用java写一个二分法

好的，下面是用Java写一个二分法的示例代码：

public class BinarySearch {
    public static int binarySearch(int[] arr, int target) {
        int left = 0;
        int right = arr.length - 1;

        while (left <= right) {
            int mid = left + (right - left) / 2;
            
            if (arr[mid] == target) {
                return mid;
            } else if (arr[mid] < target) {
                left = mid + 1;
            } else {
                right = mid - 1;
            }
        }

        return -1;
    }

    public static void main(String[] args) {
        int[] arr = {1, 2, 3, 4, 5, 6, 7, 8, 9};
        int target = 4;
        int result = binarySearch(arr, target);

        if (result == -1) {
            System.out.println("Element not present");
        } else {
            System.out.println("Element found at index " + result);
        }
    }
}

该代码会在一个有序的整数数组中查找一个给定的数，如果找到则返回其索引，否则返回 -1。二分法是通过不断缩小查找区间的范围来进行查找，时间复杂度为 O(log n)。