TIJ——Chapter Ten:Inner Classes

先提纲挈领地来个总结：

内部类（Inner Class）有四种：

1. member inner class，即成员内部类。可以访问外部类所有方法与成员变量。生成成员内部类对象的方法：OuterClass.InnerClass inner = new OuterClass().new InnerClass();
2. static inner class，即静态内部类。只能访问外部类的静态方法与静态成员变量，生成静态内部类对象的方法：OuterClass.InnerClass inner = new OuterClass.InnerClass();
3. Local Inner Class，即局部内部类。定义在方法中，只能访问方法中声明的final 类型的变量（如果作为构造体参数，则不必是final）。
4. Anonymous Inner Class，即匿名内部类。匿名内部类会隐式地继承一个父类或实现一个接口。

个人认为，内部类最常见的应用场景是：在方法中返回匿名内部类对象。

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Remember the advice: Prefer classes to interfaces. If your design demands an interface, you'll know it. Otherwise, don't put it in ultil you are forced to.

• It's possible to place a class definition within another class definition. This is called an inner class.
• when you create an inner class, an object of that inner class has a link to the enclosing object that made it, and so it can access the members of that enclosing object--without any special qualifications. In addition, inner classes have access rights to all the elements(include private member) in the enclosing class. How can this happen? The inner class secretly captures a reference to the particular object of the enclosing class that was responsible for creating it. Then, when you refer to a member of the enclosing class, the reference is used to select that member. Fortunately, the compiler takes care of all these details for you.
• 在内部类中通过外部类名.this 可以拿到外部类对象的引用。
• 外部类对象.new 语法可以用于创建外部类的内部类对象。
• It's not possible to create an object of the inner class unless you already have an object of the outer class. This is because the object of the inner class is quietly connected to the object of the outer class that is was made from. However, if you make a nested class(a static inner class), then it doesn't need a reference to the out-class object.
• Normal(non-inner) classes cannot be made private or protected; they may only be given public or package access.
• Inner classes can be created within a method or even an arbitrary scope. There are two reasons for doing this: 1. You're implementing an interface of some kind so that you can create and return a reference. 2. You're solving a complicated problem and you want to create a class to aid in your solution, but you don't want it publicly available.
• Anonymous inner classes are somewhat limited compared to regular inheritance, because they can either extend a class or implement an interface, but not both. And if you do implement an interface, you can only implement one.
• To understand the meaning of static when applied to inner classes, you must remember that the object of an ordinary inner class implicitly keeps a reference to the object of the enclosing class that created it. This is not true, however, when you say an inner class is static. A static inner class means: 1. You don't need an outer-class object in order to create an object of a nested class. 2. You can't access a non-static outer-class object from an object of a static inner class.
• Ordinary inner classes cannot have static data, static fields, or static inner classes. However, static inner classes can have all of these.
• As you've see earlier, in an ordinary(non-static) inner class, the link to the outer-class object is achieved with a special this reference. A static inner class does not have a special this reference, which makes it analogous to a static method.