用于实例化内部类的奇怪语法 [英] Strange syntax for instantiating an inner class

问题描述

我没想到在这个阶段我会在Java中遇到根本上新的语法，但是我只是遇到了一些问题：

确切的上下文和下面的代码应该做什么是非常不相关的 - 它只是为了给某种上下文。

我试图合成创建一个事件在IT Mill工具包，所以我写了这样的行：

  buttonClick（new Button.ClickEvent（button））; 
  

但是，Eclipse给了我以下错误信息：

没有可以访问类型为Button的封闭实例。必须使用Button类型的封闭实例（egxnew A（），其中x是Button的实例）限定分配。

我重写上面的行如下，它不会抱怨：

  buttonClick（button.new ClickEvent（button））; // button instanceof Button 
  

所以，我的问题是：后面的语法是什么意思， 什么是Java抱怨，它在第二个版本中做了什么？

背景info： Button 和 Button.ClickEvent 都是非抽象公开类。

b $ b

解决方案

内部类（如 Button.ClickEvent ）需要引用外部实例该语法创建一个 Button.ClickEvent

的新实例

/ code>其外部类引用设置为按钮的值。

忽略缺少封装等，它只是为了演示的目的：

  class Outer 
 {
字符串名称; 
 
 class Inner 
 {
 void sayHi（）
 {
 System.out.println（Outer name =+ name）; 
} 
} 
} 
 
 public class Test 
 {
 public static void main（String [] args）
 { 
 Outer outer = new Outer（）; 
 outer.name =Fred; 
 
 Outer.Inner inner = outer.new Inner（）; 
 inner.sayHi（）; 
} 
} 
  

请参阅规范的第8.1.3节有关内部类和封闭实例的更多信息。

I didn't imagine that I would encounter radically new syntax in Java anymore at this stage, but lo and behold, I just encountered something:

The exact context and what the code below should do is pretty irrelevant - it's there just to give some kind of context.

I'm trying to synthetically create an event in IT Mill Toolkit, so I wrote this kind of line:

buttonClick(new Button.ClickEvent(button));

But, Eclipse gives me the following error message:

No enclosing instance of type Button is accessible. Must qualify the allocation with an enclosing instance of type Button (e.g. x.new A() where x is an instance of Button).

When I rewrite the line above as follows, it doesn't complain anymore:

buttonClick(button.new ClickEvent(button)); // button instanceof Button

So, my question is: What does the latter syntax mean, exactly, and why doesn't the first snippet work? What is Java complaining about, and what's it doing in the second version?

Background info: Both Button and Button.ClickEvent are non-abstract public classes.

解决方案

Inner classes (like Button.ClickEvent) need a reference to an instance of the outer class (Button).

That syntax creates a new instance of Button.ClickEvent with its outer class reference set to the value of button.

Here's an example - ignore the lack of encapsulation etc, it's just for the purposes of demonstration:

class Outer
{
    String name;

    class Inner
    {
        void sayHi()
        {
            System.out.println("Outer name = " + name);
        }
    }
}

public class Test
{
    public static void main(String[] args)
    {
        Outer outer = new Outer();
        outer.name = "Fred";

        Outer.Inner inner = outer.new Inner();
        inner.sayHi();
    }
}

See section 8.1.3 of the spec for more about inner classes and enclosing instances.

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