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生成四位随即验证码

String str = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";

StringBuilder s = new StringBuilder();

int len = 4;

for(int i=0;i<=len;i++){

Random r = new Random();

int p = r.nextInt(str.length());

s.append(str.charAt(p));

}

System.out.println(s.toString());

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