最近，我研究了可以加，减，乘和除的简单计算器。

public static void main(String arr[]){ 
    double num1, num2;
    String ans = null;

    System.out.println("Calculator manu:");
    System.out.println("\n"+"a for adding"); 
    System.out.println("b for substracting");
    System.out.println("c for multiplying");
    System.out.println("d for dividing");

    Scanner NumInput = new Scanner(System.in);

    System.out.println("\n"+"Enter number one: ");
    num1 = NumInput.nextDouble();

    System.out.println("\n"+"Enter number two: ");
    num2 = NumInput.nextDouble();

    while(true)
    {   
    Scanner AnsInput = new Scanner(System.in);
    System.out.println("\n"+"Choose operation.");
    String answer = AnsInput.next();


    if(AnsInput.equals("1"))
    {
        answer = Double.toString(num1+num2);
        System.out.println("\n"+"The sum of the first and second number is: "+answer);
    }
    else if(AnsInput.equals("2")) 
    {
        answer = Double.toString(num1-num2);
        System.out.println("\n"+"Subtraction of the first and the second number is: "+answer);
    }
    else if(AnsInput.equals("3"))
    {
        answer = Double.toString(num1*num2);
        System.out.println("\n"+"The product of the first and second number is: "+answer);
    }
    else if(AnsInput.equals("4"))
    {
        answer = Double.toString(num1/num2);
        System.out.println("\n"+"Ratio of the first and the second number is: "+answer);
    }
    }
}

但是，如果我想让程序像普通计算器一样工作，该怎么办？它加，减，乘，除，…，但不仅是两个，而是多个。