代码：
```java
import java.util.Scanner;
public class Calculator {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
System.out.print("Enter first number: ");
double num1 = input.nextDouble();
System.out.print("Enter second number: ");
double num2 = input.nextDouble();
System.out.println("Select an operation (+, -, *, /): ");
String operator = input.next();
double result = 0;
switch(operator) {
case "+":
result = num1 + num2;
break;
case "-":
result = num1 - num2;
break;
case "*":
result = num1 * num2;
break;
case "/":
if(num2 == 0) {
System.out.println("Error: Cannot divide by zero");
return;
}
result = num1 / num2;
break;
default:
System.out.println("Invalid operator");
return;
}
System.out.println(num1 + " " + operator + " " + num2 + " = " + result);
}
}
```
作用：这个程序是一个简单的计算器，它从用户输入中获取两个数字和一个运算符，然后执行所选的运算，并输出结果。程序使用了Scanner类来读取用户输入，并使用switch语句来执行不同的运算。这个程序可以帮助初学者了解如何在Java中使用基本运算符和控制结构。
