It allows you to group classes that logically belong together and to control the visibility of one within the other.
It knows about and can communicate with the surrounding class.
Creating inner classes
You place the class definition inside a surrounding class.
More typically, an outer class will have a method that returns a reference to an inner class.
If you want to make an object of the inner class anywhere except from within a non-static method of the outer class, you must specify the type of that object as OuterClassName.InnerClassName.
The link to the outer 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.
Inner classes have access rights to all the elements in the enclosing class.
The inner class can access methods and fields from the enclosing class as if it owned them.
The inner class secretly captures a reference to the particular object of the enclosing class that was responsible for creating it.
When you refer to a member of the enclosing class, that reference is used to select that member.
An object of an inner class can be created only in association with an object of the enclosing class when the inner class is non-static.
Using .this and .new
If you need to produce the reference to the outer-class object, you name the outer class followed by a dot and this.
Sometimes you want to tell some other object to create an object of one of its inner classes.
To do this you must provide a reference to the other outer-class object in the new expression, using the .new syntax.
To create an object of the inner class directly, you must use an object of the outer class to make an object of the inner class.
It’s not possible to create an object of the inner class unless you already have an object of the outer class. because the object of the inner class is quietly connected to the object of the outer class that it was made from.
If you make a nested class (a static inner class), then it doesn’t need a reference to the outer-class object.
Inner classes and upcasting
Normal (non-inner) classes cannot be made private or protected; they may only be given public or package access.
You can’t even downcast to a private inner class (or a protected inner class unless you’re an inheritor), because you can’t access the name.
The private inner class provides a way for the class designer to completely prevent any type-coding dependencies and to completely hide details about implementation.
Inner classes in methods and scopes
Inner classes can be created within a method or even an arbitrary scope.
Anonymous inner classes
What this strange syntax means is "Create an object of an anonymous class that’s inherited from Contents."
If your base class needs a constructor with an argument, you simply pass the appropriate argument to the base-class constructor.
The semicolon at the end of the anonymous inner class doesn’t mark the end of the class body. Instead, it marks the end of the expression that happens to contain the anonymous class.
If you’re defining an anonymous inner class and want to use an object that’s defined outside the anonymous inner class, the compiler requires that the argument reference be final.
With instance initialization, you can, in effect, create a constructor for an anonymous inner class.
In effect, an instance initializer is the constructor for an anonymous inner class. you can have only one of these constructors.
Anonymous inner classes can either extend a class or implement an interface, but not both. And if you do implement an interface, you can only implement one.
Factory Method revisited
Prefer classes to interfaces. If your design demands an interface, you’ll know it. Otherwise, don’t put it in until you are forced to.
Nested classes
You don’t need an outer-class object in order to create an object of a nested class.
You can’t access a non-static outer-class object from an object of a nested class.
Fields and methods in ordinary inner classes can only be at the outer level of a class, so ordinary inner classes cannot have static data, static fields, or nested classes.
A nested class does not have a special this reference, which makes it analogous to a static method.
Classes inside interfaces
A nested class can be part of an interface, any class you put inside an interface is automatically public and static.
You can even implement the surrounding interface in the inner class.
It’s convenient to nest a class inside an interface when you want to create some common code to be used with all different implementations of that interface.
You can use a nested class to hold your test code.
Reaching outward from a multiply nested class
It doesn’t matter how deeply an inner class may be nested—it can transparently access all of the members of all the classes it is nested within.
Why inner classes?
Typically, the inner class inherits from a class or implements an interface, and the code in the inner class manipulates the outer-class object that it was created within.
You could say that an inner class provides a kind of window into the outer class.
What is it that distinguishes an inner class implementing an interface from an outer class implementing the same interface? The answer is that you can’t always have the convenience of interfaces—sometimes you’re working with implementations.
Each inner class can independently inherit from an implementation. Thus, the inner class is not limited by whether the outer class is already inheriting from an implementation.
One way to look at the inner class is as the rest of the solution of the multiple-inheritance problem.
But inner classes effectively allow "multiple implementation inheritance." That is, inner classes effectively allow you to inherit from more than one non-interface.
You’ll ordinarily have some kind of guidance from the nature of the problem about whether to use a single class or an inner class.
If you have abstract or concrete classes instead of interfaces, you are suddenly limited to using inner classes if your class must somehow implement both of the others.
Closures & callbacks
A closure is a callable object that retains information from the scope in which it was created.
An inner class is an object-oriented closure, it has permission to manipulate all the members, even private ones.
With a callback, some other object is given a piece of information that allows it to call back into the originating object at some later point.
If a callback is implemented using a pointer, however, you must rely on the programmer to behave properly and not misuse the pointer.
The closure provided by the inner class is a good solution—more flexible and far safer than a pointer.
When you create an inner class, you do not add to or modify the interface of the outer class.
The value of the callback is in its flexibility; you can dynamically decide what methods will be called at run time.
Inner classes & control frameworks
To apply an application framework, you typically inherit from one or more classes and override some of the methods.
A common problem in application programming is the graphical user interface (GUI), which is almost entirely event-driven.
Inner classes allow you to have multiple derived versions of the same base class, Event, within a single class.
Notice how inner classes almost look like multiple inheritance.
It’s more flexible to read the events from a file instead of hardcoding them.
You’ll see how elegantly inner classes are used to describe the actions of a graphical user interface.
Inheriting from inner classes
The problem is that the "secret" reference to the enclosing class object must be initialized, and yet in the derived class there’s no longer a default object to attach to.
You must use a special syntax to make the association explicit.
Can inner classes be overridden?
"overriding" an inner class as if it were another method of the outer class doesn’t really do anything.
The two inner classes are completely separate entities, each in its own namespace.
Local inner classes
A local inner class cannot have an access specifier because it isn’t part of the outer class, but it does have access to the final variables in the current code block and all the members of the enclosing class.
Local inner class can have a constructor, Anonymous inner class cannot have a named constructor, only an instance initializer.
Since the name of the local inner class is not accessible outside the method, the only justification for using a local inner class instead of an anonymous inner class is if you need a named constructor and/or an overloaded constructor, since an anonymous inner class can only use instance initialization.
Inner-class identifiers
Every class produces a .class file that holds all the information about how to create objects of this type (this information produces a "meta-class" called the Class object).
If inner classes are anonymous, the compiler simply starts generating numbers as inner-class identifiers.
