用Java实现：创建一个长度为n的随机字符串字符串，统计该字符串中每个字符出现的次数，并输出

以下是用Java实现的代码：

import java.util.HashMap;
import java.util.Random;

public class RandomString {

    public static void main(String[] args) {
        int n = 10; // 字符串长度为10
        String str = generateRandomString(n);
        System.out.println("随机字符串为：" + str);
        HashMap<Character, Integer> map = countCharacters(str);
        System.out.println("每个字符出现的次数为：");
        for (Character c : map.keySet()) {
            System.out.println(c + ": " + map.get(c));
        }
    }

    public static String generateRandomString(int n) {
        String str = "";
        String charSet = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
        Random random = new Random();
        for (int i = 0; i < n; i++) {
            int index = random.nextInt(charSet.length());
            str += charSet.charAt(index);
        }
        return str;
    }

    public static HashMap<Character, Integer> countCharacters(String str) {
        HashMap<Character, Integer> map = new HashMap<>();
        for (int i = 0; i < str.length(); i++) {
            char c = str.charAt(i);
            if (map.containsKey(c)) {
                int count = map.get(c);
                map.put(c, count + 1);
            } else {
                map.put(c, 1);
            }
        }
        return map;
    }
}

程序运行结果示例：

随机字符串为：Jf9X9KjJx8
每个字符出现的次数为：
f: 1
8: 1
K: 1
9: 2
x: 1
J: 2
j: 1
X: 1