Java随机生成密码（包含大小写字母和数字）

输入：密码的长度n

输出：随机密码，其中生成的随机密码必须包含大写字母，小写字母和数字。

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Random;

public class Main {

    static final char[] low = {'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s'
            ,'t','u','v','w','x','y','z'};
    static final char[] high = { 'A','B','C','D','E','F','G','H','I','J','K','L','M','N'
            ,'O','P','Q','R','S','T','U','V','W','X','Y','Z'};
    static final char[] num = {'0','1','2','3','4','5','6','7','8','9'};

    private static char getRandomChar(char[] str) {
        Random r = new Random();
        return str[r.nextInt(str.length)];
    }

    private static char getLowChar() {
        return getRandomChar(low);
    }

    private static char getUpperChar() {
        return Character.toUpperCase(getLowChar());
    }

    private static char getNumChar() {
        return getRandomChar(num);
    }

    private static char getRandomChar(int funNum) {
        switch (funNum) {
            case 0:
                return getLowChar();
            case 1:
                return getUpperChar();
            default:
                return getNumChar();
        }
    }

    public static String get(int len){
        List list = new ArrayList<>(len);
        list.add(getLowChar());
        list.add(getUpperChar());
        list.add(getNumChar());

        for (int i = 3; i < len; i++) {
            Random r = new Random();
            int n = r.nextInt(3);
            list.add(getRandomChar(n));
        }
        Collections.shuffle(list);
        StringBuilder ans = new StringBuilder(list.size());
        for (Character c : list) {
            ans.append(c);
        }
        return ans.toString();
    }

    public static void main(String[] args) {
        System.out.println(get(5));
    }
}