为什么不能从静态上下文引用非静态变量-reg [英] Why non-static variable cannot be reference from a static context - reg

问题描述

可能重复：

不能从静态上下文（java）引用非静态变量

public class DemoJava {

    public class Hello {

        void fun()
        {
            System.out.println("This is static fun man!!");    
        }
    }

    public static void main(String[] args) {

        Hello hello = new Hello();
        hello.fun();
    }
}

在此示例中，由于我正在尝试从静态方法访问非静态类。精细。例如，如果我在另一个文件中有相同的 Hello 类，并且我做同样的事情，则不会给我错误。

In this example it will give me an error since I am trying to access a non-static class from a static method. Fine. For instance, if I have the same Hello class in another file and I do the same thing it does not give me an error.

即使在这种情况下，我们也尝试从静态方法访问非静态类。但这不会带来任何错误。为什么？

Even in that case we are trying to access non-static class from static method. But that doesn't give any error. Why?

推荐答案

在此示例中，由于我尝试访问非

In this example it will give me an error since I am trying to access a non-static class from a static method.

不，它会给您一个错误，因为您正试图创建一个内部类的实例（隐式引用封闭类的实例），而您没有封闭类的实例。

No, it will give you an error because you're trying to create an instance of an inner class (which implicitly has a reference to an instance of the enclosing class) when you don't have an instance of the enclosing class.

The问题不是对 fun（）的调用-这是构造函数调用。

The problem isn't the call to fun() - it's the constructor call.

例如，您可以使用以下方法解决此问题：

For example, you can fix this by using:

DemoJava demo = new DemoJava();
Hello hello = demo.new Hello();

或者您可以通过更改类声明使其成为嵌套而不是内部类到：

Or you could just make it a nested but not inner class, by changing the class declaration to:

public static class Hello

阅读以下内容的第8.1.3节JLS 获取有关内部类的更多信息，以及第15.9.2节，用于确定类实例创建表达式的封闭实例：

Read section 8.1.3 of the JLS for more information on inner classes, and section 15.9.2 for determining enclosing instances for a class instance creation expression:

否则，C是内部对象成员类（第8.5节），然后：

Otherwise, C is an inner member class (§8.5), and then:

• 如果类实例创建表达式是不合格的类实例创建表达式，则：

• If the class instance creation expression is an unqualified class instance creation expression, then:

• 如果类实例创建表达式在静态上下文中发生，则将发生编译时错误。

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