哪些意想不到的空指针异常

经验丰富的程序员知道很多地方会出现空指针异常

public class Demo {
    Integer num;
    public Integer getNum() {
        return num;
    }
    public void setNum(Integer num) {
        this.num = num;
    }
    public static void main(String[] args) {
        String a = null;
        System.out.println(a.isEmpty());
        int c = new Demo().getNum();
        System.out.println(c);
    }
}
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请说出程序的执行结果