导入Math.PI作为参考或值 [英] Importing Math.PI as reference or value

问题描述

我正准备用Java进行基本认证。

I'm preparing for a basic certification in Java.

我对一个我正确的问题（！）的答案感到有些困惑： -

I'm a little confused by the answer to a question that I have got right(!):-

鉴于：

 public class Circle {
     static double getCircumference(double radius ) {
          return PI * 2 * radius;
     }
     public static double getArea(double radius) {
          return PI * radius * radius;
     }
}

哪个import语句将使代码能够编译和运行？

Which import statement will enable the code to compile and run?

import java.lang.*;

import static java.lang.Math.PI;

import java.lang.Math.*;

import java.lang.Math;

我回答了import static java.lang.Math.PI;

I answered import static java.lang.Math.PI;

但下面另外两个选项的解释让我感到困惑： -

BUT the explanation of two other options below confuses me:-

语句导入java.lang.Math ;并导入java.lang.Math。*;不会启用代码编译和运行。这些import语句只允许Math.PI作为PI常量的引用。

The statements import java.lang.Math; and import java.lang.Math.*; will not enable the code to compile and run. These import statements will only allow Math.PI as a reference to the PI constant.

我的问题是：import语句只会允许引用PI常数？该值是否未初始化为零？

My question is: what would be wrong with the import statements only allowing a reference to the PI constant? Would the value be uninitialized and zero?

推荐答案

'允许Math.PI作为PI常量的引用'表示您的代码必须看起来像这样才能工作：

'Allow Math.PI as a reference to the PI constant' means that your code will have to look like this in order to work:

static double getCircumference(double radius ) {
      return Math.PI * 2 * radius;
 }
 public static double getArea(double radius) {
      return Math.PI * radius * radius;
 }

什么 import java.lang.Math; 确实导入了类 java.lang.Math ，所以你可以用 Math 来代替限定版本 java.lang.Math 。 import java.lang.Math。*; 对 Math 和所有嵌套类执行相同操作，但不是它的成员。

What import java.lang.Math; does is importing the class java.lang.Math so you can reference it with Math instead of the qualified version java.lang.Math. import java.lang.Math.*; does the same for Math and all nested classes, but not it's members.

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