Java - Nested Classes

（本文参考：http://docs.oracle.com/javase/tutorial/java/javaOO/nested.html）

Nested Classes

class OuterClass {
    ......
    class StaticNestedClass {
        ......
    }
    ......
    class InnerClass {
    ......
}

定义

nested classes （嵌套类）分为两种：non-static与static，前者通常称作inner classes，后者称作static nested classes。

作用域与访问范围

两种nested classes都是其enclosing class的内部成员，non-static classes可以引用到包装类的private成员，static classes没有这个能力。两种nested classes都可以被声明为public/protected/private/package private。

作用场景（因原文概括性高，直接引用）

• It is a way of logically grouping classes that only used in one place.
• It increases encapsulation.
• It can lead to more readable and maintainable code.

Static Nested Classes

• static nested classes不可直接调用其外部类的方法／成员，必须通过一个外部类的实例才能访问（在这一点上，static inner class表现的如同一个top-level class）
• 用如下方式创建一个static nested class实例

  OuterClass.StaticNestedClass foo = new OuterClass.StaticNestedClass();

Inner Classes

• As with instance methods and variables, an inner class is associated with an instance of its enclosing class and has direct access to that object's methods and fields.
• Cause an inner class is associated with an instance, it cannot define any static members itself.
• An instance of InnerClass can exist only within an instance of OuterClass and has direct access to the methods and fields of its enclosing instance.
• 用如下方式创建一个inner class实例

  OuterClass.InnerClass innerObject = outerObject.new InnerClass();

• 有两种特殊的inner classes：local classes和anonymous class

Shadowing（不知确切的中文翻译是什么，“遮蔽”？）

原文用一段很简洁的代码讲清了这个问题

public class Main {
    public static void main(String[] args) throws Exception {
        (new OuterClass()).new InnerClass().printX(2);
        System.out.println("FINISH!");
    }
}

class OuterClass {
    int x = 0;
    class InnerClass {
        int x = 1;
        public void printX (int x) {
            System.out.println("x=" + x);
            System.out.println("this.x=" + this.x);
            System.out.println("OuterClass.this.x=" + OuterClass.this.x);
        }
    }
}

输出：

x=2
this.x=1
OuterClass.this.x=0
FINISH!

Serialization

• Serialization of inner classes, including local and annonymous classes, is strongly discouraged.
• 原因是编译器在compile certain constructs时，会产生synthetic constructs：these are classes, methods, fields, and other constructs that do not have a corresponding construct in the source code.
• You may have compatibility issues if you serialize an inner class and then deserialize it with a different JRE implementation.