));
}This project started as a Java fork of 30-seconds-of-code. Thanks to the project collaborators for the effort.