当返回的数字没有正确结果时，解决方案！

1.

public class Homework06 {
    public static void main(String[] args) {

        Cale cale = new Cale(2,0);
        System.out.println("和=" + cale.sum());
        System.out.println("差=" + cale.minus());
        System.out.println("乘积=" + cale.mul());
        Double divRes = cale.div();                          //用Double或者其他包装类返回一个null值！！！
        if (divRes != null) {                                //通过null来判断到底有没有正确的结果
            System.out.println("除=" + cale.div());
        }
    }
}
class Cale{
    double num1;
    double num2;
    public Cale(double num1,double num2){
        this.num1 = num1;
        this.num2 = num2;
    }
    public double sum(){
        return num1 + num2;
    }
    public double minus(){
        return num1 - num2;
    }
    public double mul(){
        return num1 * num2;
    }
    public Double div(){
        if (num2 == 0){
            System.out.println("num2不能为0");
            return null;
        }else {
            return num1 / num2;
        }
    }
}