是否可以在 Java 静态中创建匿名内部类? [英] Is it possible to make anonymous inner classes in Java static?

问题描述

在 Java 中，嵌套类可以是 static 也可以不是.如果它们是static，则它们不包含对包含实例的指针的引用(它们也不再称为内部类，它们称为嵌套类).

In Java, nested classes can be either static or not. If they are static, they do not contain a reference to the pointer of the containing instance (they are also not called inner classes anymore, they are called nested classes).

在不需要引用时忘记创建嵌套类 static 会导致垃圾收集或逃逸分析问题.

Forgetting to make an nested class static when it does not need that reference can lead to problems with garbage collection or escape analysis.

是否也可以创建一个匿名内部类 static ?还是编译器会自动计算出来(它可以，因为不能有任何子类)?

Is it possible to make an anonymous inner class static as well? Or does the compiler figure this out automatically (which it could, because there cannot be any subclasses)?

例如，如果我制作一个匿名比较器，我几乎不需要对外部的引用:

For example, if I make an anonymous comparator, I almost never need the reference to the outside:

  Collections.sort(list, new Comparator<String>(){
       int compare(String a, String b){
          return a.toUpperCase().compareTo(b.toUpperCase());
       }
  }

推荐答案

不，你不能，而且不，编译器无法弄清楚.这就是为什么 FindBugs 总是建议将匿名内部类更改为命名的 static 嵌套类的原因，如果它们不使用其隐式 this 引用.

No, you can't, and no, the compiler can't figure it out. This is why FindBugs always suggests changing anonymous inner classes to named static nested classes if they don't use their implicit this reference.

Tom Hawtin - Tackline 说如果匿名类是在静态上下文中创建的(例如在 main 方法中)，匿名类实际上是 <代码>静态.但 JLS 不同意:

Tom Hawtin - tackline says that if the anonymous class is created in a static context (e.g. in the main method), the anonymous class is in fact static. But the JLS disagrees:

匿名类永远不是抽象(第 8.1.1.1 节).匿名类始终是内部类(第 8.1.3 节)；它永远不是 static(第 8.1.1、第 8.5.1 节).匿名类总是隐式final(第8.1.1.2节).

An anonymous class is never abstract (§8.1.1.1). An anonymous class is always an inner class (§8.1.3); it is never static (§8.1.1, §8.5.1). An anonymous class is always implicitly final (§8.1.1.2).

Roedy Green 的 Java 词汇表 说 在静态上下文中允许匿名类的事实是依赖于实现:

Roedy Green's Java Glossary says that the fact that anonymous classes are allowed in a static context is implementation-dependent:

如果你想让那些维护你的代码的人感到困惑，wags 发现 javac.exe 将允许 static 初始化代码和 static 中的匿名类方法，即使语言规范说匿名类从不static.当然，这些匿名类无法访问对象的实例字段.我不建议这样做.功能 可以随时拉取.

If you want to baffle those maintaining your code, wags have discovered javac.exe will permit anonymous classes inside static init code and static methods, even though the language spec says than anonymous classes are never static. These anonymous classes, of course, have no access to the instance fields of the object. I don’t recommend doing this. The feature could be pulled at any time.

编辑 2: JLS 实际上在 §15.9.2:

Edit 2: The JLS actually covers static contexts more explicitly in §15.9.2:

让 C 成为被实例化的类，让 i 成为被创建的实例.如果 C 是一个内部类，那么 i 可能有一个直接封闭的实例.i(第 8.1.3 节)的直接封闭实例如下确定.

Let C be the class being instantiated, and let i be the instance being created. If C is an inner class then i may have an immediately enclosing instance. The immediately enclosing instance of i (§8.1.3) is determined as follows.

• 如果 C 是匿名类，则:
  • 如果类实例创建表达式出现在静态上下文中(第 8.1.3 节)，则 i 没有直接封闭的实例.
  • 否则，i 的直接封闭实例是 this.
  • If C is an anonymous class, then:
    • If the class instance creation expression occurs in a static context (§8.1.3), then i has no immediately enclosing instance.
    • Otherwise, the immediately enclosing instance of i is this.

    因此静态上下文中的匿名类大致相当于 static 嵌套类，因为它不保留对封闭类的引用，即使它在技术上不是 static 类.

    So an anonymous class in a static context is roughly equivalent to a static nested class in that it does not keep a reference to the enclosing class, even though it's technically not a static class.

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