java - static 关键字在导入 java.lang.System 类中的作用是什么？

Question

导入System类时不明白关键字static的含义:

import static java.lang.System.*

我正在阅读有关 Java 的书，里面写着:

Any import declaration that doesn't use the word static must start with the name of a package and must end with either of the following:

• The name of a class within that package
• An asterisk (indicating all classes within that package)

For example, the declaration import java.util.Scanner; is valid because java.util is the name of a package in the Java API, and Scanner is the name of a class in the java.util package.

Here’s another example. The declaration import javax.swing.*; is valid because javax.swing is the name of a package in the Java API, and the asterisk refers to all classes in the javax.swing package.

我有以下代码:

public class Addition {

   public static void main(String[] args) {
      double num;
      num = 100.53;

      num = num + 1000;

      // So when I want to import the whole package java.lang as written in the book, it doesn't work:
      // import java.lang.*;
      // or like this:
      // import static java.lang.*;
      // NetBeans in both cases doesn't see these abbreviated names `out` and throws errors. Why?
      out.print("The result is ");
      out.print(num);
      out.println(" .");
   }
}

当我以这种方式导入时它起作用了:

import static java.lang.System.out;
import static java.lang.System.*

但是当我尝试这样做时不起作用:

import java.lang.System.out;
import java.lang.System.*

在这种特殊情况下，static 关键字的含义是什么？

为什么 import java.lang.*; 没有导入包含 System 类的整个包？

Answer 1

static 导入允许您这样写:

out.print("The result is ");

而不是这个:

System.out.print("The result is ");

参见例如http://docs.oracle.com/javase/1.5.0/docs/guide/language/static-import.html .