1.static 关键字

• 修饰的成员被所有对象共享（包括成员变量和方法）。
• 修饰的成员优先于对象存在。
• 存储于方法区（共享数据区）的静态区中。
• 静态方法只能访问静态成员。
• 静态方法中不可以使用this或super关键字。
• 主函数是static，只能调用static方法。
• 静态代码块随着类的加载而运行（只执行一次）。用于给类进行初始化。

Q:

I have the following code:

1: class Hello {

2:     class Thing {

3:         public int size;

4: 

5:         Thing() {

6:             size = 0;

7:         }

8:     }

9: 

10:     public static void main(String[] args) {

11:         Thing thing1 = new Thing();

12:         System.out.println("Hello, World!");

13:     }

14: }

it refuses to compile. I get No enclosing instance of type Hello is accessible." at the line that creates a new Thing.

 

A：

You've declared the class Thing as a non-static inner class. That means it must be associated with an instance of the Hello class.

In your code, you're trying to create an instance of Thing from a static context. That is what the compiler is complaining about.

There are a few possible solutions. Which solution to use depends on what you want to achieve.

• Change Thing to be a static nested class.

  1: static class Thing

• Create an instance of Hello, then create an instance of Thing.

  1: public static void main(String[] args)

  2: {

  3:     Hello h = new Hello();

  4:     Thing thing1 = h.new Thing(); // hope this syntax is right, typing on the fly :P

  5: }

• Move Thing out of the Hello class.