java数组相关实用方法Arrays

August 26, 2022

java 数组是特殊的类。数组本身继承自 Object，因此具有 clone(), equals(), hashCode(), toString(), getClass() 方法。
操作数组需要使用实用类 java.util.Arrays

java.util.Arrays

https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/util/Arrays.html

java.util.Arrays 实用类的所有方法都是静态方法。

Arrays.asList(T... a)

此方法既可以传入一个数组，也可以传入多个同类型参数以转换为列表

java

List<Integer> even = Arrays.asList(new Integer[]{0, 2, 4, 6});
System.out.println(even); // [0, 2, 4, 6]
List<Integer> odd = Arrays.asList(1, 3, 5, 7);
System.out.println(odd); // [1, 3, 5, 7]

Collection 的 Object[] toArray() 和 T[] toArray(T[] a) 方法

这两个方法是实例方法，用于将集合转换为数组。

java

Integer[] errs = (Integer[]) even.toArray(); // 报错（ClassCastException），无法转型  class [Ljava.lang.Object; cannot be cast to class [Ljava.lang.Integer;
Object[] objs = even.toArray(); // 不传参，转换为 Object[]
Integer[] ints_plus1 = even.toArray(new Integer[even.size() + 1]); // 传入参数以指定类型，声明的数组长度若大于集合长度，则多余元素为null
Integer[] ints_minus1 = even.toArray(new Integer[even.size() - 1]); // 传入参数以指定类型，声明的数组长度若小于或等于集合长度，数组长度仍为集合长度
System.out.println(Arrays.toString(ints_plus1)); // [0, 2, 4, 6, null]
System.out.println(Arrays.toString(ints_minus1)); // [0, 2, 4, 6]

Arrays.binarySearch()

注意，二分查找需要数组已经有序!

java

int[] arr = {1, 3, 5, 7, 9};
// 指定 key
System.out.println(Arrays.binarySearch(arr, 1)); // 0
System.out.println(Arrays.binarySearch(arr, 2)); // -2
System.out.println(Arrays.binarySearch(arr, 3)); // 1
System.out.println(Arrays.binarySearch(arr, 4)); // -3
// 指定 from, to, key
System.out.println(Arrays.binarySearch(arr, 1, 3, 1)); // -2

sort parallelSort

java

int[] arr1 = {3, 9, 6, 1};
int[] arr2 = arr1.clone();
Arrays.sort(arr1); // Timsort
Arrays.parallelSort(arr2); // 简而言之，多线程排序（归并）
System.out.println(Arrays.toString(arr1)); // [1, 3, 6, 9]
System.out.println(Arrays.toString(arr2)); // [1, 3, 6, 9]

copyOf copyOfRange equals hashCode toString

java

String[] arr = {"Hello", "World"};
String[] copied = Arrays.copyOf(arr, arr.length);
System.out.println(Arrays.equals(arr, copied)); // => true
String[] hello = Arrays.copyOfRange(arr, 0, 1);
System.out.println(Arrays.toString(hello)); // => [Hello]
System.out.println(Arrays.hashCode(arr)); // => -2053301055
System.out.println(Arrays.hashCode(copied)); // => -2053301055

deepEquals deepHashCode deepToString

java

String[][] arr = {{"Hello"}, {"World"}};

System.out.println(Arrays.toString(arr)); // => [[Ljava.lang.String;@16b98e56, [Ljava.lang.String;@7ef20235]
System.out.println(Arrays.deepToString(arr)); // => [[Hello], [World]]

System.out.println(Arrays.hashCode(arr)); // => 1063928416
System.out.println(Arrays.deepHashCode(arr)); // => -2053300063

String[] s1 = {"Hello"};
String[] s2 = {"World"};
String[][] composed = {s1, s2};
System.out.println(Arrays.equals(arr, composed)); // => false
System.out.println(Arrays.deepEquals(arr, composed)); // => true

compare compareUnsigned mismatch

java

import java.util.Arrays;
import java.util.Comparator;

public class Main {
    public static void main(String[] args) {
        String[] arr1 = {"Hello"};
        String[] arr2 = {"World"};
        System.out.println(Arrays.mismatch(arr1, arr2)); // 返回两个数组中第一个不一致元素的索引
        System.out.println(Arrays.mismatch(arr1, arr2, new StringComparator())); // 数组元素一致时，返回-1
        System.out.println(Arrays.compare(arr1, arr2)); // 比较字典序，返回 -1 | 0 | 1
        System.out.println(Arrays.compare(arr1, arr2, new StringComparator())); //  给定比较器
        System.out.println(Arrays.compareUnsigned(new int[]{1}, new int[]{2})); // 仅支持数字类型数组
    }
}

class StringComparator implements Comparator<String> {
    @Override
    public int compare(String s1, String s2) {
        return 0; // 仅供测试
    }
}

stream

另见：java 流

java

int[] fib = {0, 1, 1, 2, 3, 5, 8, 13, 21, 34};
IntStream s = Arrays.stream(fib);
s.forEach(System.out::println);

fill setAll

java

int[] arr = new int[3];
System.out.println(Arrays.toString(arr)); // => [0, 0, 0]
Arrays.fill(arr, 6);
System.out.println(Arrays.toString(arr)); // => [6, 6, 6]
Arrays.fill(arr, 0, 2, 8); // fromIndex, toIndex, val
System.out.println(Arrays.toString(arr)); // => [8, 8, 6]
Arrays.setAll(arr, new IntUnaryOperator() { // 通过索引设置数组元素
    @Override
    public int applyAsInt(int operand) {
        return operand * 2; // 0, 1, 2 => 0, 2, 4
    }
});
System.out.println(Arrays.toString(arr)); // [0, 2, 4]

spliterator

分割数组，支持并行处理

java

int[] arr = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
Spliterator s = Arrays.spliterator(arr);
Spliterator s1 = s.trySplit();
Spliterator s2 = s.trySplit();
var printEach = new Consumer<Integer>() {
    @Override
    public void accept(Integer i) {
        System.out.print(i);
    }
};
System.out.print("s1: ");
s1.forEachRemaining(printEach); // s1: 12345
System.out.print("\ns2: ");
s2.forEachRemaining(printEach); // s2: 67

parallelSort parallelPrefix parallelSetAll

parallel 系列函数用于多线程时，并行运算

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