public class ArrayRandomData {
    char[] datas ;
    public ArrayRandomData() {
        String str = "abcdefghijklmnopqrstuvwxyz1234567890";
        datas = str.toCharArray();
    }
    public String getRandomData() {
        char[] rands = new char[4];
        int index;
        for(int i=0;i<4;i++) {
            index = (int)(Math.random()*datas.length);
            rands[i] = datas[index];
        }
        return new String(rands);
    }
    public static void main(String[] args) {
        ArrayRandomData ard = new ArrayRandomData();
        String randomData = ard.getRandomData();
        System.out.println("随机数是："+randomData);
    }
}