Why does Android prefer static classes

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(){}
}