“object.new”是怎么回事？工作？ （Java有.new运算符吗？） [英] How does "object.new" work? (Does Java have a .new operator?)

问题描述

今天我在阅读加速GWT（Gupta） - 第151页。

public static void getListOfBooks(String category, BookStore bookStore) {
    serviceInstance.getBooks(category, bookStore.new BookListUpdaterCallback());
}
public static void storeOrder(List books, String userName, BookStore bookStore) {
    serviceInstance.storeOrder(books, userName,    bookStore.new StoreOrderCallback());
}

这些新运营商在那里做什么？我从来没有见过这样的语法，任何人都可以解释一下吗？

What are those new operators doing there? I've never seen such syntax, can anyone explain?

有没有人知道在java规范中哪里可以找到这个？

Does anyone know where to find this in the java spec?

推荐答案

他们是内部（嵌套的非静态）类：

They're inner (nested non-static) classes:

public class Outer {
  public class Inner { public void foo() { ... } }
}

你可以这样做：

Outer outer = new Outer();
outer.new Inner().foo();

或简单地说：

new Outer().new Inner().foo();

原因是内部有对外部类的特定实例的引用。让我举一个更详细的例子：

The reason for this is that Inner has a reference to a specific instance of the outer class. Let me give you a more detailed example of this:

public class Outer {
  private final String message;

  Outer(String message) {
    this.message = message;
  }

  public class Inner {
    private final String message;

    public Inner(String message) {
       this.message = message;
    }

    public void foo() {
      System.out.printf("%s %s%n", Outer.this.message, message);
    }
  }
}

并运行：

new Outer("Hello").new Inner("World").foo();

输出：

Hello World

注意：嵌套类可以静态也是。如果是这样，他们没有隐含的这个对外部类的引用：

Note: nested classes can be static too. If so, they have no implicit this reference to the outer class:

public class Outer {
  public static class Nested {
    public void foo() { System.out.println("Foo"); }
  }
}

new Outer.Nested.foo();

静态嵌套类通常是 private 因为它们往往是实现细节和封装问题的一部分，而不会污染公共命名空间。

More often than not, static nested classes are private as they tend to be implementation details and a neat way of encapsulating part of a problem without polluting the public namespace.

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