为什么在Java中的匿名类中添加公共字段不起作用？ [英] Why does adding a public field to an anonymous class in Java not work?

问题描述

我有一个如下定义的示例类：

I have an example class defined like below:

public class FooBar {

  void method1(Foo foo){ // Should be overwritten
    ...
  }

}

稍后，当我尝试这个时：

Later, when I try this:

FooBar fooBar = new FooBar(){
  public String name = null;
  @Override
  void method1(Foo foo){
    ...
  }
};

fooBar.name = "Test";

我收到一条错误消息，指出名称字段不存在。为什么？

I get an error saying that the name field does not exist. Why?

推荐答案

因为变量的类型 fooBar是 FooBar （所述变量中对象的运行时类型是实现 FooBar 的匿名类，它也是 FooBar 的子类型 ... ...

Because the type of the variable "fooBar" is FooBar (the run-time type of the object in said variable is that of the anonymous class implementing FooBar which is also a subtype of FooBar)...

...类型 FooBar 没有所述成员。因此，编译错误。 （请记住，变量fooBar可以包含符合 FooBar 的任何对象，即使是那些没有 name ，因此编译器会拒绝非类型安全的代码。）

...and the type FooBar does not have said member. Hence, a compile error. (Remember, the variable "fooBar" can contain any object conforming to FooBar, even those without name, and thus the compiler rejects the code which is not type-safe.)

编辑：有关一个解决方案，请参阅不可信的答案，使用本地班级声明创建一个新的命名类型（替换帖子中的匿名类型）。

For one solution, see irreputable's answer which uses a Local Class Declaration to create a new named type (to replace the anonymous type in the post).

Java 不支持支持这种方式（主要是：Java不支持有用的类型推断），虽然以下工作正常，即使不是很有用：

Java does not support a way to do this (mainly: Java does not support useful type inference), although the following does work, even if not very useful:

(new foobar(){
  public String name = null;
  @Override
  void method1(Foo foo){
    ...
  }
}).name = "fred";

快乐编码。

Scala和C＃都支持所需的类型推断，因此支持局部变量的匿名类型特化。 （尽管C＃不支持匿名扩展现有类型）。但是，Java没有。

Both Scala and C# support the required type inference, and thus anonymous type specializations, of local variables. (Although C# does not support extending existing types anonymously). Java, however, does not.

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