思路：

1. 写四个方法：加减乘除
2. 使用循环和switch进行用户交互
3. 传递需要操作的两个数
4. 输出结果

import java.util.Scanner;

public class Calculate {
    public static void main(String[] args) {
        while(true){
            Scanner scanner = new Scanner(System.in);
            System.out.print("请输入第一个数：");
            double num1 = scanner.nextDouble();
            System.out.print("请输入要运用得四则运算（+，-，*，/）：");
            String ch = scanner.next();
            System.out.print("请输入第二个数：");
            double num2 = scanner.nextDouble();
            switch(ch){
                case "+":
                    System.out.println(num1+ch+num2+"="+add(num1,num2));
                    break;
                case "-":
                    System.out.println(num1+ch+num2+"="+subtract(num1,num2));
                    break;
                case "*":
                    System.out.println(num1+ch+num2+"="+multiply(num1,num2));
                    break;
                case "/":
                    if(num2 == 0.0){
                        System.out.println("除数不能为0！");
                    }else{
                        System.out.println(num1+ch+num2+"="+divide(num1,num2));
                    }
                    break;
                default:
                    System.out.println("使用的不是四则运算！");
            }

        }
    }
    //加
    public static double add(double num1, double num2){
        double result = 0.0;
        result = num1 + num2;
        return result;
    }
    //减
    public static double subtract(double num1, double num2){
        double result = 0.0;
        result = num1 - num2;
        return result;
    }
    //乘
    public static double multiply(double num1, double num2){
        double result = 0.0;
        result = num1 * num2;
        return result;
    }
    //除
    public static double divide(double num1, double num2){
        double result = 0.0;
        if(num2==0.0){//注意除数不能为0
            System.out.println("Error!");
            return 0;
        }else{
            result = num1 / num2;
            return result;
        }
    }
}