01 30 seconds of java8

Array

chunk

将数组分割成特定大小的小数组:

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, Math.min(cur + size, numbers.length)))
.toArray(int[][]::new);
}

concat

两个数组合并:

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()));
}

countOccurrences

计算数组中某个值出现的次数:

public static long countOccurrences(int[] numbers, int value) {
return Arrays.stream(numbers)
.filter(e -> e == value)
.count();
}

deepFlatten

数组扁平化:

public static int[] deepFlatten(Object[] elements) {
return Arrays.stream(elements)
.flatMapToInt(e -> {
if (e instanceof Object[]) {
return Arrays.stream(deepFlatten(((Object[]) e)));
}
return IntStream.of((Integer) e);
}).toArray();
}

difference

求两个数组中的差集,同时也可以演变成求两个数组的交集:

public static int[] difference(int[] first, int[] second) {
Set<Integer> set = Arrays.stream(second).boxed().collect(Collectors.toSet());
return Arrays.stream(first)
.filter(e -> !set.contains(e))
.toArray();
}

distinctValuesOfArray

求两个数组的并集并去重,见 union

public static int[] distinctValuesOfArray(int[] first, int[] second) {
return IntStream.concat(IntStream.of(first), IntStream.of(second))
.distinct()
.toArray();
}

everyNth

返回数组中的每个第 n 个元素:

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();
}

indexOf

查找数组中元素的索引,在不存在元素的情况下返回-1:

public static int indexOf(int[] elements, int value) {
return IntStream.range(0, elements.length)
.filter(i -> elements[i] == value)
.findFirst()
.orElse(-1);
}

lastIndexOf

查找数组中元素的最后索引,在不存在元素的情况下返回-1:

public static int lastIndexOf(int[] elements, int value) {
return IntStream.iterate(elements.length - 1, i -> i - 1)
.limit(elements.length)
.filter(i -> elements[i] == value)
.findFirst()
.orElse(-1);
}

filterNonUnique

筛选出数组中的非唯一值:

public static int[] filterNonUnique(int[] elements) {
return Arrays.stream(elements)
.filter(e -> indexOf(elements, e) == lastIndexOf(elements, e))
.toArray();
}

initializeArrayWithRange

初始化一个数组,该数组包含在指定范围内的数字,传入 start 和 end:

public static int[] initializeArrayWithRange(int start, int end) {
return IntStream.rangeClosed(start, end).toArray();
}

initializeArrayWithValues

使用指定的值初始化并填充数组:

public static int[] initializeArrayWithValues(int n, int value) {
return IntStream.generate(() -> value).limit(n).toArray();
}

intersection

返回两个数组的交集,difference 微调,和 similarity 作用大同小异:

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();
}

nthElement

返回数组的第 n 个元素:

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];
}

pick

从对象中选择与给定键对应的键值对:

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));
}

sample

从数组中返回一个随机元素:

public static <T> T sample(T[] arr) {
return arr[(int) Math.floor(Math.random() * arr.length)];
}

shuffle

数组乱序,使用洗牌算法:

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;
}

sampleSize

从数组中获取 n 个随机元素,思路是先乱序再抽取 n 个元素:

public static <T> T[] sampleSize(T[] input, int n) {
T[] arr = shuffle(input);
return Arrays.copyOfRange(arr, 0, n > arr.length ? arr.length : n);
}

similarity

返回出现在两个数组中的元素数组,见 intersection

public static <T> T[] similarity(T[] first, T[] second) {
return Arrays.stream(first)
.filter(a -> Arrays.stream(second).anyMatch(b -> Objects.equals(a, b)))
.toArray(i -> (T[]) Arrays.copyOf(new Object[0], i, first.getClass()));
}

symmetricDifference

返回两个数组之间的差集:

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()));
}

union

返回两个数组的并集,见 distinctValuesOfArray

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()));
}

without

筛选出具有指定值之一的数组的元素:

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()));
}

Maths

average

求数组平均值:

public static double average(int[] arr) {
return IntStream.of(arr)
.average()
.orElseThrow(() -> new IllegalArgumentException("Array is empty"));
}

isEven

检查数字是否是偶数。这个方法使用按位运算符,0b1 是 1 的二进制表示。数字为偶数时, 运算符将返回 0。例如,IsEven(4) 会转换成 100 & 001,结果将是 000。

public static boolean isEven(final int value) {
return (value & 0b1) == 0;
}

generateRandomInt

生成一个介于 Integer.MIN_VALUEInteger.MAX_VALUE 之间的随机数:

public static int generateRandomInt() {
return ThreadLocalRandom.current().nextInt();
}

String

anagrams

一个字符串的所有可能排列组合:

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 AbstractMap.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());
}

isNumeric

检查字符串是否为数字:

public static boolean isNumeric(final String input) {
return IntStream.range(0, input.length())
.allMatch(i -> Character.isDigit(input.charAt(i)));
}

reverseString

反转字符串:

public static String reverseString(String input) {
return new StringBuilder(input).reverse().toString();
}

splitLines

将多行字符串拆分为行数组:

public static String[] splitLines(String input) {
return input.split("\\r?\\n");
}

stringToIntegers

将由空格分隔的数字字符串转换为 int 数组:

public static int[] stringToIntegers(String numbers) {
return Arrays.stream(numbers.split(" ")).mapToInt(Integer::parseInt).toArray();
}

Enum

getEnumMap

将枚举转换为 Map,其中 key 是枚举名,value 是枚举本身:

public static <E extends Enum<E>> Map<String, E> getEnumMap(final Class<E> enumClass) {
return Arrays.stream(enumClass.getEnumConstants())
.collect(Collectors.toMap(Enum::name, Function.identity()));
}

IO

readFileAsString

将文件内容读入字符串:

public String readFileAsString(Path path) throws IOException {
return new String(Files.readAllBytes(path));
}

getCurrentWorkingDirectoryPath

获取当前工作目录:

public static String getCurrentWorkingDirectoryPath() {
return FileSystems.getDefault().getPath("").toAbsolutePath().toString();
}

stackTraceAsString

将异常堆栈跟踪转换为字符串:

public static String stackTraceAsString(final Throwable throwable) {
final StringWriter sw = new StringWriter();
throwable.printStackTrace(new PrintWriter(sw));
return sw.toString();
}

参考文章: 30 seconds of java8