Arrays的排序算法sort及方法使用

　　Java工具包中的Arrays工具类里面有数组的快速排序算法。

源码如下：

/**
     * Sorts the specified range of the array using the given
     * workspace array slice if possible for merging
     *
     * @param a the array to be sorted
     * @param left the index of the first element, inclusive, to be sorted
     * @param right the index of the last element, inclusive, to be sorted
     * @param work a workspace array (slice)
     * @param workBase origin of usable space in work array
     * @param workLen usable size of work array
     */
    static void sort(int[] a, int left, int right,
                     int[] work, int workBase, int workLen) {
        // Use Quicksort on small arrays
        if (right - left < QUICKSORT_THRESHOLD) {
            sort(a, left, right, true);
            return;
        }

        /*
         * Index run[i] is the start of i-th run
         * (ascending or descending sequence).
         */
        int[] run = new int[MAX_RUN_COUNT + 1];
        int count = 0; run[0] = left;

        // Check if the array is nearly sorted
        for (int k = left; k < right; run[count] = k) {
            if (a[k] < a[k + 1]) { // ascending
                while (++k <= right && a[k - 1] <= a[k]);
            } else if (a[k] > a[k + 1]) { // descending
                while (++k <= right && a[k - 1] >= a[k]);
                for (int lo = run[count] - 1, hi = k; ++lo < --hi; ) {
                    int t = a[lo]; a[lo] = a[hi]; a[hi] = t;
                }
            } else { // equal
                for (int m = MAX_RUN_LENGTH; ++k <= right && a[k - 1] == a[k]; ) {
                    if (--m == 0) {
                        sort(a, left, right, true);
                        return;
                    }
                }
            }

            /*
             * The array is not highly structured,
             * use Quicksort instead of merge sort.
             */
            if (++count == MAX_RUN_COUNT) {
                sort(a, left, right, true);
                return;
            }
        }

        // Check special cases
        // Implementation note: variable "right" is increased by 1.
        if (run[count] == right++) { // The last run contains one element
            run[++count] = right;
        } else if (count == 1) { // The array is already sorted
            return;
        }

        // Determine alternation base for merge
        byte odd = 0;
        for (int n = 1; (n <<= 1) < count; odd ^= 1);

        // Use or create temporary array b for merging
        int[] b;                 // temp array; alternates with a
        int ao, bo;              // array offsets from 'left'
        int blen = right - left; // space needed for b
        if (work == null || workLen < blen || workBase + blen > work.length) {
            work = new int[blen];
            workBase = 0;
        }
        if (odd == 0) {
            System.arraycopy(a, left, work, workBase, blen);
            b = a;
            bo = 0;
            a = work;
            ao = workBase - left;
        } else {
            b = work;
            ao = 0;
            bo = workBase - left;
        }

        // Merging
        for (int last; count > 1; count = last) {
            for (int k = (last = 0) + 2; k <= count; k += 2) {
                int hi = run[k], mi = run[k - 1];
                for (int i = run[k - 2], p = i, q = mi; i < hi; ++i) {
                    if (q >= hi || p < mi && a[p + ao] <= a[q + ao]) {
                        b[i + bo] = a[p++ + ao];
                    } else {
                        b[i + bo] = a[q++ + ao];
                    }
                }
                run[++last] = hi;
            }
            if ((count & 1) != 0) {
                for (int i = right, lo = run[count - 1]; --i >= lo;
                    b[i + bo] = a[i + ao]
                );
                run[++last] = right;
            }
            int[] t = a; a = b; b = t;
            int o = ao; ao = bo; bo = o;
        }
    }

　　java.util.Arrays类能方便的操作数组，它所有的方法都是静态的。

　　1.filll方法 ：给数组中的某段元素附上相同值。

　　2.sort方法：对数组中某段元素排序。

　　3.equals方法：比较两个数组，判断的是数组中元素值是否相等。

　　4.binarySearch方法：对排过序的数组进行二分法查找。

测试用例：

package recursion;

import java.util.Arrays;

/**
 * @author zsh
 * @company wlgzs
 * @create 2019-02-17 9:33
 * @Describe Arrays方法测试
 */
public class TestForArrays {

    public static void main(String[] args) {
        //填充数组，将arr[]中所有元素的值初始为0
        int[] arr = new int[5];
        Arrays.fill(arr,12);
        System.out.println(Arrays.toString(arr));
        //将arr中的第2个到第三个元素的值赋为8
        Arrays.fill(arr,1,3,8);
        System.out.println(Arrays.toString(arr));
        //对数组进行排序
        int[] arr1 = new int[]{7,6,8,5,2,9,8,1,3,5};
        //对数组的第二个到第6个元素进行排序
        Arrays.sort(arr1,1,6);
        System.out.println(Arrays.toString(arr1));
        //对整个数组进行排序
        Arrays.sort(arr1);
        System.out.println(Arrays.toString(arr1));
        //比较数组元素是否相等
        System.out.println(Arrays.equals(arr,arr1));
        //使用二分法在数组中查找指定元素所在的下标
        //数组必须是先排好序的
        System.out.println(Arrays.binarySearch(arr1,5));
        //如果不存在，就返回负数
        System.out.println(Arrays.binarySearch(arr1,20));
    }


}

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