Why does Android prefer static classes

问题:

I see a lot of java code where android prefers to have developers use static inner classes. Particularly for patterns like the ViewHolder Pattern in custom ListAdapters.

I'm not sure what the differences are between static and non-static classes. I've read about it but it doesn't seem to make sense when concerned with performance or memory-footprint.

回答1:

It's not just Android developers...

A non-static inner class always keeps an implicit reference to the enclosing object. If you don't need that reference, all it does is cost memory. Consider this:

class Outer {
    class NonStaticInner {}
    static class StaticInner {}
    public List<Object> foo(){ 
        return Arrays.asList(
            new NonStaticInner(),
            new StaticInner()); 
    }
}

When you compile it, what you get will be something like this:

class Outer {
    Outer(){}
    public List<Object> foo(){ 
        return Arrays.asList(
            new Outer$NonStaticInner(this),
            new StaticInner()); 
    }
}
class Outer$NonStaticInner {
    private final Outer this$0;
    Outer$NonStaticInner(Outer enclosing) { this$0 = enclosing; }
}
class Outer$StaticInner {
    Outer$StaticInner(){}
}

回答2:

The main difference between static and non-static inner classes is that a non-static inner class has access to other members of the outer class, even if they are private. Non-static inner classes are a "part" of the outer class. You cannot create nor can they exist without an instance of an outer class. A consequence of this is that an instance of a non-static inner classes are destroyed when the outer class's instance is destroyed.

Static inner classes, on the other hand, are just like normal outer classes. The live and die on their own. You don't need an instance of the outer class for the inner class to exist. That means they also have their own life cycle. They get destroyed when the garbage collector decides to destroy them.

How does this affect memory and/or performance? I really don't know. :)

回答3:

Static inner classes (i.e. classes declared inside another class with keyword static) are quite similar to "normal" classes except you don't pollute your package's name space. That is their (only) difference and benefit and I believe that's the reason you see it in Android.

Use static inner classes when the purpose of the class is tighten to the main class, but does not depend on its instances. This is generally considered as a good practice.

回答4:

If you decompile an inner class (or watch it using debugger) you can see that there is generated code for accessing the instance of the outer class that was used to create them. The overhead for this is more memory for the additional pointer, more cpu for garbage collection because of additional pointer to test, and if you want to nit pick, longer compile time. Creating instances of non static inner classes is a bit more complicated because you need an instance of the outer class to create them.

Visibility of both static and non-static inner classes can be controlled. Usually they are private if their implementation is strongly connnected to internal details of the outer class, and the developer doesn't think the code can be reused. In this sense they are not better than private functions. Inner classes might be public in cases like Map.Entry, where the inner class is strongly connected to the interface exposed by the class, and the developer doesn't think that Map.Entry can be used without some kind of a Map. Both types have access to private members of the outer class and the outer class has access to private members of the inner class.

Instances of static and non-static inner classes are garbage collected like every other class. There is no special connection between the grabage collection of the outer class and the garbage collection of the inner class.

In the case of UI classes implementation like swing or android you will see static inner classes because they are treated like private function. These classes are not developed for reusability outside the outer class and are strongly connected to the internal implementation of the outer class. There is no reason to expose them and to make sure they can work in more cases than the specific context of the outer class requirements.

回答5:

A non static inner class instance holds a reference to the outer class instance while a static inner class instance does not.

This is relevant for the applications memory footprint as the hidden reference may lead to memory leaks - the garbage collector cannot collect the outer class instance until no more references exist. Also the additional reference itself needs memory, this may be relevant if a high number of instances are used.

class Outer{
    class Inner{//Only works with non static inner class
          public Outer getOuter(){return Outer.this;}
    }
}

It is also relevant for its use, the reference to the outer class is a ctor argument of the inner class, to create a new non static inner class object you have to call the ctor like a memberfunction on an instance of the outer class or from within a memberfunction of the outer class. This means that you cannot have a instance of the inner class without an instance of the outer class.

Outer.Inner in = new Outer().new Inner();