java向构造方法传递字符串的问题

Question

这是我的计算器类，目的是获取键盘输入的字符串运算式并计算出结果

```java
package chap10;
import static java.lang.Integer.parseInt;

public class Calculator {
    String str;
    public Calculator(String s) {
        str = s;
    }
    String s1 = str.replaceAll(" ", "");
    String operator =s1.replaceAll("\\d","");
    String[] s2 = s1.split("[\\+\\-\\*\\/]");
    public int calculate(){
        int result;
        int num2 = parseInt(s2[1]);
        int num1 = parseInt(s2[0]);
        switch(operator.charAt(0)) {
            case '+': result=num1+num2;
                break;
            case'-':result=num1-num2;
                break;
            case'*':result=num1*num2;
                break;
            case'/':result=num1/num2;
                break;
            default:
                throw new IllegalStateException("Unexpected value: " + operator.charAt(0));
        }
        return result;
    }

之后，是测试类

```java
package chap10;
import java.util.Scanner;
public class Ex1026 {
    public static void main(String[] args) {
        Scanner sc=new Scanner(System.in);
        System.out.println("Please input the expression:");
        Calculator calculator=new Calculator(sc.nextLine());
        System.out.println(calculator.calculate());
    }
}

输入之后，报了空指针异常。请问如何修改。

Answer 1

Scanner sc=new Scanner(System.in);
这一句出现的问题，你的参数来自于标准输入，你可以理解为来自控制台，你还没有输入呢，兄弟
Scanner sc=new Scanner(System.in);
while（sc.hasNext）{
String str = sc.next
//code
}
闲着没事。。。。。