# Java 程序计算圆的面积和周长

> 原文： [https://beginnersbook.com/2014/01/java-program-to-calculate-area-and-circumference-of-circle/](https://beginnersbook.com/2014/01/java-program-to-calculate-area-and-circumference-of-circle/)

在本教程中，我们将看到**如何在 Java** 中计算圆的面积和周长。有两种方法可以做到这一点：

1）用户交互：程序将提示用户输入圆的半径
2）没有用户交互：半径值将在程序本身中指定。

**计划 1：**

```java
/**
 * @author: BeginnersBook.com
 * @description: Program to calculate area and circumference of circle
 * with user interaction. User will be prompt to enter the radius and 
 * the result will be calculated based on the provided radius value.
 */
import java.util.Scanner;
class CircleDemo
{
   static Scanner sc = new Scanner(System.in);
   public static void main(String args[])
   {
      System.out.print("Enter the radius: ");
      /*We are storing the entered radius in double
       * because a user can enter radius in decimals
       */
      double radius = sc.nextDouble();
      //Area = PI*radius*radius
      double area = Math.PI * (radius * radius);
      System.out.println("The area of circle is: " + area);
      //Circumference = 2*PI*radius
      double circumference= Math.PI * 2*radius;
      System.out.println( "The circumference of the circle is:"+circumference) ;
   }
}
```

输出：

```java
Enter the radius: 1
The area of circle is: 3.141592653589793
The circumference of the circle is:6.283185307179586
```

**计划 2：**

```java
/**
 * @author: BeginnersBook.com
 * @description: Program to calculate area and circumference of circle
 * without user interaction. You need to specify the radius value in 
 * program itself.
 */
class CircleDemo2
{
   public static void main(String args[])
   {
      int radius = 3;
      double area = Math.PI * (radius * radius);
      System.out.println("The area of circle is: " + area);
      double circumference= Math.PI * 2*radius;
      System.out.println( "The circumference of the circle is:"+circumference) ;
   }
}
```

Output:

```java
The area of circle is: 28.274333882308138
The circumference of the circle is:18.84955592153876
```